Enhanced Interior Gateway Routing Protocol

Document ID: 16406
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Contents
Introduction
EIGRP Theory of Operation
Major Revisions of the Protocol
Basic Theory
Neighbor Discovery and Maintenance
Building the Topology Table
EIGRP Metrics
Feasible Distance, Reported Distance, and Feasible Successor
Deciding if a Path is Loop−Free
Split Horizon and Poison Reverse
Startup Mode
Topology Table Change
Queries
Stuck In Active Routes
Troubleshooting SIA Routes
Redistribution
Redistribution Between Two EIGRP Autonomous Systems
Redistribution Between EIGRP and IGRP in Two Different Autonomous Systems
Redistribution Between EIGRP and IGRP in the Same Autonomous System
Redistribution To and From Other Protocols
Redistribution of Static Routes to Interfaces
Summarization
Auto−Summarization
Manual Summarization
Auto−Summarization of External Routes
Query Processing and Range
How Summarization Points Affect the Query Range
How Autonomous System Boundaries Affect the Query Range
How Distribution Lists Affect the Query Range
Pacing Packets
Default Routing
Load Balancing
Using the Metrics
Using Administrative Tags in Redistribution
Understanding EIGRP Command Output
show ip eigrp traffic
show ip eigrp topology
show ip eigrp topology <network>
show ip eigrp topology [active | pending | zero−successors]
show ip eigrp topology all−links
Related Information

Introduction
Enhanced Interior Gateway Routing Protocol (EIGRP) is an interior gateway protocol suited for many
different topologies and media. In a well designed network, EIGRP scales well and provides extremely quick
convergence times with minimal network traffic.

EIGRP Theory of Operation
Some of the many advantages of EIGRP are:
• very low usage of network resources during normal operation; only hello packets are transmitted on a
stable network
• when a change occurs, only routing table changes are propagated, not the entire routing table; this
reduces the load the routing protocol itself places on the network
• rapid convergence times for changes in the network topology (in some situations convergence can be
almost instantaneous)
EIGRP is an enhanced distance vector protocol, relying on the Diffused Update Algorithm (DUAL) to
calculate the shortest path to a destination within a network.

Major Revisions of the Protocol
There are two major revisions of EIGRP, versions 0 and 1. Cisco IOS versions earlier than 10.3(11), 11.0(8),
and 11.1(3) run the earlier version of EIGRP; some explanations in this paper may not apply to that earlier
version. We highly recommend using the later version of EIGRP, as it includes many performance and
stability enhancements.

Basic Theory
A typical distance vector protocol saves the following information when computing the best path to a
destination: the distance (total metric or distance, such as hop count) and the vector (the next hop). For
instance, all the routers in the network in Figure 1 are running Routing Information Protocol (RIP). Router
Two chooses the path to Network A by examining the hop count through each available path.

Since the path through Router Three is three hops, and the path through Router One is two hops, Router Two
chooses the path through One and discards the information it learned through Three. If the path between
Router One and Network A goes down, Router Two loses all connectivity with this destination until it times
out the route of its routing table (three update periods, or 90 seconds), and Router Three re−advertises the

route (which occurs every 30 seconds in RIP). Not including any hold−down time, it will take between 90 and
120 seconds for Router Two to switch the path from Router One to Router Three.
EIGRP, instead of counting on full periodic updates to re−converge, builds a topology table from each of its
neighbor's advertisements (rather than discarding the data), and converges by either looking for a likely
loop−free route in the topology table, or, if it knows of no other route, by querying its neighbors. Router Two
saves the information it received from both Routers One and Three. It chooses the path through One as its best
path (the successor) and the path through Three as a loop−free path (a feasible successor). When the path
through Router One becomes unavailable, Router Two examines its topology table and, finding a feasible
successor, begins using the path through Three immediately.
From this brief explanation, it is apparent that EIGRP must provide:
• a system where it sends only the updates needed at a given time; this is accomplished through
neighbor discovery and maintenance
• a way of determining which paths a router has learned are loop−free
• a process to clear bad routes from the topology tables of all routers on the network
• a process for querying neighbors to find paths to lost destinations
We will cover each of these requirements in turn.

Neighbor Discovery and Maintenance
To distribute routing information throughout a network, EIGRP uses non−periodic incremental routing
updates. That is, EIGRP only sends routing updates about paths that have changed when those paths change.
The basic problem with sending only routing updates is that you may not know when a path through a
neighboring router is no longer available. You can not time out routes, expecting to receive a new routing
table from your neighbors. EIGRP relies on neighbor relationships to reliably propagate routing table changes
throughout the network; two routers become neighbors when they see each other's hello packets on a common
network.
EIGRP sends hello packets every 5 seconds on high bandwidth links and every 60 seconds on low bandwidth
multipoint links.
• 5−second hello:
♦ broadcast media, such as Ethernet, Token Ring, and FDDI
♦ point−to−point serial links, such as PPP or HDLC leased circuits, Frame Relay
point−to−point subinterfaces, and ATM point−to−point subinterface
♦ high bandwidth (greater than T1) multipoint circuits, such as ISDN PRI and Frame Relay
• 60−second hello:
♦ multipoint circuits T1 bandwidth or slower, such as Frame Relay multipoint interfaces, ATM
multipoint interfaces, ATM switched virtual circuits, and ISDN BRIs
The rate at which EIGRP sends hello packets is called the hello interval, and you can adjust it per interface
with the ip hello−interval eigrp command. The hold time is the amount of time that a router will consider a
neighbor alive without receiving a hello packet. The hold time is typically three times the hello interval, by
default, 15 seconds and 180 seconds. You can adjust the hold time with the ip hold−time eigrp command.
Note that if you change the hello interval, the hold time is not automatically adjusted to account for this
change − you must manually adjust the hold time to reflect the configured hello interval.

It is possible for two routers to become EIGRP neighbors even though the hello and hold timers do not match.
The hold time is included in the hello packets so each neighbor should stay alive even though the hello
interval and hold timers do not match.
While there is no direct way of determining what the hello interval is on a router, you can infer it from the
output of show ip eigrp neighbors on the neighboring router.
If you have the output of a show ip eigrp neighbors command from your Cisco device, you can use Output
Interpreter (registered customers only) to display potential issues and fixes. To use Output Interpreter, you
must have JavaScript enabled.
router# show ip eigrp neighbors
IP−EIGRP neighbors for process 1
H
Address
Interface
Hold Uptime
SRTT
(sec)
1
10.1.1.2
Et1
13 12:00:53
12
0
10.1.2.2
S0
174 12:00:56
17

RTO

Q Seq
(ms)
300 0 620
200 0 645

rp−2514aa# show ip eigrp neighbor
IP−EIGRP neighbors for process 1
H
Address
Interface
Hold Uptime
SRTT
(sec)
1
10.1.1.2
Et1
12 12:00:55
12
0
10.1.2.2
S0
173 12:00:57
17

RTO Q
(ms)
300 0
200 0

Seq

rp−2514aa# show ip eigrp neighbor
IP−EIGRP neighbors for process 1
H
Address
Interface
Hold Uptime
SRTT
(sec)
1
10.1.1.2
Et1
11 12:00:56
12
0
10.1.2.2
S0
172 12:00:58
17

