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Enhanced Interior Gateway Routing Protocol, or EIGRP, is a Cisco proprietary, advanced

distance vector dynamic routing protocol.

EIGRP Characteristics
Fast Convergence

EIGRP uses the DUAL algorithm to converge very quickly. It does this by knowing neighbor
router’s routing tables and predefining primary and secondary routes to every destination
Triggered Updates

EIGRP uses partial triggered updates to its directly connected neighbors rather than periodically
sharing its entire routing table. This saves link bandwidth because updates are only sent if a
change is incurred, only the changes are sent in the update, and lastly – the updates are only sent
to a routers’s affected neighbors. Very efficient!
Protocol Independent

Enhanced Interior Gateway Routing Protocol supports more than just IPv4. It supports IPv4,
IPv6, IPX, and AppleTalk.

EIGRP sends route updates, hellos, and queries to its neighbors using the multicast address so end hosts are not affected. Hellos are sent out every 5 seconds by default to learn
about new neighbors and make sure existing neighbors are still available.

Variable length subnet masking is supported by EIGRP because it is a classless routing protocol.
That means subnet masks are included in route updates.

Feasible and Advertised Distance

EIGRP’s DUAL algorithm determines the best route to a particular network by using distance
information, known as cost or metric. DUAL determines the lowest cost path by adding up the
cost to the destination network. Neighbors exchange the cost to every route they know of when a
neighbor adjacency is formed. A router then uses that information to calculate their own cost to
the same network by adding the cost between themselves and their neighbor, then adding that to
the neighbor’s advertised cost.

So, (the cost between neighbors) + (the neighbor’s cost to the destination network) = the total
cost to the remote network, or the feasible distance. The cost the neighbor advertised to the
remote network is known as the advertised distance.
See the diagram below.


Think of the successor as the active, or primary, route to a destination for EIGRP. The successor
is actually the neighbor router that has the least-cost path to a destination network (a.k.a. has the
lowest feasible distance). Successor routes are added directly to the routing table. You should
also know that multiple successors can exists if they have identical feasible distance values.
Feasible Successor

This is more like the backup route EIGRP chooses to a destination network. The feasible
successor feature is what makes EIGRP convergence so unique and so fast. It always tries to find
a backup route. In the event that the successor fails, it can immediately switch over to the
feasible successor (backup) route with very little delay. To qualify as a feasible successor, the AD
must be less than the successor’s FD. This helps ensure a loop-free layer 3 path.

Neighbor Table

EIGRP discovers neighbors by sending out hellos every 5 seconds. When a routers receives a
hello with the same AS number defined, it forms an adjacency and adds the local interface it used
to reach it as well as the neighbor’s IP address to the EIGRP neighbor table.
Topology Table

When routers form an adjacency, they exchange route information. That information is
transferred to the EIGRP topology table, which contains all the destinations advertised by a
router’s neighbors.
There are two different types of entries in the topology table, active and passive. Now you may
think that the active entry is the preferred or “actively-in-use” route, but surprisingly, the
opposite is true. The route in the topology table that is in the active state signifies that it is
“actively” looking for an alternative path to a destination because the successor has failed and no
FS exists. Obviously this is not an ideal scenario.
If a router’s successor route becomes unavailable, but has a feasible successor – the FS will
immediately become the successor and there is almost no delay incurred. This is the primary
reason EIGRP convergence times tend to be some of the fastest of all the dynamic routing
If, however, a router’s successor becomes unavailable and does not have a FS to the destination,
it will send query messages to all of its neighbors asking if they know of a path to the
destination. The neighbors will either respond with a path or forward the query to all of their
neighbor routers until a path is identified and relayed back to the original requester or no more
neighbor routers exist. During the time the router is waiting back for a response, it is unable to
forward traffic to the destination network, which can hurt EIGRP’s convergence time.
Passive entries represent routes that have at least a single successor and perhaps a feasible
successor. They are what you should see in a normal, stable topology. Notice the “P’s” in the
output from the show eigrp topology command below. They indicate that the entries in the
EIGRP topology table are in the passive (read: normal) state.
R1#sh ip eigrp topology IP-EIGRP Topology Table for AS(1)/ID( Codes:
P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - reply Status, s
- sia Status P, 1 successors, FD is 156160 via
(156160/128256), FastEthernet0/0 P, 1 successors, FD is 156160 via (156160/128256), FastEthernet0/0 via (2297856/128256),
Serial1/0 P, 1 successors, FD is 128256 via Connected, Loopback1 P, 1 successors, FD is 156160 via (156160/128256),
FastEthernet0/0 P, 1 successors, FD is 28160 via Connected,
FastEthernet0/0 P, 1 successors, FD is 2169856 via Connected,

EIGRP Messages

EIGRP hello packets are sent out every 5 seconds by default using multicast address
to maintain and discover neighbor relationships. On slower (T1 and below) and NBMA links,
hellos are sent every 60 seconds to conserve bandwidth.
EIGRP hello packets also contain a hold timer which lets the router know if a neighbor is down.
The hold timer is set to 15 seconds normally (~3 unresponsive hellos), and 180 seconds for
slower WAN links. When a router receives a hello packet from another router with the same AS
(Autonomous System) number, it automatically forms a neighbor relationship (also known as an

During the EIGRP start-up process on a router, an update message is sent out to its neighbors
containing the contents of the router’s routing table. The only other time an update packet is sent
is when network changes occur on a router and it then sends out an update message to its
neighbors who the route change would affect.

When EIGRP looses its successor route and does not have a FS, it sends out a query message to
all of its neighbors asking if they know a path. (See topology section above)

Acknowledgement packets are sent in response to update, query, or reply packets.

When a router responds to a neighbor router looking for a route (query), it sends it in the form of
a reply.

Graceful Shutdown
When an EIGRP process is shut down, the router sends out “goodbye” messages to its neighbors
(ironically in the form of hello packets). The neighbors can then immediately begin recalculating
paths to destinations that went through the shutdown router without having to wait for the hold
timer to expire.

EIGRP Metrics
There are 5 descriptives EIGRP uses to calculate its metric, although Cisco generally does not
recommend tuning these metrics unless you have a very specific purpose. You should be aware
that only the bandwidth and delay numbers factor into the default formula.

Bandwidth – the lowest bandwidth value between the source and destination

Delay – the cumulative delay along a series of links




EIGRP Configuration
Step 1. Define EIGRP as the routing protocol with a predefined Autonomous System ID. Routers
will not form a neighbor relationship if their AS numbers do not match.
R3(config)# router eigrp 1

Step 2. Define the attached networks you want to participate in EIGRP
Add each network to the EIGRP process with the network prefix mask command for each
network. The mask is an inverted mask, like ACLs use. Example, a /24 mask would be
The network prefix mask command tells the router which local interfaces will then participate in
EIGRP. This can be very useful if you do not want specific interfaces to participate in EIGRP.
Using the mask statement will define how you want the routes summarized if you turn off auto
summarization. If you choose not to use the mask, EIGRP will assume the networks are part of
the major networks (class A,B,C boundaries) and could cause potential problems.

4 0. R .Reply. FD is 128256 via Connected..1.3.100. Q .0. r . 0.100.0 0.100. R3#sh run | begin router eigrp 1 .sia Status P 192.1 Fa0/0 14 00:12:29 535 3210 0 22 show ip eigrp topology – displays the output of the EIGRP topology tables including successor and feasible successor routes. router eigrp 1 network R3(config-router)# network 192.reply Status.Example Configuration R3(config-router)#router eigrp 1 R3(config-router)# network 10.0.3 network 192.0/ Fa0/0 13 00:12:23 737 4422 0 21 0 s . Loopback3 P 10.Query. Loopback15 P 10.3 R3(config-router)# no auto-summary The output of R3′s running configuration can be seen below. R3#sh ip eigrp neighbors IP-EIGRP neighbors for process 1 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 1 10.1.0 network 192.0. FD is 156160 via 10.0. U .0.1.2 .0/24. A .0.Update.5) Codes: P .168. 1 successors.100.Active. no auto-summary EIGRP Verification show ip eigrp neighbors – displays EIGRP neighbors a router has discovered.0 0.100.0.Passive.4/30. FD is 128256 via Connected. 1 successors.225 R3(config-router)# network 192.. R3#sh ip eigrp topology IP-EIGRP Topology Table for AS(1)/ID(192.0. 1 successors.100.

FastEthernet0/0 D 10.1. FD is 2172416 via 10.0. This must be configured on every router that will use that default route.100. EX .1.1. Any network that is reachable within the local router’s routing table is eligible to be used by EIGRP as a default route.EIGRP external.OSPF NSSA external type 1.0/30 is subnetted. Loopback11 show ip route eigrp – displays the EIGRP routes that the routing table is using. U .per-user static route o .100.0/24 is subnetted.OSPF. FastEthernet0/0 P 10.static. FastEthernet0/0 [90/2172416] via 10.OSPF inter area N1 .0 is directly connected.1. FastEthernet0/0 [90/2172416] via 10.0 [90/156160] via 10.IS-IS summary.100. One option is to use a static default route with the ip route 0.OSPF NSSA external type 2 E1 .1. su .168. in conjunction with a static route – you will have to first redistribute the static route into EIGRP. FastEthernet0/0 D 10.100.IS-IS inter area. EIGRP will advertise the route to its EIGRP neighbors as a default route.1.100.1. R .RIP. . 2 subnets C 192.0/24.1 (156160/128256). B . Once configured. FD is 156160 via 10.200.200. R3#sh ip route Codes: C .1.0 [90/156160] via 10.OSPF external type 2 i .0.BGP D . [90/2172416] via FastEthernet0/0 C 10. They can decrease the size (and complexity) of the routing table by providing a path to all unspecified destinations.100. S .periodic downloaded static route Gateway of last resort is not set 10. O . 00:16:49.1.OSPF external type 1.100. 00:14:55.4 is directly connected.1 (2172416/2169856).0 is directly connected.0. 00:14:46.100. 5 subnets C 10. E2 .1.100. 1 successors.1.1.IS-IS level-1.2 (2172416/2169856). R3#sh ip route eigrp 10. 2 successors.2.168.1. 00:16: 5 subnets D 10.1.0 0.0. 00:16:49. FastEthernet0/0 P ** If you want to use this method.100. FastEthernet0/0 P 10.0/24. Another option if you are running EIGRP is to use the ip default-network network-number command in global configuration mode.0.0 [90/156160] via 10. 00:14:46.1. 1 successors.0. FastEthernet0/0 00:14:46. FastEthernet0/0 via 10. Loopback11 P 10.IS-IS.100. M .ODR. FD is 28160 via Connected. All internal EIGRP routes will be marked with a D (as in DUAL) at the beginning. L2 IS-IS level-2 ia . L1 . Loopback15 C 192.candidate default. is subnetted.1.100. * . N2 .(156160/128256).1.0. FastEthernet0/0 D 10.connected.100.168.0/30.0.100.EIGRP. FastEthernet0/0 show ip route – shows the ip routing table entries for all routing protocols. FastEthernet0/0 D [90/2172416] via 10. P . FastEthernet0/0 EIGRP Default Routes Defaults routes make life easier in many situations.1.0 [90/156160] via 10.100. IA .0 interface/address statement as discussed in the Routing Fundamentals page.0 is directly connected.1. 00:16:49.3. FD is 128256 via Connected. 1 successors. Loopback3 D 10.1.

encapsulates it into a MPLS packet. .1. or provider’s edge routers. To disable automatic summarization: R1(config)# router eigrp 1 R1(config-router)# no auto-summary It is also possible to manually summarize routes with EIGRP out specific interfaces. PE East strips off the MPLS information. re-encapsulates it into an Ethernet frame and forwards it on to CE East. This transparent transport allows an EIGRP neighbor relationship to form between the two customer routers. Summarization EIGRP summarizes routes by their major classful boundaries. The CE routers appear to each other as directly connected peers. PE West intercepts the data.255. Under the interface configuration mode. you must use the no ip route command instead of no ip default-network. That means in order to remove the default route.0 255. strips the Ethernet frame.2. R1(config)# intferface s0/0/0 R1(config-if)# ip summary-address eigrp 1 10.0 EIGRP over WAN Networks EIGRP + MPLS MPLS defines the customer’s WAN routers as CE. When CE West sends information to CE East. and forwards it over the service provider’s network to PE East.255. which can be problematic and cause specific subnets to not be advertised correctly. or customer edge routers and the carrier’s border routers as PE.** Once you use the ip default-network command to define a default route for EIGRP. use the ip summary-address eigrp autonomous-system command. the router creates a static route in the configuration without notifying you.

The subinterface is marked down whenever its local DLCI goes down. 15 second dead timer). frame relay is a dying WAN technology. The broadcast descriptive is required at the end of the statement because frame relay defaults to a non-broadcast medium. static mappings can be applied to both multipoint interfaces as well as subinterfaces on a single physical port. Frame relay is able to emulate point-to-point links by using multiple subinterface on a single physical interface (often used on hub-and-spoke topologies). but Cisco thinks it’s important for us to understand the underlying framework of how frame relay works. virtual circuits through the service provider network. The default timers are shorter (5 sec hold timer. The layer 2 virtual circuit must then be mapped to a layer three neighbor. Each VC is identified with a locally-significant DLCI. Also. configurations must be done on the interface level. which can be either dynamic or static.EIGRP + Frame Relay Let’s face it. 2. more current WAN options like MPLS and metro Ethernet have taken over. Other. Static To configure frame relay statically. R1(config-if)# frame-relay map ip remote-ip-address loacl-dlci broadcast . or Data-Link Connection Identifier. This allows neighbor’s to be identified as down much more quickly for two reasons: 1. One of the advantages of Frame Relay is that it allows multiple logical circuits to be configured on a single physical interface. Frame relay works using switched.

The first is that EIGRP assumes that WAN interfaces run at T1 speed (1544 kbs). The second is that EIGRP will allocate up to 50% of a link’s bandwidth for EIGRP control traffic. To remedy the situation. EIGRP messages could choke out data traffic quickly. In this case. split horizon must be disabled on the R2 EIGRP process. R2(config-if)# no ip split-horizon EIGRP as-number Managing EIGRP Bandwidth There are two important points to remember when running EIGRP over WAN links. Split-horizon is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned. but R3 and R1 are unable to communicate with each other. These two combined can be problematic on links that are slower than a T1 (like a 64k fractional T1 for example). if R2 learns routes from R1. No IP split horizon When running EIGRP on frame relay multipoint subinterfaces. it cannot then pass those on to R3 because split horizon would prevent the advertisement from going out the same physical interface.Dynamic Dynamic mappings use inverse ARP. When a hub and spoke frame relay topology exists. a major communication problem can occur. The issue is that split horizon is enabled by default. multipoint subinterfaces are configured on the hub router. This results in R2 being able to communicate with the spoke router’s networks. the bandwidth command should be used in WAN links to tell EIGRP what the actual link bandwidth is. In that situation. routers only form EIGRP neighbor relationships with other routers they connect to using a frame relay virtual circuit. . so in the example below. To control that.

For EIGRP.0. however. Other EIGRP Options Passive Interfaces Not to be confused with the passive (healthy) topology table entries. EIGRP will automatically load balance across equal-cost paths with no special configuration. it only works when the cost of the path is lower than the variance number multiplied by the best metric. in its ability to load balance across unequal-cost paths with a single command. this means that the router will not form adjacencies with connected routers on that particular port. EIGRP load balancing Out of the box. To configure it: R1(config)#router eigrp 1 R1(config-router)# neighbor ip-address The IP address used must be in one of the same subnet ranges as one of the router’s interfaces. . Here is an example scenario. You should be aware that EIGRP can also use a unicast address when communicating with a specific neighbor. R1(config)# router eigrp 1 R1(config-router)# passive-interface gig 3/1 Unicast Neighbors EIGRP uses multicast address 224. The variance command allows unequal-cost load balancing over up to 6 different paths.10 when sending messages to its neighbors.R1(config)# int serial 0/0/0 R1(config-if)# bandwidth 64 EIGRP is often used on frame relay for this reason alone. interfaces with the passiveinterface command applied do not allow any routing updates or hellos out the interface.0. But here’s the key. EIGRP is unique. The ability to control the routing protocol’s usable bandwidth so simply makes it a popular choice.

When configured. This will load balance the traffic in proportion to each path’s metric. Cisco IOS will load balance across 4 equal-cost paths only. the packet is silently dropped.000) by 2 (20. R1(config)#router eigrp 1 R1(config-router)# maximum-paths number-of-paths EIGRP Authentication EIGRP supports authentication of its messages using an MD5 hash. . Setting it to 1 disables the load balancing.000). if an incoming EIGRP packet’s hash does not match the local hash.000) and will begin load balancing across all paths with a FD less than that – which includes the path through R2(15. you can configure the router to load balance over up to 16 paths. we can use the following command: R1(config)#router eigrp 1 R1(config-router)# variance 2 The variance command multiplies the best cost (10. To enable unequalcost load balancing. Authentication configuration steps: 1.R1 will by default use the path through R3 because it has the lowest metric. Using the maximumpaths command. Configure a key chain to group the keys (read: passwords). Maximum-paths By default.