RTO Q
(ms)
300 0
200 0

Seq

Type
Cnt Num

Type
Cnt Num

620
645

Type
Cnt Num

620
645

The value in the Hold column of the command output should never exceed the hold time, and should never be
less than the hold time minus the hello interval (unless, of course, you are losing hello packets). If the Hold
column usually ranges between 10 and 15 seconds, the hello interval is 5 seconds and the hold time is 15
seconds. If the Hold column usually has a wider range − between 120 and 180 seconds − the hello interval is
60 seconds and the hold time is 180 seconds. If the numbers do not seem to fit one of the default timer
settings, check the interface in question on the neighboring router − the hello and hold timers may have been
configured manually.
Note:
• EIGRP does not build peer relationships over secondary addresses. All EIGRP traffic is sourced from
the primary address of the interface.
• When configuring EIGRP over a multi−access Frame Relay network (point−to−multipoint, and so
on), configure the broadcast keyword in the frame−relay map statements. Without the broadcast
keyword the adjacencies would not establish between two EIGRP routers. Refer to Configuring and
Troubleshooting Frame Relay for more information.
• There are no limitations on the number of neighbors that EIGRP can support. The actual number of
supported neighbors depends on the capability of the device, such as:
♦ memory capacity
♦ processing power
♦ amount of exchanged information, such as the number of routes sent
♦ topology complexity
♦ network stability

in Figure 2 below. Router One is computing the best path to Network A. including: • lowest bandwidth on the path to this destination as reported by the upstream neighbor • total delay • path reliability • path loading • minimum path maximum transmission unit (MTU) • feasible distance • reported distance • route source (external routes are marked) Feasible and reported distance are discussed later in this section. from which it installs routes in the routing table. EIGRP Metrics EIGRP uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics. it builds a second table. The bandwidth and delay metrics are determined from values configured on the interfaces of routers in the path to the destination network. we do not recommend it. unlike RIP and IGRP. To use Output Interpreter. Although you can configure other metrics. For instance. the topology table. what are they talking about? Their topology tables.0T and 12. It starts with the two advertisements for this network: one through Router Four. does not rely on the routing (or forwarding) table in the router to hold all of the information it needs to operate. To see the basic format of the topology table on a router running EIGRP. with a minimum bandwidth of .1. issue the show ip eigrp topology command. RIP maintains its own database from which it installs routes into the routing table. Instead. as it can cause routing loops in your network. you can use Output Interpreter (registered customers only) to display potential issues and fixes. If you have the output of a show ip eigrp topology command from your Cisco device. you must have JavaScript enabled.Building the Topology Table Now that these routers are talking to each other. of course! EIGRP. The topology table contains the information needed to build a set of distances and vectors to each reachable network. Note: As of Cisco IOS versions 12.

the formula reduces to Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 − load) + k3 * delay]. Throughout this paper. Note: If K5 = 0. In this example. you need to round down to the nearest integer to properly calculate the metrics. the total cost through Router Four is: In this example. Router One chooses the path with the lowest metric. EIGRP calculates the total metric by scaling the bandwidth and delay metrics. you can simplify the formula as follows: metric = bandwidth + delay Cisco routers do not perform floating point math. on the route to the destination network. EIGRP uses the following formula to scale the delay: • delay = delay(i) * 256 where delay(i) is the sum of the delays configured on the interfaces. so at each stage in the calculation. The default values for K are: • K1 = 1 • K2 = 0 • K3 = 1 • K4 = 0 • K5 = 0 For default behavior. we use delay as it is configured and shown on the interface. so you must divide by 10 before you use it in this formula. EIGRP uses the following formula to scale the bandwidth: • bandwidth = (10000000/bandwidth(i)) * 256 where bandwidth(i) is the least bandwidth of all outgoing interfaces on the route to the destination network represented in kilobits.56 and a total delay of 2200. Let us compute the metrics. Mismatched K values prevent a neighbor relationship from being built. The delay as shown in the show ip eigrp topology or show interface commands is in microseconds. and the other through Router Three. in tens of microseconds. with a minimum bandwidth of 128 and a delay of 1200. the total cost through Router Four is: minimum bandwidth = 56k total delay = 100 + 100 + 2000 = 2200 [(10000000/56) + 2200] x 256 = (178571 + 2200) x 256 = 180771 x 256 = 46277376 And the total cost through Router Three is: . which can cause your network to fail to converge. EIGRP uses these scaled values to determine the total metric to the network: • metric = [K1 * bandwidth + (K2 * bandwidth) / (256 − load) + K3 * delay] * [K5 / (reliability + K4)] Note: These K values should be used after careful planning.

EIGRP chooses the route through Router Three as the best path. Note the bandwidth and delay values we used are those configured on the interface through which the router reaches its next hop to the destination network. Router Four added the delay configured on its Ethernet. In other words. A feasible successor is a path whose reported distance is less than the feasible distance (current best path). The network converges instantly. Router Two advertised Network A with the delay configured on its Ethernet interface. . Reported Distance. • The route through Router Four has a cost of 46277376 and a reported distance of 307200. Router One examines each path it knows to Network A and finds that it has a feasible successor through Router Four. Figure 3 illustrates this process: Router One sees that it has two routes to Network A: one through Router Three and another through Router Four. For example. and Feasible Successor Feasible distance is the best metric along a path to a destination network. Router One considers the path through Router Four a feasible successor. and Router One added the delay configured on its serial. shown in Figure 4. Router One uses this route. Feasible Distance. the reported distance from Router Four is the metric to get to Network A from Router Four. Let us look at a more complex scenario. Since the reported distance to this network through Router Four is less than the feasible distance.minimum bandwidth = 128k total delay = 100 + 100 + 1000 = 1200 [(10000000/128) + 1200] x 256 = (78125 + 1200) x 256 = 79325 x 256 = 20307200 So to reach Network A. Router One chooses the route through Router Three. including the metric to the neighbor advertising that path. Reported distance is the total metric along a path to a destination network as advertised by an upstream neighbor. and uses the metric through Router Three as the feasible distance. Note that in each case EIGRP calculates the reported distance from the router advertising the route to the network. and the reported distance from Router Three is the metric to get to Network A from Router Three. using the metric through Router Four as the new feasible distance. and updates to downstream neighbors are the only traffic from the routing protocol. • The route through Router Three has a cost of 20307200 and a reported distance of 307200. When the link between Routers One and Three goes down.

but because of the rules for determining feasible successors. Let us look at the metrics to see why: . four).There are two routes to Network A from Router One: one through Router Two with a metric of 46789376 and another through Router Four with a metric of 20307200. and this metric becomes the feasible distance. Since Router One no longer has the better route through Router Four. four). and not a loop? In Figure 4a. you can see the routes that are not feasible successors using show ip eigrp topology all−links ). Let us suppose that the link between Router One and Router Four goes down. Router Three believes it can get to Network A through one of the other paths. (In reality there are two entries in the topology table at Router One. Since Router Two does have a route to Network A. let us look at the path through Router Two to see if it qualifies as a feasible successor. you would only see one entry for Network A − through Router Four. three. Deciding if a Path is Loop−Free How does EIGRP use the concepts of feasible distance. which is higher than the feasible distance − so this path is not a feasible successor. Router One sees that it has lost its only route to Network A. it results in a routing loop. Next. Router Three thinks it can get to Network A through Router Two. If Router Three accepts all of these routes. if these are multipoint Frame Relay interfaces). only Router Two) to see if they have a route to Network A. and feasible successor to determine if a path is valid. reported distance. it accepts this route through Router Two to Network A. The reported distance from Router Two is 46277376. Since split horizon is disabled (for example. so the other will not be displayed in show ip eigrp topology. If you were to look in the topology table of Router One at this point (using show ip eigrp topology). Router One chooses the lower of these two metrics as its route to Network A. one. If the connection between Router Four and Router Three goes down. two. it will never use these paths as alternates. but the path through Router Two passes through Router Three to get to Network A. Router Three examines routes to Network A. and through Router One (path is one. through Router Two (path is two. and queries each of its neighbors (in this case. three. it responds to the query. Router Three shows three routes to Network A: through Router Four. but only one will be a feasible successor.