Example R1(config)# key chain TEST R1(config-keychain)# key 1 R1(config-keychain-key)# key-string samplepassword R1(config-keychain-key)# exit R1(config)# interface gig 1/12 R1(config-if)# ip authentication mode eigrp 10 md5 R1(config-if)# ip authentication key-chain eigrp 10 TEST EIGRP Stub Routing If a router is a spoke in a hub-and-spoke router topology. The router will look inside the keychain and compare the keys against incoming packets. 4.2. R1(config)#router eigrp 1 R1(config-router)# eigrp stub [receive-only | connected | static | summary | redistributed] Options Receive-only Connected Static Summary Result Router will not advertise any networks (including its own) Router will advertise connected routes (enabled by default) Router will advertise static routes Router will advertise summary routes (enabled by default) Router will advertise routes that have been redistributed into EIGRP from another Redistributed routing protocol or AS EIGRP Best Practices  Summarize routes when possible  Limit the network depth to 7 hops  Limit the scope of EIGRP queries . Within EIGRP you can define a router as a stub router to limit the EIGRP queries. If you have many spoke routers. it is considered a stub router. It is not a transit router and usually has only a single neighbor router. This saves bandwidth and prevents neighbor routers from requesting alternate routes when a path fails. The EIGRP stub router still receives all route updates from its neighbor(s) by default. Enable authentication and assign a key to an interface. this can dramatically improve EIGRP reconvergence time. Indicate MD5 as the authentication type. 3. sometimes two. Create a key(s) inside the keychain.

so routers only compute paths within their own area. . sometimes referred to as the backbone area and every additional area must be physically connected to area 0. A router running OSPF creates its own database which contains information on the entire OSPF network. This allows the router to make intelligent choices about path selection on its own instead of relying exclusively on neighbor information. it begins sending LSAs. Every OSPF network must contain an area 0. I apologize if the acronyms are starting to pile up. Those values are then sent to the adjacency table. they are then eligible to be added to the routing table. A router stores all of its LSA information (including info it receives from incoming LSAs) in the Link State Database (LSDB). to compute internally the best path to any given route. OSPF routers do form neighbor relationships though. Note that the SPF algorithm only runs within a single area.OSPF. They exchange hellos with neighboring routers and in the process learn their neighbor’s Router ID (RID) and cost. architecturally speaking. is more complicated than its counterpart EIGRP – and the long list of acronyms and definitions is part of that. OSPF. open-standard. This helps partition routers into manageable groups if the layer 3 network begins to get large. or Link State Advertisements. OSPF uses an algorithm known as SPF. LSAs contain the RID and costs to the router’s neighbors. From there. Once the SPF algorithm selects the best paths. using cost as it’s metric. or Dijkstra’s Shortest Path First. other areas are optional. Areas OSPF is different from EIGRP in that it uses areas to segment routing domains. LSAs are shared with every other router in the OSPF domain. not simply neighbor’s routes like EIGRP. It all starts with area 0. Every router is responsible for computing its own best paths to all destinations within an OSPF domain. dynamic routing protocol. Inter-area routes are passed using border routers. or Open Shortest Path First. is a link-state. Link State Database Once a router has exchanged hellos with its neighbors and captured Router IDs and cost information. OSPF is classless and converges fairly quickly.

Area Types Backbone area Another name for area 0 . After you reach about 50 routers. the more LSA advertisements that must be sent out. This means that the more OSPFenabled routers are configured for the same area. The following three factors determine the maximum number of routers:  How easily the area’s subnets can be summarized  The type of areas being used  The number of external LSAs being injected An added bonus of partitioning out your OSPF network into areas is that it is a natural fit for a hierarchical IP scheme. That is why Cisco recommends limiting an OSPF area to no more than 50-100 routers. the high levels of LSA traffic and numerous routing table entries can become a problem.All link state databases must match within an OSPF area.

optionally summarizing routes.000 = . The formula is: Cost = (100Mbs/bandwidth) Higher bandwidth means a lower cost.000 / 100. and optionally sourcing default routes.000 / 1544 = 64 10 Mbps | 100. and optionally a default route Totally Stubby NSSA Cisco proprietary option for NSSA Router Roles Internal: All interfaces in a single area (routers 1. 4.000.000 = 1 1000 Mbps | 100. 2 .Regular area Non-backbone area. 5 in diagram above) Backbone: At least one interface assigned to area 0 (routers 1. OSPF Metric Each interface is assigned a cost value based purely on bandwidth. with both internal and external routes Stub area Contains only internal routes and a default route Totally Stubby Area Cisco proprietary option for a stub area Not-So-Stubby area (NSSA) Contains internal routes. redistributed routes. Let’s run through some common examples quickly: T1 line | 100.000 / 10. Autonomous System Boundary Router (ASBR): Has at least one interface in an OSPF area and at least one interface outside of an OSPF area. separating LSA flooding between areas.1 1 (OSPF still uses 1 for this.000 = 10 100 Mbps | 100.000 / 1.3 in diagram above) Area Border Router (ABR): Have interfaces in two or more areas (routers 2 and 3 in diagram above) ABRs contain a separate Link State Database. see explanation below) .

it discards the LSA and sends back the newer version to the original sender. LSDB Overload In large OSPF networks. OSFP was written when 100Mbs was considered fast.  If it receives an older version (according to its sequence number). the router will enter the ignore state – dropping all adjacencies and clearing the OSPF database. it checks it against its internal Link State Database (LSDB).  If it is new. starting with 0×80000001. if major network changes occur. entries with an older sequence number are discarded. it assigns the same cost to any interface with speeds higher than 100Mbs. you can use the auto-cost command under the OSPF process. When an LSA enters a router.  If it contains a Router ID (RID) that is already in the database. a flood of LSAs will immediately hit the entire network. To fix that problem. To mitigate that scenario. The number of incoming LSAs to each router could be substantial and bring the CPU and memory to its knees. both a cost of 1. To OSPF. Because of that. . Unfortunately.535). The sequence number increases if:  a route is added or deleted  a LSA ages out The largest sequence number is always the most current. a Fast Ethernet interface is weighted the same as a Gigabit Ethernet interface. if the defined threshold is exceeded over one-minute time period. it is added to the LSDB and the SPF algorithm is re-run. The command show ip ospf database will display the sequence numbers and age (in seconds) for each entry.The cost is then accrued at each hop along the path based on the link’s bandwidth. Cisco offers what it refers to as Link Sate Database Overload Protection. The default time that LSAs are aged out is 30 minutes. Sequence numbers are 32 bits. R1(config-router)# auto-cost reference-bandwidth 1000 Another option is to simply change the cost on a per-interface basis with the ip ospf cost command (using any number between 1-65. Once enabled. R1(config-if)# ip ospf cost 35 Link State Advertisements LSAs contain a sequence number and a Router ID.

R1(config-router)# max-lsa number LSA Definitions LSA Type Name 1 Router LSA 2 Network LSA 3 Summary LSA 4 Summary LSA 5 External LSA 6 Multicast LSA 7 NSSA LSA 8 9-11 External Attributes LSA Opaque LSAs Description • Inter-area route advertisements • Produced by each OSPF router • Flooded within an area • Produced by routers on a multi-access link • Produced by DRs • Flooded within an area • Advertises inter-area routes • Produced by an ABR • Flooded to adjacent areas • Advertises routes to an ASBR • Produced by an ABR • Flooded to adjacent areas • Advertises routes in another routing domain • Produced by an ASBR router • Flooded to adjacent areas • Used in multicast OSPF environments • Advertises routes in another routing domain • Produced by an ASBR within a NSSA • Used in OSPF/BGP convergence • Used only for specific applications .Know that this is a drastic response because routing will be disrupted during that period.

0.0. Link State Request (LSR) Requests a Link State Update (LSU). Link State Update (LSU) Contains one or more complete LSAs. . including RIDs and sequence numbers.5 is used if sending to all routers. address 224. Link State Acknowledgement (LSAck) Acknowledges all other OSPF packets (except hellos).0. Database Description (DBD) Contains a summary of the LSDB. Multicast address 224.0. see below. OSPF sends the five packet types listed above over IP directly.6 is used for sending to all OSPF DRs.OSPF Packet Types Hello Discovers neighbors and works as a keepalive. using IP port 89 with an OSPF packet header.

adjacencies are only formed between the router and the DR and BDR. the router is considered down. if four consecutive hellos are not received (the dead time). 120 second dead timer OSPF States There are 7 different OSPF states when forming neighbor relationships. Two-way State A hello is received from another router with its own RID in the neighbor field. All of the following fields in an OSPF hello message must match for an adjacency to form:  hello timer  dead timer  area ID  authentication type  password  stub area flag As with many network protocols. The router forms an adjacency with a peer router when it sees its own Router ID in the neighbor field of another router’s hello message. . 3. With OSPF. Pointpoint interfaces: hellos every 10 seconds. That indicates there is direct. Note: On multi-access links. bi-directional communication on the same subnet. Down State OSPF has not started and no hellos have been sent. 1.OSPF Neighbors Hellos are sent out periodically using multicast on OSPF enabled routers. All other required elements match and the routers become neighbors. 40 second dead timer Nonbroadcast multiaccess (NBMA) interfaces: hellos every 30 seconds. Take the time to learn the states and their corresponding functions. 2. hellos act as a form of keepalive or heartbeat. Init State Hellos are sent out all OSPF-participating interfaces.

0. R1(config)# router ospf process-id The next step is to determine which router interfaces you want participating in OSPF.255.102. interfaces in the the subnet wildcard mask in the network statement is not optional because OSPF is classless by default. the LSUs are acknowledged.0 area 1 In the example above.1/24 GigabitEthernet 0/2: 192.0.5/30 The simplest way to configure OSPF an all interfaces into area 0 would be to use this command: R1(config-router)# network 0.9.1. Let’s do another example. GigabitEthernet 0/1: 192.100. and 192. LSRs are sent out for missing or outdated LSAs. Each router then responds to the LSRs with a Link State Update. networks into different statements: . 6.0/24 subnet will participate in OSPF area 0.1/30 Serial 1/1: 10.101.0/24 subnet will participate in OSPF area 1.168. R1 has six interfaces.1/24 GigabitEthernet 0/3: 192. Loading State Routers compare the DBD to their LS database.0. but has some important syntax distinctions from EIGRP. First. all within area 0: GigabitEthernet 0/0: 192. the network statements define which local router interfaces will participate. Full State The LSDB is completely synchronized with the OSPF neighbor.1/24 Serial 1/0: 10.0 0.168.1. it is configured from router configuration mode and requires a process ID appended to the router ospf command.100. Finally. 7.0. Interfaces in the 10. 5.9.255 area 0 A second option is to break up the 10. Exchange State Routers exchange DBDs.9.4. OSPF Configuration OSPF configuration is not too complicated.103. R1(config)# router ospf 10 R1(config-router)# network 10. Unlike EIGRP. Exstart State Routers determine which one will begin the route exchange process with the other.0 255. so don’t worry if it doesn’t match on other OSPF routers.255 area 0 R1(config-router)# network 10.168. The process ID is only locally significant.100. Just like EIGRP.1.