however. The only other entry in the topology table is from Router One. it examines its topology table and notes that the destination is marked as unreachable. When Router One receives the Router Two query. When Router Two receives the Router One query. The network has converged. Router Two replies to Router One that Network A is unreachable. Before dealing with the details of how EIGRP uses split horizon. it will not advertise the route to Network A back out the same interface to Router Three. for a path to Network A. and Router One learned about Network A from Router Two. Split Horizon and Poison Reverse In the previous example. Router Three has queried each of its neighbors in an attempt to find a route to Network A.• total metric to Network A through Router Four: 20281600 • total metric to Network A through Router Two: 47019776 • total metric to Network A through Router One: 47019776 Since the path through Router Four has the best metric. Because the reported distance through Router One is not less than the last known feasible distance. this route is not a feasible successor. For instance. Router Three does not install either route as a feasible successor for Network A. with a reported distance equal to the last known best metric through Router Three. we assumed that split horizon was not in effect to show how EIGRP uses the feasible distance and the reported distance to determine if a route is likely to be a loop. and queried each of their remaining neighbors in an attempt to find a path to Network A. in Figure 4a. Router Two marks the route as unreachable and queries each of its neighbors − in this case. Now Routers One and Two have both concluded that Network A is unreachable. only Router One − for a path to Network A. This is the first level of queries. In turn. In some circumstances. Router One examines its topology table and finds that the only other path to Network A is through Router Two with a reported distance equal to the last known feasible distance through Router Three. Router Two receives the query and. . Because both of these metrics are greater than the feasible distance. because the query is from its successor. since the reported distance through Router Two is not less than the last known feasible distance. if Router One is connected to Routers Two and Three through a single multipoint interface (such as Frame Relay). and they reply to the original Router Three query. searches each of the other entries in its topology table to see if there is a feasible successor. EIGRP uses split horizon to prevent routing loops as well. Once again. Routers One and Two have marked the route unreachable. Router One assumes that Router Three would learn about Network A directly from Router Two. Router Three installs this route in the forwarding table and uses 20281600 as its feasible distance to Network A. Router Three queries each of its neighbors for an alternative route to Network A. it also sends back a reply that Network A is unreachable. Router Two. Router Three then computes the reported distance to Network A through Routers Two and One: 47019776 for the path through Router Two. let us review what split horizon is and how it works. Router One marks the route as unreachable and queries its only other neighbor. and all routes return to the passive state. The split horizon rule states: • Never advertise a route out of the interface through which you learned it. and 47019776 for the path through Router One. Router Three also sends a query for Network A to Router One. Suppose that the link between Routers Three and Four goes down.

Poison reverse is another way of avoiding routing loops. if it shows any path to Network A through Router One. Router Three. Let us say the routers in Figure 4a have poison reverse enabled. For each table entry a router receives during startup mode. it advertises Network A as unreachable through its link to Routers Two and Three. and the 128k link between Routers Three and Four (see the Load Balancing section for a discussion of variance). they exchange topology tables during startup mode. it advertises the same entry back to its new neighbor with a maximum metric (poison route). advertise it as unreachable back through that same interface. Router One uses variance to balance the traffic destined to Network A between the two serial links − the 56k link between Routers Two and Four. When Router One learns about Network A from Router Two. Its rule states: • Once you learn of a route through an interface. Topology Table Change In Figure 5. EIGRP uses split horizon or advertises a route as unreachable when: • two routers are in startup mode (exchanging topology tables for the first time) • advertising a topology table change • sending a query Let us examine each of these situations. Startup Mode When two routers first become neighbors. EIGRP combines these two rules to help prevent routing loops. removes that path because of the unreachable advertisement. .

1.1. Queries Queries result in a split horizon only when a router receives a query or update from the successor it is using for the destination in the query. However.2. it sends a query to each of its neighbors. There are two possible solutions to this type of problem. it turns off split horizon and poison reverses the old route out all interfaces. The most basic SIA routes occur when it simply takes too long for a query to reach the other end of the network and for a reply to travel back. because Router One is its successor to this network. This is known as a stuck in active (SIA) route. When a router changes its topology table in such a way that the interface through which the router reaches a network changes. Routers One. and Four. Router Two turns off split horizon for this route. So long. For instance. that the router that issued the query gives up and clears its connection to the router that is not answering. however.0/24. If the link between Routers Two and Four goes down. effectively restarting the neighbor session. Two. In this case. Router Two would not normally send an update. Router Three receives a query concerning 10.2. and advertises Network A as unreachable. Since the split horizon rule states that you should never advertise a route out the interface through which you learned about it. this leaves Router One with an invalid topology table entry. Stuck In Active Routes In some circumstances. in Figure 7. Instead. in this case. Router One is recording a large number of SIA routes from Router Two. Router Two simply re−converges on the path through Router Three. in fact. After some investigation. it does not send a query back to Router One. Let us take a look at the network in Figure 6. Router Three receives a query or update (such as a metric change) from Router One for the destination 10.0/24 (which it reaches through Router One) from Router Four. If Three does not have a successor for this destination because a link flap or other temporary network condition. it only sends queries to Routers Two and Four. If. the problem is narrowed down to the delay over the satellite link between Routers Two and Three. it takes a very long time for a query to be answered. Router One hears this advertisement and flushes its route to Network A through Router Two from its routing table. The first is to increase the amount of .Router Two sees the path through Router Three as a feasible successor.

Find the router that is consistently failing to answer queries for these routes. is not a common reason for reported SIA routes. The command to gather this information is show ip eigrp topology active: Codes: P − Passive. Serial1 1 replies. If you run into a situation where it seems that the query range is the problem. Query range in itself.2. or slow convergence. FD is 512640000.1.1. however. some router on the network can not answer a query for one of the following reasons: • the router is too busy to answer the query (generally due to high CPU utilization) • the router is having memory problems. query−origin: Local origin via 10. The better solution. Serial3 Remaining replies: via 10.2 (Infinity/Infinity). If you are logging console messages. To avoid these problems. Note that these neighbors may not show up in the Remaining replies section. A − Active.2 (Infinity/Infinity). Find the routes that are consistently being reported as SIA. Pay particular attention to routes that have outstanding replies and have been active for some time. More often. or other problems with this neighbor. This setting can be changed using the timers active−time command. Q 1 replies. but some queries or replies are getting lost between the routers • unidirectional links (a link on which traffic can only flow in one direction because of a failure) Troubleshooting SIA Routes Troubleshooting SIA routes is generally a three−step process: 1. is to redesign the network to reduce the range of queries (so very few queries pass over the satellite link).4. Repeat this process until you find the router that is consistently not answering queries. such as below−optimal routing.1. The second step is more difficult. Examine this neighbor to see if it is consistently waiting for replies from any of its neighbors. memory or CPU utilization.2.1. however. U − Update. r. generally two to three minutes. and cannot allocate the memory to process the query or build the reply packet • the circuit between the two routers is not good − enough packets are getting through to keep the neighbor relationship up. r − Reply status A 10. Q − Query. Please note that the examples below show the minimum required to configure redistribution. Run this command several times and you begin to see which neighbors are not responding to queries (or which interfaces seem to have a lot of unanswered queries). Query range is covered in the Query Range section.0/24. they may appear among the other RDBs. active 00:00:01. 3. please see "Avoiding . R − Reply.3. 0 successors.2. it is always best to reduce the query range rather than increasing the SIA timer. a quick perusal of the log indicates which routes are most frequently marked SIA. active 00:00:01. query−origin: Local origin via 10. 2. Redistribution can potentially cause problems. Find the reason that router is not receiving or answering queries. routing loops. Serial0 Any neighbors that show an R have yet to reply (the active timer shows how long the route has been active). The first step should be fairly easy. You can look for problems on the link to this neighbor. Redistribution This section examines different scenarios involving redistribution. r.time the router waits after sending a query before declaring the route SIA.

0.255 Router Three is advertising the network 10. When routes from EIGRP 2000 are redistributed back to EIGRP 1000.255 Router Two router eigrp 2000 redistribute eigrp 1000 route−map to−eigrp2000 network 172.0.0.255.0 0.1. Router Two is redistributing this route into autonomous system 2000 and advertising it to Router One.0. Note: The routes from EIGRP 1000 are tagged 1000 before redistributing them to EIGRP 2000.Problems Due to Redistribution" in Redistributing Routing Protocols.16. Redistribution Between Two EIGRP Autonomous Systems In Figure 8. For more information on redistribution among routing protocols.255.0 0.16. the routes with 1000 tags are denied to ensure a loop−free topology. please see .255 ! router eigrp 1000 redistribute eigrp 2000 route−map to−eigrp1000 network 10.0 0. the routers are configured as follows: Router One router eigrp 2000 !−−− The "2000" is the autonomous system network 172.1.0.2.1.0.255 route−map to−eigrp1000 match tag 1000 ! route−map to−eigrp1000 set tag 2000 ! route−map to−eigrp2000 match tag 2000 ! route−map to−eigrp2000 set tag 1000 deny 10 permit 20 deny 10 permit 20 Router Three router eigrp 1000 network 10.1.0 0.0.0.1.0/24 to Router Two through autonomous system 1000.

we have changed the configurations as follows: Router One router eigrp 2000 network 172.0 Router Two router eigrp 2000 redistribute igrp 1000 route−map to−eigrp2000 network 172.0 IP−EIGRP topology entry for 10.1.Redistributing Routing Protocols. from 20.255. the minimum bandwidth shown in this topology table entry is 56k. Redistribution Between EIGRP and IGRP in Two Different Autonomous Systems In Figure 9. 1 Successor(s). Query origin flag is 1.2.0.1. This means that EIGRP preserves all metrics when redistributing between two EIGRP autonomous systems.255.1.16. external metric is 46251776 Administrator tag is 1000 (0x000003E8) Notice that although the link between Routers One and Two has a bandwidth of 1.1.2.1.1 (Serial0).544Mb. FD is 46763776 Routing Descriptor Blocks: 20. On Router One.0 ! router igrp 1000 redistribute eigrp 2000 route−map to−igrp1000 network 10.2.1.1. Route is External Vector metric: Minimum bandwidth is 56 Kbit Total delay is 41000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 2 External data: Originating router is 10.1.0.1.0/24 State is Passive.1.0 ! .16. we see: one# show ip eigrp topology 10.0 255. Send flag is 0x0 Composite metric is (46763776/46251776).1 AS number of route is 1000 External protocol is EIGRP.

but is learning about this directly−connected interface through redistribution from IGRP.1. it advertises the route with a metric of 1.1.0/24 is directly connected to Router Two. On Router One.1. Send flag is 0x0 Composite metric is (46763776/46251776).1 (Serial0). from 20. FD is 46763776 Routing Descriptor Blocks: 20.1.1 AS number of route is 1000 External protocol is IGRP.2.route−map to−igrp1000 deny 10 match tag 1000 ! route−map to−igrp1000 permit 20 set tag 2000 ! route−map to−eigrp2000 deny 10 match tag 2000 ! route−map to−eigrp2000 permit 20 set tag 1000 Router Three router igrp 1000 network 10. EIGRP is not routing for this network. There is one caveat to redistribution between IGRP and EIGRP that should be noted.0 IP−EIGRP topology entry for 10. and IGRP is routing for this network (there is a network statement under router IGRP that covers this interface).0/24 State is Passive. the topology table entry for 10.1. 1 Successor(s).0/24 shows: one# show ip eigrp topology 10.0 255.1. For example.1.255.1.1.255.0.1.1. Query origin flag is 1. Query origin flag is 1.1. from 20.1. the network 10.0/24 State is Passive.1. Route is External Vector metric: Minimum bandwidth is 1544 Kbit Total delay is 20000 microseconds Reliability is 0/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 .1.0 The configuration for Router One is shown below: one# show ip eigrp topology 10.255. If the network is directly connected to the router doing the redistribution. Route is External Vector metric: Minimum bandwidth is 56 Kbit Total delay is 41000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 External data: Originating router is 10.1 (Serial0).0. Send flag is 0x0 Composite metric is (2169856/1).1.1.0 IP−EIGRP topology entry for 10. 1 Successor(s).1.1. external metric is 180671 Administrator tag is 1000 (0x000003E8) IGRP metrics are preserved when routes are redistributed into EIGRP with a different autonomous system.1.0 255.1. but they are scaled by multiplying the IGRP metric by the constant 256. FD is 2169856 Routing Descriptor Blocks: 20.2.255.1.

" Redistribution Between EIGRP and IGRP in the Same Autonomous System The following changes are made to the router configurations in Figure 10: Router One router eigrp 2000 network 172. is 1.16.1.0 IP−EIGRP topology entry for 10.0 And Router One is configured as follows: one# show ip eigrp topology 10.1.1 AS number of route is 1000 External protocol is IGRP.255.0 Router Three router igrp 2000 network 10. 1 Successor(s).255.0/24 State is Passive.0.16.2.1.0.0 Router Two router eigrp 2000 network 172.1. Query origin flag is 1.1.1.0 255. FD is 46763776 Routing Descriptor Blocks: 20.External data: Originating router is 10.1.0.1. from 20. external metric is 0 Administrator tag is 1000 (0x000003E8) Note that the reported distance from Router Two. Route is External Vector metric: Minimum bandwidth is 56 Kbit Total delay is 41000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 External data: .1.0 ! router igrp 2000 network 10.1.2.1. which is bolded.0.1 (Serial0). Send flag is 0x0 Composite metric is (46763776/46251776).

Let us examine these caveats in Figure 11: Router One advertises 10.255. Query origin flag is 1. external metric is 180671 Administrator tag is 0 (0x00000000) This configuration looks amazingly like the earlier output when we were redistributing between two different autonomous systems running IGRP and EIGRP.1 AS number of route is 2000 External protocol is IGRP.1.1.255. external metric is 0 Administrator tag is 0 (0x00000000) So this network. Router Two runs both EIGRP and IGRP in autonomous system .1 (Serial0).0/24 State is Passive. Route is External Vector metric: Minimum bandwidth is 1544 Kbit Total delay is 20000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 External data: Originating router is 10. FD is 2169856 Routing Descriptor Blocks: 20.1.1.0/24 in IGRP autonomous system 100.1. Send flag is 0x0 Composite metric is (2169856/1).1.1.1.1.1.0/24 network is handled the same way in both scenarios: one# show ip eigrp topology 10.Originating router is 10. which is directly connected to Router One.1.1.1 AS number of route is 2000 External protocol is IGRP.1. There are two caveats with EIGRP/IGRP redistribution within the same autonomous system: • Internal EIGRP routes are always preferred over external EIGRP or IGRP routes. Router Four advertises 10.1.1.4. 1 Successor(s). • External EIGRP route metrics are compared to scaled IGRP metrics (the administrative distance is ignored).4.1. is redistributed from IGRP to EIGRP with a metric of 1 − the same metric we see when redistributing between two different autonomous systems.1. The directly attached 10.0/24 as an external in EIGRP autonomous system 100.0 IP−EIGRP topology entry for 10.0 255. from 20.

an IGRP metric.1.4. via Serial1 Route metric is 12001. The router always prefers the path with the lowest cost metric and ignores the administrative distance. is through Router Four.2. 00:53:59 ago. eigrp 100 Advertised by igrp 100 (self originated) eigrp 100 Last update from 10.1. Hops 1 Note the metrics for these two routes are the same after being scaled from IGRP to EIGRP (see the Metrics section): • 12001 x 256 = 3072256 where 12001.1.1.1. from 10. Router Two prefers the EIGRP external route with the same metric (after scaling) and a higher administrative distance. type external Redistributing via igrp 100.1.2. metric 12001 Redistributing via igrp 100. traffic share count is 1 Total delay is 20010 microseconds.2 on Serial0. . minimum MTU 1 bytes Loading 1/255. for example − works in the same way as all redistribution.0 Routing entry for 10. Redistribution To and From Other Protocols Redistribution between EIGRP and other protocols − RIP and OSPF.100. minimum MTU 1 bytes Loading 1/255.1. traffic share count is 1 Total delay is 20010 microseconds.1. Hops 0 Note the administrative distance is 100.1. for instance).2.2. metric 3072256. When we add the EIGRP route.4.2. 00:00:42 ago Routing Descriptor Blocks: * 10. • External EIGRP routes have an administrative distance of 170. It is always best to use the default metric when redistributing between protocols. via Serial0 Route metric is 3072256. You should be aware of the following two issues when redistributing between EIGRP and other protocols: • Routes redistributed into EIGRP are not always summarized − see the Summarization section for an explanation. an EIGRP metric. distance 100.1. eigrp 100 Last update from 10.2.1. from 10.1. 00:53:59 ago Routing Descriptor Blocks: * 10. Router Two shows: two# show ip route 10.0 Routing entry for 10.4. 00:00:42 ago. is through Router One.1. If we ignore the EIGRP route advertised by Router Four (by shutting down the link between Routers Two and Four. distance 170. This is true whenever automatic redistribution occurs between EIGRP and IGRP within the same autonomous system.0/24 Known via "igrp 100". minimum bandwidth is 1000 Kbit Reliability 1/255. and 3072256.4.0/24 Known via "eigrp 100". Router Two shows: two# show ip route 10.2 on Serial1.2. minimum bandwidth is 1000 Kbit Reliability 1/255.