0 0.0.255 area 0 R1(config-router)# network 192.1 0.0.R1(config-router)# network 10.1 The configuration you choose is up to you.255.0. Loopbacks are preferred for use as a router ID because they are virtual interfaces and are not affected by links going up and down. it will use the highest IP address assigned to an active interface when the OSPF process begins.0 0.0.0 area 0 All three approaches achieve the exact same result. first create it and assign it an IP address.0.0 area 0 R1(config-router)# network area 0 R1(config-router)# network 192.1 Note: The clear ip ospf process command will also force the OSPF process to restart.0 area 0 R1(config-router)# network 255.0. The ip ospf process-id area area-id command takes precedence over the more common network commands. If no loopback is configured. area 0 The third way to configure the interfaces to participate in OSPF: R1(config-router)# network 10.1 0. of course.0. This helps keep the network stable and happy. Interface Configuration An alternative configuration option is to configure an interface to participate in OSPF directly. even if another interface with a higher IP address comes online unless the OSPF process is restarted.1 0. To configure a loopback interface.5 0.168. The problem. The designers of OSPF decided to use the highest IP address assigned to a loopback interface as the Router ID (RID) by default. R1(config)# int gig 0/1 R1(config-if)# ip ospf 10 area 0 Router ID The SPF algorithm uses a Router ID to identify hops along a path. OSPF will not change the RID.100. R1(config)# int loopback 0 R1(config-if)# ip address 10.3. is that routers don’t have a generic “router ID” built in.0. .0 area 0 R1(config-router)# network 10.0 area 0 R1(config-router)# network 192.103. but will cause an outage – so use it with caution.100.255 Static RIDs It is also possible to manually define a static Router ID within OSPF with the router-id command.100.101.

network LSAs to advertise the segment over multicast address 224. 2. A router with two different interfaces connected to two different multiaccess links will have separate DR elections for each segment. When the OSPF process on a router starts up. It doesn’t work well.0. The Non-Designated routers then use IP address 224. This whole system works really well when routers are connected with point-to-point links and OSPF traffic is simply sent using multicast address 224.5. the router with the highest OSPF priority is elected as the DR. In the case that the DR goes down.0.1 DRs & BDRs SPF works by mapping all paths to every destination on each router. If two routers happen to have the same OSPF priority. so they are only significant on an interface level.0. If hellos are received before the dead time expires. It uses the RID to identify hops along each path and uses bandwidth as a metric between those hops. Next.0. however. it will not participate in the elections. To set the OPSF priority. it listens for hellos.5. OSPF Election Process 1. it elects itself the DR. Remember that DRs are only used on multiaccess links. 3. . Note: If a router’s OSPF priority is set to 0.100.100. On that specific multiaccess segment. when a router is connecting to multiaccess networks like an Ethernet VLAN. Multiaccess OSPF links require a Designated Router (DR) be elected to represent the entire segment.0. routers only form adjacencies with the DR and BDR. or BDR. use the ip ospf priority command on the interface connected to the multiaccess segment. Once a DR is elected. the router with the highest Router ID will become DR. Be aware that a router with a non-zero priority that happens to boots first can become the DR just because it did not receive any hellos when the OSPF process was started – even though it may have a low OSPF priority. The DR uses type 2. elections cannot take place again until either the DR or BDR go down. the BDR automatically is assigned the DR role and a new BDR election occurs.R1(config)# router ospf 10 R1(config-router)# router-id 10. This essentially means that there is no OSPF DR preemption if another router comes online with a higher OSPF priority. Values can be between 0-255. If it does not receive any within its dead time.0.6 to communicate directly with the DR. The same is true for BDR. Another router is then elected as the Backup Designated Router. The default OSPF priority is 1 and Cisco recommends manually changing that on routers you want to become the DR and BDR. the same process happens to elect the BDR.

It also consolidates many routes in to a single statement. The new summary route’s cost will be equal to the lowest cost route within the summary range. the router will automatically create a static route pointing to Null0. This really only works well if the networks contained within an area are subnetted contiguously so that they can be easily summarized into a single statement. Avoiding running the algorithm whenever it isn’t required is a big win.100.100.255. reducing the memory load and database size on OSPF-enabled routers. Inter-area Summarization (LSA Type 3) This occurs on ABRs to summarize routes between areas.R1(config)# int gig 0/1 R1(config-if)# ip ospf priority 255 OSPF over the WAN Routing protocols assume both broadcast capabilities and full mesh connectivity on multiaccess networks. For OSPF. it’s important to note that running the SPF algorithm on a router is extremely taxing on CPU resources and can easily consume them all.0/16 is summarized from area 2.0. It prevents topology changes from being passed outside an area – thus reducing the number of routers re-running the SPF algorithm. The reason is because OSPF has to compute the best path to every destination within its area. there are a few points to consider:  Full mesh environments can use physical interfaces. Inter-area Summarization Example: ABR-R1(config)# router ospf 10 ABR-R1(config-router)# area 2 range 10. the summary network 10.0 255. but often times subinterfaces are used  Partial mesh environments should be configured using point-to-point subinterfaces  Hub-and-spoke environments should elect the hub as the DR or use point-to-point subinterfaces – which don’t require a DR  Frame Relay and ATM maps should include the broadcast attribute  In multiaccess environments.0. After the command is entered.0. Summarization has two important benefits for OSPF. the DR and BDR should have full virtual circuit connectivity to all other routers Summarization First. inter-area and external.0 In this example. There are two types of route summarization. .

External Summarization Example: ASBR-R1(config)# router ospf 10 ASBR-R1(config-router)# summary-address 192. but Cisco recommends using the default-information originate command under the OSPF routing process. OSPF Default Routes Default routes are injected into OSPF via type 5 LSAs. the router will automatically create a static route pointing to Null0. OSPF Passive Interfaces Like EIGRP. After the command is entered. thus disabling the interface from forming adjacencies out that interface.0. like a static route.0/16 and is injected into OSPF via a single type 5 LSA. R1(config)# router ospf 10 R1(config-router)# default-information originate [always] [metric metric] If the always keyword is not used. In this example.External Summarization (LSA Type 5) This occurs on ASBRs for routes that are injected into OSPF via route redistribution.0 255. Another option is to use the area range and summary-address commands discussed in the summarization section above. The passive-interface interface command disables OSPF hellos from being sent out. a default route will be advertised regardless if the route exists in the routing table. There are multiple ways to inject default routes into OSPF. OSPF will advertise a default route learned from another source. OSPF supports the use of passive interfaces. an external network has been summarized into 192.255. Using these will result in the router advertising a default route pointing to itself. Stub and Not-So-Stubby Areas .0. If the always keyword is present.

Configuration Example: R3(config)# router ospf 10 R3(config-router)# area 2 stub no-summary R3(config-router)# area 2 stub default-cost 8 . If a router needs to send traffic to a route outside of its own area. they do not know about any non-OSPF route information outside their own area. That is.0. TSAs do not accept any external routes from non-OSPF sources AND they do not accept routes from other areas within their OSPF autonomous system. The ABR in a stub area drops all external routes and instead uses a default route of 0. A Cisco proprietary version of a stub area is a Totally Stubby Area. ABRs use default routes in Stub and Totally Stubby areas. it sends the traffic using a default route. or TSA. Stubby areas are made into Totally Stubby Areas by appending the no-summary keyword to the ABR. Area 2 in the diagram above shows an example.0.Stub areas are another way to simplify route information that gets advertised.0 (R3 in this example).

connected area 0s. A Totally Stubby NSSA does not accept external or summary routes from other areas. Using the no-summary keyword turns the area into a Totally Stubby NSSA. The default-cost command is optional and in this case changed the default route cost from 1 to 8. More specifically. not a permanent design. Virtual links were developed as a band-aid to situations that temporarily must violate those requirements. Not-So-Stubby Areas. type 7. NSSAs are very similar to stubby areas. Lastly.The example above sets area 2 as a totally stubby area. . NSSA is configured using the area area-number nssa command as can been seen in the example below. External routes are advertised by the ASBR as type 7 LSAs and the ABR then converts them into type 5 external LSAs when it advertises them to adjacent areas. but one does not have the stub statement configured. the hello packets will be dropped and they will not form a neighbor adjacency. OSPF Stub Limitations  Virtual links cannot be included  Cannot include an ASBR  The stub configuration must be applied to every router within the stubby area  Area 0 cannot be a stub Bullet point 3 is extremely important. The default-information-originate option does just that. R4(config)# router ospf 10 R4(config-router)#area 1 nssa [no-summary] [default-information-originate] OSPF Virtual Links OSPF has strict rules around how areas connect and where they can be located. or NSSAs were an addendum to the original OSPF RFC and defined a new special LSA. every area must be physically connected to area 0 and area zero must be ‘contiguous’ – meaning it cannot broken into multiple. the NSSA ABR does not by default advertise a default route back into the area. but they allow the use of ASBRs in the area – something stub areas prohibit. It can also connect two area 0s together! Keep in mind that Cisco recommends virtual links be a temporary workaround to a short-term problem. If two routers are connected. Virtual links connect areas that do not connect directly to area 0.