1. in Figure 13. Serial0 Note the route to 172. because the interface Router Two uses to reach Router One is in a different major network.1. 00:00:47.0 network 172.0/24 [90/2169856] via 10.0.0. For example. 00:00:47.. 2 masks C 10. because EIGRP considers this a directly attached network. 2 subnets.1.2.. .1.0.16.1. On Router Two.255. 10.0/24 appears as an internal EIGRP route on Router Two.1. this looks as follows: two# show ip route .1.0 no auto−summary Router One redistributes this route.1.0/8 is variably subnetted. EIGRP redistributes this route as if it were a directly connected interface. Summarization There are two forms of summarization in EIGRP: auto−summaries and manual summaries. Router One has a static route to the network 172.0. Serial0 D 10.1.Redistribution of Static Routes to Interfaces When you install a static route to an interface.255.1.0/24 is subnetted. 1 subnets D 172.0/24 is directly connected.0 Serial0 And Router One also has a network statement for the destination of this static route: router eigrp 2000 network 10. Router Two advertises only the 10. and configure a network statement using router eigrp.1.16.0/24 configured through interface Serial 0: ip route 172.16.16.1.16.0.16.. Let us look at the network in Figure 12.0. which includes the static route. Serial0 172.0/8 network to Router One.0. even though it is not redistributing static routes.0.0 [90/2169856] via 10.1. Auto−Summarization EIGRP performs an auto−summarization each time it crosses a border between two different major networks.0 255.

0/8. from 0.0.0.3. Route is Internal Vector metric: Minimum bandwidth is 256 Kbit Total delay is 20000 microseconds Reliability is 255/255 Load is 1/255 . Serial1 10. Serial2 10.0 IP−EIGRP topology entry for 10. The metric is the best metric from among the summarized routes.0.0.0/8 State is Passive.0. The topology table entry for this summary route looks like the following: two# show ip eigrp topology 10.0.1.0/24 is directly connected.0. 1 Successor(s).0. Route is Internal Vector metric: Minimum bandwidth is 256 Kbit Total delay is 40000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 This route is not marked as a summary route in any way. although there are links in the 10.0 (Null0).0. On the router doing the summarization.0.0.0.2.0 here means this route is originated by this router) Composite metric is (10511872/0). 00:23:24.0/8 State is Passive.16.1. it looks like an internal route.2.0.0 network that have a bandwidth of 56k.2.0.0. 4 known subnets Attached (2 connections) Variably subnetted with 2 masks Redistributing via eigrp 2000 C D D C 10. Query origin flag is 1.0.1.1.0.0.0.0 Routing entry for 10. 00:23:20.On Router One.0. Serial1 The route to 10.0. Send flag is 0x0 (note: the 0.0/8 is a summary.1 (Serial0). Query origin flag is 1.1. this looks like the following: one# show ip eigrp topology 10. Note that the minimum bandwidth on this route is 256k.1.0/8 is marked as a summary through Null0.0. from 172.1.0/24 [90/10537472] via 10.0.1. FD is 10511872 Routing Descriptor Blocks: 0.0.0/24 is directly connected.16.0. Null0 10. a route is built to null0 for the summarized address: two# show ip route 10. Send flag is 0x0 Composite metric is (11023872/10511872).0 IP−EIGRP topology entry for 10. FD is 11023872 Routing Descriptor Blocks: 172. 1 Successor(s).

0 network 10.2. FD is 46354176 via 20.0.1.3.1. Router One now sees all of the components of the 10.1. Serial0 There are some caveats when dealing with the summarization of external routes that are covered later in the Auto−Summarization of External Routes section. 1 successors.1 (11023872/10511872).3.. 1 successors.1 (11049472/10537472).1. FD is 11049472 via 20.0/24. 1 successors.1.0/24.1.0.1. R − Reply. and 192. Serial0 P 10. . Serial0 P 10.0. 1 successors. in Figure 14.Minimum MTU is 1500 Hop count is 0 To make Router Two advertise the components of the 10.16.1. U − Update.1. Serial0 P 172.16.0/24. A − Active..0.0.1. Router Two is summarizing the 192.0/24. FD is 11023872 via 20. FD is 2169856 via Connected. For example.0.0.0 no auto−summary With auto−summary turned off. 192. r − Reply status P 10.1.0 network: one# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive.0/24 into the CIDR block 192..1.1.2.1.1.1 (46354176/45842176). Manual Summarization EIGRP allows you to summarize internal and external routes on virtually any bit boundary using manual summarization.0/24.0/22.0/24. configure no auto−summary on the EIGRP process on Router Two: On Router Two router eigrp 2000 network 172. The configuration on Router Two is shown below: two# show run .0 network instead of a summary.1. Q − Query.0.

! interface Serial0 ip address 10.1.1. U − Update. Serial1 P 192. Q − Query. FD is 2169856 via Connected.0.0/24. two# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive. FD is 11023872 via 10. Serial0 P 10.50.255.50. FD is 45842176 via Connected.1. To illustrate.10.1.1 (46354176/45842176). let us look at Figure 15.0/24.1 255. Loopback0 P 10.2. r − Reply status P 10.0/22. A − Active. FD is 10639872 via 192. we see this as an internal route: one# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive. U − Update.1. 1 successors.0/24. Serial1 Note the ip summary−address eigrp command under interface Serial0.10.0/24.1.1. 1 successors. Serial0 P 192. 1 successors.0 ip summary−address eigrp 2000 192. R − Reply. 1 successors. Serial1 P 192.0/22.1. Q − Query. FD is 46354176 via 10. 1 successors.1 (10639872/128256). A − Active..1.255.0 255.1.0/26 and 192. as shown in the configurations below.1 (10537472/281600).1.0 no ip mroute−cache ! . r − Reply status P 10.50.64/26 into EIGRP using the redistribute connected command.1. FD is 10511872 via Connected.3. Null0 P 192.50.1.1. 1 successors. 1 successors. R − Reply. .1 (11023872/10511872).1..252.0/24. On Router One. Serial0 Auto−Summarization of External Routes EIGRP will not auto−summarize external routes unless there is a component of the same major network that is an internal route.1. FD is 10511872 via Summary (10511872/0).0.255.50.. Router Three is injecting external routes to 192. FD is 10537472 via 192. 1 successors. Serial0 P 192.1.0/24.0.1.2.1. and the summary route via Null0.2. FD is 2169856 via Connected.0/24. 1 successors.1.

if you reconfigure the link between Routers Two and Three to 192.2. 00:00:53. 00:00:36.0/8 is subnetted..1 255.1.1.255.255.1.2.. Serial0 D 192.0/26 and 192. Null0 .1.1.50.2.2.. 2 subnets D 10.0/24 auto−summary is then generated on Router Two.. 10.50.0.128/26. 00:06:48. 10.1. and add network statements for this network on Routers Two and Three.1.0/24 [90/11023872] via 10.1.65 255.0/8 is subnetted. the 192. And Router One shows only the summary route: one# show ip route .1..Router Three interface Ethernet0 ip address 192.1.2. it does not do this because both routes are external. 1 subnets D EX 192.2.1. Router Three interface Ethernet0 ip address 192.50.1.2.1..2. Serial0 D EX 192.1. 00:00:53.2.0/24 is a summary.0 !router eigrp 2000 redistribute connected network 10.1.0 Now Router Two generates the summary for 192.255. Serial0 192.0 is directly connected. 1 subnets C 10.0.2.2.255.0/24: two# show ip route .255. D 192..255.2.1.2.50.0 default−metric 10000 1 255 1 1500 With this configuration on Router Three.255. 00:02:03.64/26 routes into one major net destination (192.0.255.255..1.1.1 255.1.1.0 [170/11049472] via 10.2. Serial0 Although auto−summary normally causes Router Three to summarize the 192..64 [170/11049472] via 10. However.1 255.65 255.0 [90/11023872] via 10.192 ! interface Ethernet1 ip address 192.1.1.2.255.0.192 ! interface Serial0 ip address 192.1.0/24).1.255. the routing table on Router One shows: one# show ip route .2.2.192 ! interface Ethernet2 ip address 10.255.2. Serial0 C 10. Serial0 .192 ! interface Ethernet1 ip address 192.0.0 is directly connected.0.2..1.192 ! router eigrp 2000 network 192.2.0/26 is subnetted.2.2...130 255.2.50.1.

mark destination unreachable and query all neighbors except the previous successor The actions in the table above impact the range of the query in the network by determining how many routers receive and reply to the query before the network converges on the new topology. if not successful. let us look at the network in Figure 16. reply with an unreachable if there is a good successor. the following rules apply: Query from neighbor (not the current successor) successor any neighbor any neighbor Route state passive passive Action reply with current successor information attempt to find new successor. reply with new information. which is running under normal conditions. . if successful. reply with new information. mark destination unreachable and query all neighbors except the previous successor no path through this neighbor before query reply with best path currently known not known before query reply that the destination is unreachable neighbor (not the current active successor) if there is no current successor to this destinations (normally this would be true). To see how these rules affect the way queries are handled. if successful.Query Processing and Range When a router processes a query from a neighbor. reply with the current path information successor active attempt to find new successor. if not successful.

3.0/24 as unreachable. and queries Router Four: Router Four.168.0/24: ♦ through Router Two with a distance of 46533485 and a reported distance of 20307200 ♦ through Router Three with a distance of 20563200 and a reported distance of 20307200 • Router One chooses the path through Router Three and keeps the path through Router Two as a feasible successor • Routers Two and Three show one path to 192. Router Five marks 192.0/24 fails.168.0/24 as unreachable and query Routers Two and Three: . What activity can we expect to see on this network? Figures 16a through 16h illustrate the process.3.3. It does not find one.3. upon receiving a query from its successor. so it marks 192.3.0/24 (far right side): • Router One has two paths to 192.168.168.3.168.We can expect the following to happen regarding network 192.168. attempts to find a new feasible successor to this network.0/24 through Router Four Suppose that 192.

let us assume that Router One receives the query from Router Three first. see that they have lost their only feasible route to 192.3. in turn. and marks the route as unreachable.168.Routers Two and Three. Router One then receives the query from Router Two. Although another order is possible. they will all have the same final result.0/24. and mark it as unreachable. they both send queries to Router One: For simplicity. .

0/24: .Router One replies to both queries with unreachables. Routers Two and Three are now passive for 192.168.3. Router One is now passive for 192.168.3.0/24: Routers Two and Three reply to the query from Router Four.

0/24 when that link goes down. It is important to understand that although there may be other query paths or processing orders.1.Router Five.1. Router Five sends updates back to Router Four so the route is removed from the topology and routing tables of the remaining routers. removes network 192. • Router Three has a topology table entry for the 10.0/24 network with a cost of 46251885 through Router One.0. upon receiving the reply from Router Four.1. . Router Five is now passive for network 192. How Summarization Points Affect the Query Range Now let us look at the paths to 10.1.3. • Router Four has a topology table entry for the 10.3.0/24 from its routing table. all routers in the network process a query for network 192.1. so the path through Router Two is not a feasible successor). This is a good example of an unbounded query in an EIGRP network.168. Some routers may end up processing more than one query (Router One in this example).168. In fact.0/24 in the same network: • Router Two has a topology table entry for the 10.0/8 network (because Routers Two and Three are autosummarizing to the major network boundary) through Router Three with a metric of 20307200 (the reported distance through Router Two is higher than the total metric through Router Three. if the queries were to reach the routers in a different order. some would end up processing three or four queries.0/24 network with a cost of 20281600 through Router One.0/24.3.168.1.0.

marks the route as unreachable (because the query is from its successor) and then queries Routers Four and Three: . and then queries each of its neighbors (Routers Two and Three) for a new path to that network: Router Two.1. on receiving the query from Router One.0/24 goes down.If 10. Router One marks it as unreachable.1.

when it receives the query from Router One.0.1.0.0/24 is unreachable (note that Router Four has no knowledge of the subnet in question. marks the destination as unreachable and queries Routers Two and Four: Router Four.0/8 route): . replies that 10.Router Three.1. since it only has the 10. when it receives the queries from Routers Two and Three.

1.1.1. and is not involved in the re−convergence of the network.0/24 is unreachable: Since Routers Two and Three now have no outstanding queries. in this case.1. Router Five does not participate in the query process. and distribution lists. For example. Queries can also be bound by manual summarization. How Autonomous System Boundaries Affect the Query Range If a router is redistributing routes between two EIGRP autonomous systems.0/24 is unreachable: The query.Routers Two and Three reply to each other that 10. they both reply to Router One that 10. if the link to the network attached to Router Three goes down. autonomous system borders. Router Three marks the route unreachable and queries Router Two for a new path: . it replies to the query within the normal processing rules and launches a new query into the other autonomous system. is bounded by the autosummarization at Routers Two and Three.

. Router Three is now passive for this network: Router One replies to Router Two. Once Router Three receives the reply to its original query.Router Two replies that this network is unreachable and launches a query into autonomous system 200 toward Router One. and the route goes passive: While the original query did not propagate throughout the network (it was bound by the autonomous system border). Let us use Figure 19 as an example. this method of bounding a query may worsen the problem by preventing the auto−summarization of routes that would otherwise be summarized (external routes are not summarized unless there is an external component in that major network). it removes the route from its table. but it does not solve the overall problem that each router must process the query. In fact. distribution lists in EIGRP mark any query reply as unreachable. This technique may help to prevent stuck in active (SIA) problems in a network by limiting the number of routers a query must pass through before being answered. the original query leaks into the second autonomous system in the form of a new query. How Distribution Lists Affect the Query Range Rather than block the propagation of a query.

Router Three does not advertise a path to Network A because of the distribution list on its serial ports. Router Three marks the route as unreachable. it marks the route as unreachable and sends a query to Router Three.In the figure above: • Router Three has a distribute−list applied against its serial interfaces that only permits it to advertise Network B. then queries Router Two: . • Routers One and Two do not know that Network A is reachable through Router Three (Router Three is not used as a transit point between Routers One and Two). When Router One loses its connection to Network A. • Router Three uses Router One as its preferred path to Network A. and does not use Router Two as a feasible successor.

even though Router Three has a valid route to Network A: . Note the query was not affected by the distribution list in Router Three: Router Two replies that Network A is reachable. but the distribution list causes Router Three to send a reply that Network A is unreachable.Router Two examines its topology table and finds that it has a valid connection to Network A. Router Three now has a valid route: Router Three builds the reply to the query from Router One.