support two message authentication options:  Simple Authentication (using plaintext keys)  MD5 Authentication Matching authentication methods and keys must configured on each interface on a segment. Simple Authentication Example R1(config)# int fa0/1 R1(config-if)# ip ospf authentication-key KEY123 R1(config-if)# ip ospf authentication R1(config-if)# exit R1(config)# router ospf 10 R1(config-router)# area 0 authentication MD5 Authentication Example R1(config)# int fa0/1 R1(config-if)# ip ospf message-digest-key 1 md5 KEY123 R1(config-if)# ip ospf authentication message-digest R1(config-if)# exit R1(config)# router ospf 10 R1(config-router)# area 0 authentication messagedigest .The diagram below illustrates an example when a virtual link could be used. Let’s pretend Company ABC and Company XYZ just announced a merger and now their corporate networks must do the same. however. In this case.30. Also.30. OSPF does.50 OSPF Authentication Out of the box. Note that the area used in the command is the transit area that the virtual link resides in. both routers R1 and R2 have now become ABRs and the virtual link configuration will be applied to them.50.50.30 R1(config-router)# exit R2(config)# router ospf 20 R2(config-router)# area 1 virtual-link 10. The command area area-number virtual-link routerid is applied to each ABR. Theoretically. different passwords could be applied to different router interfaces – the routers on the other ends of those links would just be required to have matching information. the RID identifies the RID of the OTHER router at the end of the link! R1(config)# router ospf 20 R1(config-router)# area 1 virtual-link 10. OSPF does not authenticate its protocol’s messages or route updates.

as well as DR and BDR assignments. use the show ip ospf interface command . and timers  To see which router interfaces are participating in OSPF (and their area assignments). status of neighbor adjacencies.NOTE: The 1 in the ip ospf message-digest-key 1 md5 KEY123 statement above is a key number. OSPF Verification The OSPF neighbor table can be viewed using the show ip ospf neighbor command. issue the show ip route ospf command  The show ip ospf command displays the RID.  To show which OSPF routers are being used by the routing table. It shows the status of the OSPF database loading process. counters.

EIGRP Redistribution Example R1(config)# router eigrp 10 R1(config-router)# redistribute ospf 20 metric 1000 100 255 1 1500 The example above shows OSPF being redistruted into EIGRP with a metric of 1000 100 255 1 1500. To deal with this dilemma. say RP and BGP. R1(config-router)# redistribute protocol [AS/process-ID] [metric metricvlaue] Both RIP and EIGRP require the use the metric keyword. the redistribute protocol command is used under the routing protocol that recieves the routes. and so must their networks  OSPF or EIGRP is used at the access and distribution layer of an enterprise and BGP is used in the core The challenge to redistributing routing protocols is that each routing protocol uses it own metric and they are not compatible with each other. This can happen in a number of situations. but some examples include:  Organizations transitioning routing protocols  Businesses merge. That is a lot of different numbers for an EIGRP cost! That’s because EIGRP redistribution metric requires you to input all of the metric calculation manually:  bandwidth  delay . a new seed metric is used as a staring point when redistribution is configured. Configuring Redistribution To configure redistribution between routing protocols. there is no magic algorithm than can automatically translate metrics between.Redistribution is necessary when routing protocols connect and must pass routes between the two. Furthermore.

Determine if you want to filter updates coming into the router or leaving the router. Make sure to include it. Incoming Distribute Lists: R1(config-router)# distribute-list {acl-number | name} in [interface-type number] Outgoing Distribute Lists: R1(config-router)# distribute-list {acl-number | name} out [interface-name | routing-process | AS-number] Route Maps . Step 3. If you don’t use it the IOS will even give you a warning. determining which networks are allowed into the routing table or included in updates. The subnets keyword at the end of the redistribute command is extremely important! Without this keyword. Distribute Lists Distribute lists are access lists applied to the routing process. OSPF Redistribution Example: R1(config)# router ospf 100 R1(config-router)# redistribute eigrp 10 subnets The example above redistributes EIGRP routes into OSPF. OSPF will redistribute networks at their classful boundaries – not something most administrators want. reliability  loading  mtu You can perform a show interface on the outgoing router interface prior to implementing the redistribution to see what values the router is currently using. Identify the network addresses to be filtered and create an ACL – permitting the networks you want to be advertised. use the following steps: Step 1. Assign the ACL using the distribute-list command. They essentially act as a filter. Step 2. An access list applied to routing = distribute lists When creating a distribute list.

The diagram below outlines those steps and serves as a foundation for the rest of this route redistribution and filtering section. Route maps are extremely flexible and are used in a number routing scenarios including:  Controlling redistribution based on permit/deny statements  Defining policies in policy-based routing (PBR)  Add more mature decision making to NAT decisions than simply using static translations  When implementing BGP PBR . a series of steps occur to process it correctly.When a routing update arrives at an interface.

Permit means that any traffic matching the match statement that follows is processed by the route map. The router stops reading statements when it reaches its first matching statement. the statement matches anything (similar to a ‘permit any’). the router would interpret ‘match a b c’ as ‘a or b or c’. For example. If traffic does not match any statement. the router would interpret the following commands as match a and b and c: route-map EXAMPLE permit 5 match a match b match c Important route redistribution match conditions ip address Refers to an access list that permits or denies networks ip address prefix-list Refers to a prefix list that permits or denies prefixes ip next-hop Refers to an access list that permits or denies ip next hops IP addresses . Know the difference. it is interpreted as a logical AND. it matches traffic against conditions and sets options for that traffic. which is read from lowest to highest. Basic Route Map Configuration R1(config)# route-map {tag} permit | deny [sequence_number] That is how all route maps begin. the statement is simply permitted or denied with no additional changes made. it is interpreted as a logical OR. In other words. In other words. Understand that there is an implicit deny included in all route maps. For example. Match & Set Conditions If no match condition exists. Route maps uses logic similar to if/then statements in simple scripting. If multiple match conditions are used on consecutive lines. If multiple match conditions are used on the same line. If no set condition exists. it is denied. Each statement in a route map has a sequence number.Route maps allow an administrator to define specific traffic and then take automated actions against it to control how routing information is processed and forwarded. all conditions must be true before a match is made. if one condition is true. Deny means that any traffic matching the match statement that follows is NOT processed by the route map. a match is made. In route map terms.

ip route-source Refers to an access list that permits or denies advertising router IP addresses length Permits or denies packets based on length (in bytes) metric Permits or denies routes with specific metrics from being redistributed route-type Permits or denies redistribution based on the route type listed tag Routes can be labeled with a number that identifies it Important Route Redistribution Set Conditions metric Sets the metric for redistributed routes tag Tags a route with a numbered identifier Route Map Verification Use the show route-map command to verify route maps and PBR entries are filtering as expected. .

BGP has two separate databases – a neighbor database and a BGP-specific database. When an update leaves an AS.  BGP is sometimes referred to as a “path-vector” protocol because its route to a network uses AS numbers on the path to the destination. iBGP is 200.BGP. . when route path manipulation is needed.  The administrative distance for eBGP routes is 20. or Border Gateway Protocol is an external. dynamic routing protocol. not speed – as a result it behaves very differently than most other routing protocols. the AS number is prepended to the update along with all the other AS numbers that have spread the update. Scalability and stability are BGP’s focus.  When a BGP router receives an update. BGP is literally the routing protocol of the Internet because it connects independent networks together. BGP Databases Like most modern routing protocols. 1-64511 are public.  BGP is used to connect IGPs. BGP is recommended whenever multihoming is a requirement (dual ISP connections to different carriers).  BGP neighbors are called “peers” and must be statically assigned.  Peers receive incremental. interior gateway protocols like OSPF and EIGRP. Routing between Autonomous Systems is referred to as interdomain routing.  BGP uses it’s path-vector attributes to help in loop prevention. triggered updates as well as keepalives using TCP port 179. and in transit Autonomous Systems. A Quick Overview  Routers running BGP are called BGP speakers. 64512-65535 are private. If it sees it own AS number. the update is discarded. It is most often used between ISPs and between enterprises and their service providers.  BGP uses autonomous system numbers to keep track of different administrative domains. it first scans through the list of AS numbers. enabling end-to-end transport.

Keepalive BGP peers send keepalive messages every 60 seconds by default to maintain active neighbor status. Open After a BGP neighbor is configured. External iBGP. . or internal BGP is a peering relationship between BGP routers within the same autonomous system.Neighbor Database Lists all of the configured BGP neighbors Router# show ip bgp summary BGP Database Lists all networks known by BGP along with their attributes. a notification message is sent and the session is closed. R1 and R2 are eBGP peers. R2 and R3 and iBGP peers. Notification If a problem occurs and a BGP peer connection must be dropped. Update The type of message used to transfer routing information between peers. or external BGP describes a peering relationship between BGP routers in different autonomous systems. Internal vs. In the diagram below. Router# show ip bgp BGP Message Types There are four different BGP message types. the router sends an open message to establish peering with the neighbor. It is an important distinction to make. eBGP.