it uses the following formula to determine how long to wait before sending the packet: • (8 * 100 * packet size in bytes) / (bandwidth in kbps * bandwidth percentage) For instance. The pacing timer determines when the packet is sent. the largest packet that can be sent over the interface.1463 seconds This allows a packet (or groups of packets) of at least 512 bytes to be transmitted on this link before EIGRP sends its packet. but this can be changed with the following command: router(config−if)# ip bandwidth−percent eigrp 2 ? <1−999999> Maximum bandwidth percentage that EIGRP may use Essentially. each time EIGRP queues a packet to be transmitted on an interface. as shown below: router# show ip eigrp interface IP−EIGRP interfaces for process 2 Interface Se0 Se1 router# Peers 1 1 Xmit Queue Un/Reliable 0/0 0/0 Mean SRTT 28 44 Pacing Time Un/Reliable 0/15 0/15 Multicast Flow Timer 127 211 Pending Routes 0 0 The time displayed is the pacing interval for the maximum transmission unit (MTU). and is typically expressed in milliseconds. There is a field in show ip eigrp interface that displays the pacing timer. EIGRP avoids this congestion by pacing the speed at which packets are transmitted on a network. The pacing time for the packet in the above example is 0. The default configuration for EIGRP is to use up to 50 percent of the available bandwidth. if EIGRP queues a packet to be sent over a serial interface that has a bandwidth of 56k.Pacing Packets Some routing protocols consume all of the available bandwidth on a low bandwidth link while they are converging (adapting to a change in the network). . and the packet is 512 bytes. thereby using only a portion of the available bandwidth. EIGRP waits: • (8 * 100 * 512 bytes) / (56000 bits per second * 50% bandwidth) (8 * 100 * 512) / (56000 * 50) 409600 / 2800000 0.1463 seconds.

and 3.0. The only way to configure a default route on a router using this method is to configure a static route to 0.0 0. which is greater than the metric through path 3.x (next hop to the internet) ! router eigrp 100 redistribute static default−metric 10000 1 255 1 1500 The static route that is redistributed into EIGRP does not have to be to network 0.x.0 Load Balancing EIGRP puts up to four routes of equal cost in the routing table. Since summaries are configured per interface. For example: ip route 0.0.1. however. Note that a summary to 0. which the router then load−balances. Let us say there are four paths to a given destination. because 1100 x 2 = 2200.0. you can use the variance command to instruct the router to also place traffic on paths 3 and 4.0.0.0.1 frame−relay interface−dlci 10 ip summary−address eigrp 100 0.x.0. EIGRP. and the metrics for these paths are: • path 1: 1100 • path 2: 1100 • path 3: 2000 • path 4: 4000 The router. you do not need to worry about using distribute−lists or other mechanisms to prevent the default route from being propagated toward the core of your network.0 ! interface serial 0 encapsulation frame−relay no ip address ! interface serial 0.0/0. you can also configure an administrative distance on the end of the ip summary−address eigrp command.0.0.0. so the local summary does not override the 0. can also load−balance over unequal cost links. The type of load balancing (per packet or per destination) depends on the type of switching being done in the router.0.0.0/0. you can configure EIGRP to use up to six routes of equal cost. to also add path 4.0. Refer to How Does Unequal Cost Path .Default Routing There are two ways to inject a default route into EIGRP: redistribute a static route or summarize to 0. use variance 2. The variance is a multiplier: traffic will be placed on any link that has a metric less than the best path multiplied by the variance.0. router eigrp 100 network 10.1.0 x. Similarly.0.0.0. Summarizing to a default route is effective only when you want to provide remote sites with a default route. (Beginning in Cisco IOS Software 12.0. Use the first method when you want to draw all traffic to unknown destinations to a default route at the core of the network. you must use the ip default−network command to mark the network as a default network.0(4)T. issue variance 4 under the router eigrp command.1 point−to−point ip address 10. 2. If you use another network.0. Note: Using max−paths.0.0. Using EIGRP. To load balance over paths 1. This method is effective for advertising connections to the Internet. Refer to Configuring a Gateway of Last Resort for further information.0 0.0/0 route). by default. places traffic on both path 1 and 2.0/0 overrides a default route learned from any other routing protocol.

0/24 Known via "igrp 100".4.2.1. eigrp 100 Advertised by igrp 100 (self originated) eigrp 100 Last update from 10. In order to raise the metric.1. At lower bandwidths. at higher bandwidths. do not change the . If it is necessary to influence the path EIGRP chooses.2. How does the router divide the traffic between these paths? It divides the metric through each path into the largest metric. It is not possible to modify the administrative distance for a default gateway that was learned from an external route because. and so on.Load Balancing (Variance) Work in IGRP and EIGRP? for more information. minimum MTU 1 bytes Loading 1/255. EIGRP will not send traffic over an unequal cost path if the reported distance is greater than the feasible distance for that particular route. it is important that these be set correctly. and the next packet over path 4. and Feasible Successors section for more information. the next three packets over path 2. Reported Distance. minimum bandwidth is 1000 Kbit Reliability 1/255. 00:00:42 ago. the traffic share counts are: • for paths 1 and 2: 4000/1100 = 3 • for path 3: 4000/2000 = 2 • for path 4: 4000/4000 = 1 The router sends the first three packets over path 1. 00:00:42 ago Routing Descriptor Blocks: * 10. in EIGRP. the modification of the administrative distance only applies for internal routes. router# show ip route 10. Refer to the Feasible Distance. and uses this number as the traffic share count.1. distance 100.2. Using Administrative Tags in Redistribution External administrative tags are useful for breaking redistribution routing loops between EIGRP and other protocols. By tagging the route when it is redistributed into EIGRP.1. the next two packets over path 3. metric 12001 Redistributing via igrp 100. use a route−map with prefix−list. always use delay to do so. the delay has more influence over the total metric. rounds down to the nearest integer.0 Routing entry for 10.4. from 10.2. Because EIGRP uses the interface bandwidth to determine the rate at which to send packets. multipoint serial links and other mismatched media speed situations are the exceptions to this rule. Hops 0 For this example. via Serial1 Route metric is 12001.2.1. Using the Metrics When you initially configure EIGRP. traffic share count is 1 Total delay is 20010 microseconds. • The delay should always be used to influence EIGRP routing decisions. Note: Even with variance configured. the bandwidth has more influence over the total metric.2 on Serial1. remember these two basic rules if you are attempting to influence EIGRP metrics: • The bandwidth should always be set to the real bandwidth of the interface. you can block redistribution from EIGRP into the external protocol. The router then restarts by sending the next three packets over path 1.

administrative distance. interface Loopback0 ip address 172. Serial0 via Redistributed (2169856/0) P 172.. FD is 128256..17. Router Three. A basic example of configuring these tags follows.255..255.1 255.1.255..0 ! interface Ethernet0 ip address 172.0/24.. 1 successors.. three# show ip eigrp topo IP−EIGRP Topology Table for process 444 Codes: P − Passive. FD is 2169856 via Connected. access−list 10 permit 172.0 .16.0 default−metric 10000 1 255 1 1500 . r − Reply status P 172.0.17.0. A − Active.0 loopback no keepalive ! interface Serial0 ip address 172. U − Update. shows: three# show run .19. Q − Query.17.. which is redistributing routes connected into EIGRP.255... R − Reply.1.255.0/24. router eigrp 444 redistribute connected route−map foo network 172.1.0 0.1 255.0/24.255 route−map foo permit 10 match ip address 10 set tag 1 .1.1 255...19.0.1.255.. but this example does not show the entire configuration used for breaking redistribution loops.255. FD is 281600 via Redistributed (281600/0) P 172. tag is 1 via Redistributed (128256/0) .19.1. 1 successors. 1 successors.16.

one# show ip route .2 255. U − Update.0 no fair−queue clockrate 1000000 router rip network 172.2 255.1.. FD is 2195456 via 172. A − Active.1..255..255.18.. interface Serial0 ip address 172.1.0 .0 . shows: two# show run .. FD is 2169856 via Connected.255.18.3 255.0/24.255 Serial0 route−map foo deny 10 match tag 1 ! route−map foo permit 20 .0/24.0. shows: one# show run .Router Two. two# show ip eigrp topo IP−EIGRP Topology Table for process 444 Codes: P − Passive.1. which is receiving the RIP routes redistributed by Router 2..18.1. r − Reply status P 172.0/24.0 ! interface Serial1 ip address 172..1. Serial0 P 172...0/24. tag is 1 via 172.17..255... 1 successors. router eigrp 444 network 172.255.1 255.255. Serial0 P 172. interface Serial0 ip address 172.0 network 172. FD is 2297856. 1 successors. Router One.255.18.17.17.0.1.17. 1 successors...1.1.19.0.1.255.0. Q − Query.1 (2195456/281600)..0.17. R − Reply.16. Serial0 Note the tag 1 on 172.0 default−metric 1 ! no ip classless ip route 1.1. which is redistributing routes from EIGRP into RIP.0 ! router rip redistribute eigrp 444 route−map foo network 10.1 (2297856/128256).19.