1. it must pass the update to its iBGP neighbors with-out modifying the next hop attribute.1. In this example.1 serial interface.2 next-hopself R2(config)# exit BGPs Synchronization Rule The BGP synchronization rule states that a BGP router cannot use or forward new route updates it learns from iBGP peers unless it knows about the network from another source.1 when it passes the update along to R3. R2(config)# router bgp 65300 R2(config-router)# neighbor 10. The problem is that R3 does not know about 10. its iBGP peer.2. let’s say R1 sends an update to R2 from its 10. like an IGP or static route.2.1.BGP Next-Hop Self When you have BGP neighbors peering between autonomous systems like R1 and R2 above. For example. BGP uses the the IP address of the router the update was received from as its “next hop”. The next-hop IP address is the IP address of the edge router belonging to the next-hop autonomous system. . R2 must keep the next-hop IP set as 10.1.1. The neighbor [IP address] next-hop-self command solves the problem by advertising itself as the next-hop address. it would be applied to R2 so any updates passed along to R3 would use an R2 address as the next-hop.1 and so it cannot use it as its next hop address. When a router receives an update from an eBGP neighbor.1.

There are three ways to do this:  Hard reset  Soft reset  Route refresh The hard and soft reset options aren’t discussed here because they are not directly relevant to the exam. All existing routes stay unchanged. fastest route. To make matters worse. like a route map. The router would have to check the filter against every possible route and attribute combination.The idea is to prevent using or forwarding on information that is unreliable and cannot be verified. The following command performs the BGP route refresh: Router# clear ip bgp [ * | neighbor-address] BGP Configuration Enabling BGP Like other routing protocols. Those changes could affect many of the routes already in the routing table from BGP. BGP must be enabled with the router command. recent versions of IOS have it disabled by default. Because BGP’s network list is usually very long. To remove the limitation. use the no synchronization command under BGP configuration mode. those changes could then cause another reconvergence – and on and on. applying a route map or prefix list after BGP has converged can be disastrous. Make sure to include the AS number. that both options are extremely memory-taxing on the router as all the routes must be recomputed. BGP will only apply attribute and network changes to routes AFTER the filter has been applied. then the BGP instance must be reset – forcing the entire BGP table to pass through the filter. Resetting BGP Sessions Internet routers running BGP have enormous routing tables. You should know though. if it were to apply the filters and pull routes back from neighbors. If the network administrator decides that the filter needs to be applied to all routes. but it is important topic to understand. Route refresh was developed to solve the high memory problems. In an effort to avoid that scenario (BGP loves stability). BGP prefers reliability and stability over using the newest. When a filter is applied. changes to BGP attributes occur. . while still forcing a reset. Remember. This means that iBGP peers will not update each other unless an IGP is running under the hood.

1 remote-as 65300 Peer group configuration example: R1(config)# router bgp 65300 R1(config-router)# neighbor MINE peer-group R1(config-router)# neighbor MINE remote-as 65300 R1(config-router)# neighbor 10.1.1.R1(config)# router bgp autonomous-system-number BGP Peering Each neighbor must be statically assigned using the neighbor command.3. Peer groups BGP Peer groups are groups of peer neighbors that share a common update policy. the BGP configuration can start to get long and difficult to follow – especially as neighbor policies are applied. Peer groups not only reduce the number of lines of configuration. Much easier for large BGP networks. If a peer group is configured.1.1 remote-as 65300 R1(config-router)# neighbor 10. A BGP update process normally runs for each neighbor. Notice that this means that all of the router inside a peer group must be either all iBGP or eBGP neighbors.2. .1.1. it is an iBGP connection.3.1 remote-as 65300 R1(config-router)# neighbor 10. It can peer to the physical interface IP address.1. Basic neighbor configuration example: R1(config)# router bgp 65300 R1(config-router)# neighbor 10.1.168. 192. a single update process runs for all routers in the group.1. Updating an entire group of neighbor statements can then be done with one command.2 or it can peer to R2′s loopback interface.2.1.2. If the AS number is different. 10.1. If the AS number matches the local router’s.1 peer-group MINE R1(config-router)# neighbor 10. R1(config-router)# neighbor ip-address remote-as autonomous-system-number If a router has a long list of directly connected neighbors. Think of a peer group as a logical grouping of routers that are grouped under a single name to make changes faster and configurations shorter.1 peer-group MINE BGP Source Address R1 in the diagram below has two different options when it comes to peering to R2.1 peer-group MINE R1(config-router)# neighbor 10.2. Like OUs in Active Directory. Peer groups solve that. but they reduce the ease the overhead of the router. it is an eBGP connection.

1.168.2 update-source loopback0 R2(config)# router bgp 65400 R2(config-router)# neighbor 192.1 remote-as 65400 R2(config-router)# neighbor 192. This way.2 interface fails.168. even if R2′s 10.2. R2 will still be reachable.1.2 remote-as 65400 R1(config-router)# neighbor interface drops. it still has connectivity to R2′s networks via R3 and R2′s other physical interface. the BGP peer relationship would drop because R1 cannot reach its peering address with R2.1. Here’s an example: R1(config)# router bgp 65400 R1(config-router)# neighbor 192. if R2′s 10. Even though an IGP would still show R2′s network as accessible. problems can occur if the interface goes down. Remember that the loopback address must be added to the IGP running for this to work.2. The update-source command accomplishes this. Most implementations recommend using a loopback address as the BGP source address for this reason.1.1 update-source loopback0 Defining Networks .If a peer relationship is made using the physical interface as the source address.1.1. In this scenario.

Oldest route 10.255. This is why iBGP is not a good replacement for an IGP like EIGRP and OSPF.1.Network statements in BGP are used differently than in other routing protocols like EIGRP or OSPF. Highest local preference 3. Instead. Each of these attributes can be administratively tuned for extremely granular control of route selections.255.1. Neighbor with the lowest router ID . BGP uses network statements to define which networks the local router should advertise.1. BGP assigns a long list of attributes to each path. but the network must exist in the routing table. Route with nearest IGP neighbor (lowest IGP metric) 9. Highest weight 2. Each network doesn’t have to be originating from the local router. The optional mask keyword is often recommended as BGP supports subnetting and supernetting. Path with the shortest AS path 5. BGP uses the criteria in the following order: 1. Example: R1(config)# router bgp 65300 R1(config-router)# neighbor 10.0 Understand that by default a BGP router will not advertise a network learned from one iBGP peer to another.1 remote-as 65300 R1(config-router)# network 10. To select the best route. BGP also does not load balance across links by default.1 remote-as 65300 R1(config-router)# network 10. Choose routes originated locally 4.1.0 R1(config-router)# neighbor 10.2. Lowest origin code ( i < e < ? ) 6.255.0 255. EIGRP and OSPF use the network statements to define which interfaces you want to participate in the routing protocol process.2. eBGP route over iBGP route 8.0 255. BGP Path Selection Unlike most other routing protocols.1. BGP is not concerned with using the fastest path to a given destination. Lowest MED 7.255.1.

and MED. weight is the most influential BGP attribute. In the example below. The weight attribute is proprietary to Cisco and is normally used to select an exit interface when multiple paths lead to the same destination. local preference is propagated to iBGP neighbors.294. let’s first discuss the three prominent attributes: weight.2.295.535. It can be a value between 0-65.1. local preference. .1 remote-as 65100 R2(config-router)# neighbor 10.1. Weight On Cisco routers. Local preferences can range from 04.2. 0 is the default. with 100 being the default value. R2(config)# router bgp 65100 R2(config-router)# neighbor 10.967.0 then the weight attribute could raised on R2 for R1. if you want R2 to prefer to use R1 when sending traffic to 192.1.1 remote-as 65100 R2(config-router)# neighbor weight 100 Local Preference Local preference is not proprietary to Cisco and can be used in a similar fashion to weight.1. This allows specific attributes to be changed on specific routes. Weight is local and is not sent to other routers. It can be set for the entire router or for a specific prefix. Neighbor with the lowest IP address Controlling Path Selection The most common method of controlling the attributes listed above is to use route maps. Before we get into route maps. Unlike weight.20.

30. but local preference is not shared with routers outside an AS. A common scenario for MED is when a company has two connections to the same ISP for internet.255 R7(config)# route-map med_example permit 10 R7(config-rmap)# match ip address 7 R7(config-rmap)# set metric 50 R7(config-rmap)# exit R7(config)# route-map med_example permit 20 R7(config-rmap)# set metric 150 BGP Verification It’s important that you understand and are able to interpret to results of the show ip bgp command output. MED could be set on one router so ISP routers prefer that path in.Using the diagram above. The default MED value is 0 and a lower value is preferred.168.30.1 remote-as 200 R7(config-router)# neighbor 10.10. route maps are usually the best option. AS path length.1 remote-as 100 R7(config-router)# neighbor 10. MED is also much farther down on the attribute list. the configuration would look something like this: R1(config)# router bgp 65100 R1(config-router)# bgp default local-preference 500 After the local preference is raised on R1.30.255 R7(config)# route-map lp_example permit 10 R7(config-rmap)# match ip address 7 R7(config-rmap)# set local-preference 300 R7(config-rmap)# exit R7(config)# route-map lp_example permit 20 R7(config-rmap)# set local-preference 100 MED The MED attribute.0/24 using MED: R7(config)# router bgp 200 R7(config-router)# neighbor 10.20.including the attributes assigned to each network.0. To set the MED on all routes: R1(config-router)# default-metric value Here’s an example using a route map to influence incoming paths to route-map lp_example in R2(configrouter)# exit R7(config)# access-list 7 permit or multi-exit discriminator is used to influence which path external neighbors use to enter an AS. It displays the contents of the local BGP topology database.0.0 0. It is perhaps the most important BGP verification and troubleshooting tool! . Weight or local preference could be used to send outgoing traffic on the higher bandwidth link. if an administrator wanted R2 to use R1 when sending traffic to 192.0. so attributes like weight.0. If you want to set the local preference on specific prefixes.30.10. it will be shared with R2 and R2 will begin using it as its best path to the distant network (assuming the weight is the same of course). Below is an example of the local preference being set using a route map: R7(config)# router bgp 200 R7(config-router)# neighbor 10.0. local preference.1 route-map med_example out R2(config-router)# exit R7(config)# access-list 7 permit 10.10.0 0. and origin are used first.10.30.

0.22.25 10 0 25 ? *> 0.1. usually because it is part of a summarized route.24 Status codes: s suppressed.25 10 0 25 ? *> 0.0.0 0.IGP. it means the network was learned from an iBGP neighbor. Let’s start with IPSec. but not in great detail.0.incomplete Network Next Hop Metric LocPrf Weight Path *> 10. lower is preferred. R1# show ip bgp BGP table version is 21.0.0/16 10.0. e . > best. > – Indicates the best route for a particular destination. d damped.0 10. You’ll need to know enough to verify a sample configuration and answer straightforward questions on both technologies.0.The last column displays information on how BGP originally learned the route.0. the output of the show ip bgp command can be a bit overwhelming if you don’t know what you are looking for.0 0 32768 ? *> 192.Because BGP uses many attributes and sources routes in a number of ways. h history.168. i (internal) – If the third column has an i in it.25 10 0 25 ? Attributes Here’s a breakdown of some important fields you should consider remembering: * – An asterisk in the first column means that the route has a valid next hop. i/?. ? is used for each route meaning they were all redistributed routes into BGP from an IGP.22.0 0 32768 ? * 10.0.0. * valid.0. A 0. Recall that 0 is the default and if another value exists. 10.0. ? .0. s (suppressed) – BGP is not advertising the network.0 0 32768 ? * 10.0. which indicates that network commands were used to configure the route. In the example above. Because the . The other option is a question mark. The IPSec protocol sets up a unidirectional SA (security association between the two endpoints).0.0 indicates the local router originated the route (examples include a network command entered locally or a network an IGP redistributed into BGP on the router) Metric (MED value) – The column titled Metric represents the configured MED values. If it is blank.internal Origin codes: i . local router ID is 10. VPN tunnels and IPSec are two topics covered on the exam.EGP.22. it means the network was learned from an external source.0 – The fifth column shows the next hop address for each route.22.0. i . 0.0. IPSec Basics IPSec allows the establishment of a secure connection between two hosts.2. These will end up in the routing table.

an SA is created on both ends. a “tunnel within a tunnel” approach can be used. which creates a problem. then the tunnel needs to allow routing information to pass through. an IPSec tunnel can be configured to automatically be established over the internet to the remote site. GRE tunnels support many layer 3 protocols but perhaps most importantly allow . Of course dynamic routing protocols use broadcast or multicast to send hellos and updates. but with an administrative distance higher than that of the WAN routing protocol’s. resulting in two SAs per IPSec tunnel. the IPSec tunnel is disconnected. A generic tunnel can be configured within the IPSec tunnel to allow routing protocol information (along with all the other traffic). the static route becomes active. There are generally four ways to do this paired with IPSec: DMVPN and GET VPN Both allow the creation of secure. If the primary route goes down. Virtual Tunnel Interface (VTI) A secure. If you want to use an IPSec VPN in an “always on” fashion. The idea is to configure the IPSec VPN as a static route. make sure you define a higher administrative distance value at the end of the statement: R1(conf)# ip route prefix mask address|interface distance_value VPN Tunnels One major problem with standard IPSec sessions is that they do not support broadcast or multicast traffic. When the primary WAN circuit comes back up. If the primary route is active. multipoint tunnels. “always-on” tunnel that supports multicast traffic. This allows routing protocols to operate within it.association is unidirectional. Generic Routing Encapsulation (GRE) GRE tunnels may be the most common of the bunch – they are also the default tunnel mode on Cisco routers. IPSec tunnels are often used as a backup to a WAN link failure. To configure a floating static route. If a point-to-point WAN circuit drops. “on-demand”. the backup link is not placed into the routing table because it has a higher administrative distance. Floating Static Routes Configuring an IPSec tunnel to activate when a primary link drops is commonly implemented as a floating static route. To get around this issue.

Branch Office Connectivity The CCNP ROUTE exam covers several unusual topics related to managing and configuring the connectivity between an HQ site and a branch office. You need to be familiar with some of the underlying technologies used. which a router can process. PPoA Point-to-Point Protocol over ATM is less common and routes PPP traffic over an ATM network between the customer and the DSL service provider. PPPoE is especially helpful because it frees the local office’s computers from running PPPoE Cable Broadband cable providers also provide internet connectivity which can be used for WAN backup or provide internet connectivity for telecommuters. The international standard for sending data over a cable system is Data Over Cable Service Interface Specification (or DOCSIS). Be aware that GRE tunnels add an additional 20 byte IP header as well as a 4 byte GRE tunnel header. In smaller offices. or Digital Subscriber Line. . Cable system connections are typically not terminated directly into a Cisco router. DSL uses frequencies not used by TDM phone systems on a phone line – allowing the extra bandwidth to be used for data connectivity. In that case.multicast traffic accross the tunnel – permitting dynamic routing protocol traffic. Asymmetrical DSL has higher downstream bandwidth than upstream. know that an Ethernet Switch Module would be required for the ISR router. Cisco ISR routers are often a good choice for branch sites as they support a wide variety of incoming services. Many different versions of the standard are used throughout the world. The internet signal is carried on the same line that the television is carried. but a cable modem allows the data traffic to be separated. can be used as a backup WAN connection to a branch office. Instead. DSL DSL. a cable modem demodulates the incoming signal and converts the traffic to Ethernet frames. Both options can be configured on a Cisco router to terminate the DSL connectivity. while with symmetric DSL they are both the same rate. a single ISR may be used for a both remote connectivity and inter-VLAN routing. There are two primary methods for pushing L2 data across a DSL line: PPPoE Point-to-Point Protocol over Ethernet is the most common method and encapsulates PPP traffic into Ethernet frames.

Leading zeros can be removed in any section. IPv6 Basics IPv4 addresses are 32 bits long and are represented in dotted-decimal format.IPv6 is an important topic – and not just for the exam. IPv6 addresses are 128 bits and are in hexadecimal format. There are two ways to condense an IPv6 address: 1. especially on web-facing networks. For example. The first 64 bits of an IPv6 address are reserved for the network portion and the last 64 bits are used for the host portion. The growth of web-based services and diminishing IPv4 addressing will continue to push organizations towards IPv6. IPv6 Shorthand The ability to shorten IPv6 addresses is very important to understand because it makes reading and writing them much easier. 0021:0001:0000:030A:0000:0000:0000:0987E can be abbreviated as: 21:1:0:30A:0:0:0:987E .

global unicast and link-local unicast. This can only be used once per address. Be aware that with IPv6. & Anycast Unicast Unicast is for sending traffic to a single interface. an interface can be assigned multiple addresses. This is used for sending traffic to a group of devices. it is used for sending traffic to the nearest interface in a group. it has replaced it with multicast (which is a more efficient variation). Manual Address Configuration .2. Instead. In IPv6 there are actually two different unicast types. it can be further shortened to: 21:1:0:30A::987E Unicast. only one will be used per packet sent. Here is the list:  Unicast address  Link-local address  loopback (::1/128)  All nodes multicast (FF00::1)  Site-local multicast (FF02::2)  Solicited-nodes multicast  Default Route (::/0) IPv6 Address Assignment There are three different ways devices are assigned an IPv6 address: manual configuration. Sequential sections of all zeros can be shortened to a single double colon. Technically. stateless autoconfiguration. Multicast. IPv6 addressing does not support broadcasts. Multicast Unlike IPv4. Anycast IPv6 supports another new packet type – anycast. or DHCPv6. Anycast allows the same address to be used on multiple devices for load balancing and redundancy. Using the same example address above. While multiple devices may be running the same anycast address.

In this case. it flips the 7th bit and inserts 0xFFFE into the middle of the MAC address. it uses NDP to make sure it is actually unique within the local network. Once a router has created an IPv6 link-local address using stateless autoconfiguration.The first thing to know about manual IPv6 address configuration is that addresses assigned to a router interface use the address/prefix-length notation instead of the address mask notation. most IGPs use link-local addresses for neighbor relationships and the link-local address is listed as the next-hop address in the routing table. it works well for auto-generated local IP addresses. and in this case stateless autoconfiguraiton will generate a link-local address in addition to any other manually assigned addresses. Since every MAC address should be unique. add the keyword eui-64 to the end of the ipv6 address statement. R1(config)# int gig 1/1 R1(config-if)# ipv6 address autoconfig . An example of an interface configured with an IPv6 address: R1# conf t R1(config)# ipv6 unicast-routing R1(config)# int gig 1/1 R1(configif)# ipv6 address 21:1:0:30A::987E/64 Manual Network Assignment Another way to manually configure an IPv6 address is to configure the network and allow the host portion to be auto-populated based on the device’s MAC address. To configure stateless autoconfiguration. use the ipv6 address autoconfig command. make sure you first enable IPv6 routing with the ipv6 unicast-routing global configuration command. Stateless Autoconfiguration Stateless autoconfiguration allows a device to self-assign an IP address for use locally without any outside information. Link-local addresses are not routable within packets and are used for administrative purposes within the local segment. Link-local addresses are created using the prefix FE80:: and appending the device’s MAC address. To do this.255… after every IP address! Also. Use the ipv6 address ipv6-address/prefix-length command to assign an address. For example. NDP stands for Neighbor Discovery Protocol and uses ICMP packets as part of the neighbor discovery process. Remember that interfaces using IPv6 will often have more than one IPv6 address assigned. This can work well because MAC addresses are 64 bits long – the exact same length as the host portion of an IPv6 address! An example configuration with the network portion defined: R1(config)# int gig 1/1 R1(config-if)# ipv6 address 21:1:0:30A::/64 Note: Some systems have a 48 bit MAC address. This is so much easier than typing 255. This modified version is called an EUI-64 address.