0. An explanation of each output field follows the table. 00:00:15. Serial0 [120/1] via 172. Serial0 Note that 172.1.Gateway of last resort is not set R R C 172. • SIA−Replies sent/received displays the number of stuck in active reply packets sent and received (sent−0/received−0). .0/16 172.17.3.18.3. • Hello Process ID is the hello process identifier (270). • Updates sent/received displays the number of update packets sent and received (sent−20/received−39). • Input Queue shows the EIGRP Hello Process to EIGRP PDM socket queue counters (current−0/max−2000/highest−1/drops−0). • Acks sent/received stands for the number of acknowledgment packets sent and received (sent−66/received−41).19.1.1.0/24 is missing.18. Serial0 is subnetted. 00:00:15. The output of this command shows the information that has been exchanged between the neighboring EIGRP router.0. 1 subnets is directly connected.0. • Socket Queue displays the IP to EIGRP Hello Process socket queue counters (current−0/max−2000/highest−1/drops−0). • PDM Process ID stands for protocol−dependant module IOS process identifier (251). • SIA−Queries sent/received means number of stuck in active query packets sent and received (sent−0/received−0).0/24 172. • Queries sent/received means the number of query packets sent and received (sent−10/received−18). Understanding EIGRP Command Output show ip eigrp traffic This command is used to display information about EIGRP named configurations and EIGRP autonomous−system (AS) configurations.0/16 172.18.18. • Replies sent/received shows the number of reply packets sent and received (sent−18/received−16).1.0 [120/1] via 172. show ip eigrp traffic Configuration Explanations • Hellos sent/received shows the number of hello packets sent and received (sent −1927/received − 1930).16.

• (Infinity/Infinity) shows the metric to reach this path through this neighbor in the first field. Serial1 shows we are using this route (indicates which path the next path/destination will take when there are multiple routes of equal cost). • 0 successors shows how many successors (or paths) are available for this destination. • query origin: Local origin shows this route originated the query. • via 10.2. • Serial1 is the interface through which this neighbor is reachable.2 shows that we learned of this route from a neighbor whose IP address is 10. • FD is 512640000 shows the feasible distance. if the network is directly connected to this router. • Q means a query is pending. meaning there is an update pending. if this is a summary route generated on this router.2. if this route is being redistributed into EIGRP on this router. for reply pending.4. • r shows that we have queried this neighbor about the route and have not yet received a reply.1. • tag is 0x0 can be set and/or filtered using route maps with the set tag and match tag commands. To display all entries in the topology table.2.2 (512640000/128256). meaning the successor originated the query. . • Serial0 is the interface through which this neighbor is reachable. show ip eigrp topology Configuration Explanations • A means active. if successors is capitalized.0/24 is the destination or mask.2. • 10. meaning passive. • Via 10.show ip eigrp topology This command only displays feasible successors.1. meaning that multiple neighbors have sent queries on this destination. • active 00:00:01 shows how long this route has been active. the route is in transition. or Summary. or Successor origin. or R. • Q is the send flag for this route. and the reported distance through this neighbor in the second field. use the show ip eigrp topology all−links command. • r shows that we have queried this neighbor and are waiting for a reply.1. for update pending. which is the best metric to reach this destination or the best metric known when the route went active. • Via 10. An explanation of each output field follows the table. Redistributed.2 shows the neighbor from which we are waiting for a reply.2. This field can also be: Connected. This field can also be: U. This could also show a P.1. but not the successor. • 1 replies shows the number of outstanding replies. This field can also be: U. or R.1. meaning there is a query pending. This field can also be: Multiple origins. meaning there is a reply pending.

this indicates the metric of the path we were previously using to route packets to this network. ♦ from 10. we are not looking for a path to this network. ♦ 4: Multiple query sources for this route. • Query origin flag is 1 If this route is active.2 (Ethernet1) is the next hop to the network and the interface that next hop is reached through. Similar to 2.1. not just feasible successors. If the route is active. ♦ 0: This route is active but no query has been originated for it (we are searching for a feasible successor locally). ♦ 2: Multiple diffusing computations for this query. ♦ Send flag is: .2. Routes are almost always in a passive state in stable networks.1. This router has received more than one query for this route from more than one source.show ip eigrp topology <network> This command displays all entries in the topology table for this destination. • FD is 307200 shows the best current metric to this network. ♦ 10. An explanation of each output field follows the table.1.2 is the source of this path information. or. • Routing Descriptor Blocks Each of the following entries describes one path to the network. in other words. ♦ 3: The router that we learned the path to this network from is querying for another route. but it also means there is a query origin string which describes the queries outstanding for this path. • 2 Successor(s) means there are two feasible paths to this network. ♦ 1: This router originated the query for this route (or the route is passive). this field provides information on who originated the query. show ip eigrp topology network Configuration Explanations • State is Passive means the network is in passive state. including the router through which we learned this route.

but does limit the maximum size of an EIGRP AS. ◊ 0x2: This route is active. • Composite metric is (307200/281600) shows the total calculated costs to the network. and is not used by default when EIGRP calculates the cost to use this path. The maximum number of hops that EIGRP will accept is 100 by default. and each router computes the cost and reported distance individually. ♦ Load is 1/255 indicates the amount of load the link is carrying. This number is calculated dynamically. ♦ Minimum MTU is 1500 This field is not used in metric calculations. the following information is included. The second number in the parentheses is the reported distance. • External AS shows the Autonomous System this route came from (if there is one). EIGRP does not propagate total cost information throughout the network. this indicates the type of packet. but it also shows some portion of the topology table. • external metric shows the internal metric in the external protocol. the vector metrics are propagated. the cost the next hop router uses. ♦ Minimum bandwidth is 10000 Kbit shows the lowest bandwidth on the path to this network. ♦ Reliability is 0/255 shows a reliability factor. ◊ 0x3: This route has changed. this field would indicate that the route is External. ♦ Total delay is 2000 microseconds shows the sum of the delays on the path to this network. An explanation of each output field follows the table. in other words. . If the route was redistributed into this EIGRP AS. • Route is Internal means this route was originated within this EIGRP autonomous system (AS). including the cost to the next hop. If the route is external. although the maximum can be configured to 220 with metric maximum hops. ◊ 0x1: This router has received a query for this network.◊ 0x0: If there are packets that need to be sent in relation to this entry. or. and a multicast query should be sent. • Vector metric shows the individual metrics used by EIGRP to calculate the cost to a network. ♦ Hop count is 2 This is not used in metric calculations. The first number in the parentheses is the total cost to the network through this path. • Administrator Tag can be set and/or filtered using route maps with the set tag and match tag commands. show ip eigrp topology [active | pending | zero−successors] Same output format as show ip eigrp topology . This number is calculated dynamically. and needs to send a unicast reply. but is not used by default in metric calculations. and a multicast update should be sent. • External Protocol shows the protocol this route came from (if there is one). External Route Configuration Explanations • Originating Router shows that this is the router that injected this route into the EIGRP AS.

Inc. Terms & Conditions | Privacy Statement | Cookie Policy | Trademarks of Cisco Systems.show ip eigrp topology all−links Same output format as show ip eigrp topology . Inc. All rights reserved. Updated: Sep 09. 2005 Document ID: 16406 . Related Information • EIGRP Support Page • EIGRP Command Reference Guide • IP Routing Support Page • Technical Support & Documentation − Cisco Systems Contacts & Feedback | Help | Site Map © 2013 − 2014 Cisco Systems. rather than just feasible successors. but it also shows all links in the topology table.