Instead. except for the ipv6 route keywords instead of ip route. To configure IPv6 EIGRP: R1(config)# ipv6 unicast-routing ! R1(config)# ipv6 router eigrp AS R1(configrtr)# router-id ipv4-address R1(config-rtr)# no shut R1(config-rtr)# exit ! R1(config)# interface type number R1(config-if)# ipv6 eigrp AS OSPFv3 OSPFv3 is an updated version of OSPF designed to accommodate IPv6 natively. Perhaps the biggest difference is that there is no network command.  Also. but when running EIGRP with IPv6 addresses it uses the multicast address FF02::A.IPv6 Routing Static Routes The configuration for IPv6 static routes is identical to IPv4.  It still sends hellos out every 5 seconds to its neighbors. IPv6 EIGRP There are many differences in the way EIGRP is configured for IPv6. EIGRP running IPv6 requires a router ID be configured. Most of the configuration and function is identical to its predecessor. . but a few changes were made starting with messaging. An example of a static IPv6 default route: R1(config)# ipv6 route ::/0 serial1/1 An example of an IPv6 static route with a next-hop address: R1(config)# ipv6 route 2003:2:1:A::/64 2003:2:1:F::1 To view the IPv6 routes in the routing table.  The last major change is that the EIGRP process starts in the shutdown state. Other than that. EIGRP routing is enabled on each participating interface. The format is that of an IPv4 address – 32 digits and it can be a private address (non-routable) with no issues. You have to issue a no shut to bring it up on the router. use the command show ipv6 route. it is exactly the same.  EIGRP messages are exchanged using the link-local address as the source address.

10 R3(config-rtr)# neighbor 2003:76:1:1::10 remote-as 700 R3(config-rtr)# address-family ipv6 unicast R3(config-rtr-af)# neighbor 2003:76:1:1::10 activate R3(config-rtr-af)# network 2003:2:2::/48 R3(configrtr-af)# exit R3(config-rtr)# exit Migrating to IPv6 Three options exist for transitioning from IPv4 to IPv6: dual stack. tunneling.10.  OSPFv3 has dropped it’s native authentication options.1 R2(config-rtr)# area 1 stub no-summary R2(configrtr)# exit ! R2(config)# interface gig1/1 R2(config-if)# ipv6 address 2003:2:1:2::1/64 R2(config-if)# ipv6 ospf 100 area 0 ! R2(config)# interface gig1/2 R2(config-if)# ipv6 address 2003:2:1:A::1/64 R2(config-if)# ipv6 ospf 100 area 1 R2(config-if)# ipv6 ospf priority 30 MP-BGP MP-BGP. or multiple protocol BGP. or NAT. was outlined in RFC 2858 and includes extensions to the original BGP standard that allows support for other protocols – one of which is IPv6! The command address-family was added to specify which new protocol functionality is being configured and is used when applying IPv6 addressing. it still is done in router configuration mode. Dual Stack This involves running IPv4 alongside IPv6 on the same system. a 32 bit router ID must be manually created. It will not automatically create one based on highest loopback or interface address. but like EIGRP it now uses its link-local address as the source address in advertisements. The major difference is that neighbors must be first defined under router BGP configuration mode and then “activated” under IPv6 address-family mode submode. an IPv4 address must be configured as a router ID. The RID that is assigned will then be used to determine the DR and BDR on a segment (highest wins). like IPSec.10.10. R3(config)# ipv6 unicast-routing ! R3(config)# router bgp 600 R3(config-rtr)# router-id 10. The configuration is now done on each individual interface. The following is an example configuration: R2(config)# ipv6 unicast-routing ! R2(config)# ipv6 router ospf 100 R2(configrtr)# router-id 10. Instead. The BGP configuration is not done at the interface level.  It is possible to run multiple instances of OSPFv3 on each link. . OSPFv3 uses the multicast address FF02::5 and FF02::6.10. it relies on the underlying authentications built into IPv6.  Like the IPv6 implementation of EIGRP. Networks and other parameters are also configured under IPv6 address-family mode submode. Like EIGRP and OSPFv3.

the network core does not support IPv6 or it has not been implemented. IPv6 tunnels solve this problem by allowing IPv6 islands to exist and bridges them over IPv4 systems. Dual Stack configuration example: R1# config t R1(config)# ipv6 unicast-routing R1(config)# ipv6 cef ! R1(config)# interface serial1/0/1 R1(config-if)# ip address 192.3.1 R1(config-if)# ipv6 address 2001:1:3:1::1/64 IPv6 Tunneling Dual-stacking IPv4 alongside IPv6 on systems works well. but it requires most of your infrastructure to support IPv6. it does not matter what specific IPv4 transport is used.1. GRE tunnels are the default tunnel method on Cisco routers.168. IPv6 will be used.1. In many cases. Router B then decapsulates the packets and forwards them on to their destination using native IPv6.Tunneling This option allows you to encapsulate IPv6 traffic within an IPv4 header. NAT A new network translation extension. Both protocols can run concurrently and neither communicating with the other. NAT-PT allows IPv6-to4 translation. Because IPv6 tunnels provide virtual IPv6 connectivity through an IPv4 transport. Using the Router A/B example above. . Dual Stack Using a dual-stack transition allows servers. If both IPv4 and IPv6 are running on a server for example. Manual Tunnels The tunnels discussed here are from one router to another. clients. then forwards it to the other end of the tunnel (Router B). The source router (RouterA) encapsulates the IPv6 traffic in IPv4 headers. and applications to be slowly moved to IPv6. GRE tunnels are very flexible and work over most protocols.1 RouterA(config-if)# tunnel destination 10. the configuration on Router A would look something like this: RouterA(config)# interface tunnel0 RouterA(config-if)# ipv6 address 2001:2:0:7::/64 RouterA(config-if)# tunnel source 10. The only requirement is that there is end-to-end IPv4 connectivity between both ends.3. Manual IPv6 tunnels are easy to configure using the tunnel mode ipv6ip command.1.1 RouterA(config-if)# tunnel mode ipv6ip RouterA(config-if)# exit GRE Tunnels First.

but you do not have to specify the tunnel mode. Define the address mappings (either static or dynamic) using the options discussed below. 1.The configuration is exactly the same as the manual configuration example above. Only traffic matching the prefix will be translated. To apply it globally on the router. NAT Translation is a unique solution because it allows IPv4 devices to communicate with IPv6 devices without the dual stack requirement. using the IPv4 address as the source. use the command tunnel mode ipv6ip 6to4. 2. To enable NAT-PT IPv4 to IPv6 translation on a router. so be careful. routing protocols can be enabled on GRE tunnel interfaces just as if they were physical interfaces. 6to4 tunnels use 2002::/16 IPv6 addresses in front of the 32 bit IPv4 address of the edge router – creating a 48 bit prefix. The second step is to define at least one NAT-PT prefix. To apply it to traffic on a specific interface. Static NAT-PT For an IPv6 to IPv4 static mapping: R1(config)# ipv6 nat v6v4 source ipv6_address ipv4_address For an IPv4 to IPv6 static mapping: R1(config)# ipv6 nat v4v6 source ipv4_address ipv6_address Dynamic NAT-PT There are many ways to implement dynamic NAT using IPv6. 3. enter ipv6 nat prefix/prefix_length in global configuration mode.The tunnel requires an IPv6 address using the method just described. enter ipv6 nat prefix/prefix_length in interface configuration submode. For an IPv4 to IPv6 static mapping: . but at its most basic level a pool of addresses is created and the router temporarily assigns them to hosts as they need them. Configure the tunnel as if it was a manual tunnel. Also. the first step is to use the ipv6 nat command on each interface participating in the translation. NAT-PT allows bidirectional translation services. Finally. Each router on both sides of the tunnel needs a route to its peer. but don’t enter a destination. 6to4 Tunnels 6to4 tunnels are similar to the manual tunnel. They only support static and BGP routes. but set up the tunnel dynamically.

R1(config)# ipv6 nat v4v6 pool name beginning_ipv6 ending_ipv6 prefix-length prefix-length R1(config)# ipv6 nat v4v6 source list (access-list_number | name) pool name For an IPv6 to IPv4 static mapping: R1(config)# ipv6 nat v6v4 pool name beginning_ipv4 ending_ipv4 prefix-length prefix-length R1(config)# ipv6 nat v6v4 source list (access-list_number | name) pool name .