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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

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

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 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. 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: . The delay as shown in the show ip eigrp topology or show interface commands is in microseconds. Let us compute the metrics. we use delay as it is configured and shown on the interface. 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. on the route to the destination network. with a minimum bandwidth of 128 and a delay of 1200. In this example. and the other through Router Three. so you must divide by 10 before you use it in this formula. The default values for K are: • K1 = 1 • K2 = 0 • K3 = 1 • K4 = 0 • K5 = 0 For default behavior. which can cause your network to fail to converge. so at each stage in the calculation. the total cost through Router Four is: In this example. Throughout this paper. you can simplify the formula as follows: metric = bandwidth + delay Cisco routers do not perform floating point math. the formula reduces to Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 − load) + k3 * delay]. in tens of microseconds. Router One chooses the path with the lowest metric. EIGRP calculates the total metric by scaling the bandwidth and delay metrics. Note: If K5 = 0. you need to round down to the nearest integer to properly calculate the metrics. Mismatched K values prevent a neighbor relationship from being built.

shown in Figure 4. Since the reported distance to this network through Router Four is less than the feasible distance. and Router One added the delay configured on its serial. the reported distance from Router Four is the metric to get to Network A from Router Four.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. and uses the metric through Router Three as the feasible distance. Router One examines each path it knows to Network A and finds that it has a feasible successor through Router Four. Router One considers the path through Router Four a feasible successor. Let us look at a more complex scenario. Router One chooses the route through Router Three. Reported distance is the total metric along a path to a destination network as advertised by an upstream neighbor. For example. 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. The network converges instantly. Reported Distance. using the metric through Router Four as the new feasible distance. Router Four added the delay configured on its Ethernet. 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. In other words. and Feasible Successor Feasible distance is the best metric along a path to a destination network. • The route through Router Three has a cost of 20307200 and a reported distance of 307200. including the metric to the neighbor advertising that path. Router One uses this route. Feasible Distance. and updates to downstream neighbors are the only traffic from the routing protocol. A feasible successor is a path whose reported distance is less than the feasible distance (current best path). Router Two advertised Network A with the delay configured on its Ethernet interface. . When the link between Routers One and Three goes down. EIGRP chooses the route through Router Three as the best path. 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. • The route through Router Four has a cost of 46277376 and a reported distance of 307200.

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

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

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

Router Two sees the path through Router Three as a feasible successor. in Figure 7. However. this leaves Router One with an invalid topology table entry.2. Two. The first is to increase the amount of . Router One is recording a large number of SIA routes from Router Two.0/24. Stuck In Active Routes In some circumstances. Router Two would not normally send an update. in this case. So long. effectively restarting the neighbor session. in fact. If. it turns off split horizon and poison reverses the old route out all interfaces. Router Three receives a query or update (such as a metric change) from Router One for the destination 10. Instead. Let us take a look at the network in Figure 6.2. There are two possible solutions to this type of problem. 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.1.1. and advertises Network A as unreachable. 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. After some investigation. If Three does not have a successor for this destination because a link flap or other temporary network condition. that the router that issued the query gives up and clears its connection to the router that is not answering. If the link between Routers Two and Four goes down. Since the split horizon rule states that you should never advertise a route out the interface through which you learned about it. In this case. the problem is narrowed down to the delay over the satellite link between Routers Two and Three. When a router changes its topology table in such a way that the interface through which the router reaches a network changes. it does not send a query back to Router One. because Router One is its successor to this network. This is known as a stuck in active (SIA) route. Router Three receives a query concerning 10. 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. Router Two simply re−converges on the path through Router Three. it only sends queries to Routers Two and Four. Routers One. For instance. and Four. however. it sends a query to each of its neighbors. Router Two turns off split horizon for this route.0/24 (which it reaches through Router One) from Router Four.

3. Serial0 Any neighbors that show an R have yet to reply (the active timer shows how long the route has been active). Repeat this process until you find the router that is consistently not answering queries. The command to gather this information is show ip eigrp topology active: Codes: P − Passive.1. or other problems with this neighbor. routing loops. U − Update. This setting can be changed using the timers active−time command. r. query−origin: Local origin via 10. Serial3 Remaining replies: via 10.2. To avoid these problems. Query range is covered in the Query Range section. 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. The first step should be fairly easy.4. The second step is more difficult. 0 successors. Serial1 1 replies. it is always best to reduce the query range rather than increasing the SIA timer. 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. 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). however. FD is 512640000. Query range in itself. a quick perusal of the log indicates which routes are most frequently marked SIA. is not a common reason for reported SIA routes. generally two to three minutes.2. Find the routes that are consistently being reported as SIA.1.2 (Infinity/Infinity). Pay particular attention to routes that have outstanding replies and have been active for some time.2 (Infinity/Infinity). If you are logging console messages.2. Redistribution This section examines different scenarios involving redistribution. You can look for problems on the link to this neighbor. such as below−optimal routing. Redistribution can potentially cause problems. Q − Query. Please note that the examples below show the minimum required to configure redistribution. query−origin: Local origin via 10. memory or CPU utilization. please see "Avoiding . Find the reason that router is not receiving or answering queries. r. they may appear among the other RDBs.0/24.time the router waits after sending a query before declaring the route SIA. r − Reply status A 10. 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.3. R − Reply. however. active 00:00:01. Q 1 replies. The better solution. is to redesign the network to reduce the range of queries (so very few queries pass over the satellite link). Note that these neighbors may not show up in the Remaining replies section. More often.1. Find the router that is consistently failing to answer queries for these routes. Examine this neighbor to see if it is consistently waiting for replies from any of its neighbors.1. 2. If you run into a situation where it seems that the query range is the problem. active 00:00:01. or slow convergence. A − Active.

Router Two is redistributing this route into autonomous system 2000 and advertising it to Router One.0. For more information on redistribution among routing protocols.1.16.0.1.2. Redistribution Between Two EIGRP Autonomous Systems In Figure 8. the routers are configured as follows: Router One router eigrp 2000 !−−− The "2000" is the autonomous system network 172.0.Problems Due to Redistribution" in Redistributing Routing Protocols.1.1. When routes from EIGRP 2000 are redistributed back to EIGRP 1000.0.0.0 0.255.16.0. the routes with 1000 tags are denied to ensure a loop−free topology.255 Router Two router eigrp 2000 redistribute eigrp 1000 route−map to−eigrp2000 network 172.0 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 10.255 Router Three is advertising the network 10.1. Note: The routes from EIGRP 1000 are tagged 1000 before redistributing them to EIGRP 2000.0.255.0 0.0 0.255 ! router eigrp 1000 redistribute eigrp 2000 route−map to−eigrp1000 network 10. please see .0/24 to Router Two through autonomous system 1000.0.

from 20. Redistribution Between EIGRP and IGRP in Two Different Autonomous Systems In Figure 9. the minimum bandwidth shown in this topology table entry is 56k.2. we see: one# show ip eigrp topology 10.2.1. 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 10.0 ! router igrp 1000 redistribute eigrp 2000 route−map to−igrp1000 network 10. Send flag is 0x0 Composite metric is (46763776/46251776).2. 1 Successor(s). FD is 46763776 Routing Descriptor Blocks: 20.0/24 State is Passive. we have changed the configurations as follows: Router One router eigrp 2000 network 172.1.1 (Serial0).16.1.544Mb.Redistributing Routing Protocols.255.0. Query origin flag is 1. This means that EIGRP preserves all metrics when redistributing between two EIGRP autonomous systems.0.1.1. On Router One.0 IP−EIGRP topology entry for 10.1.16.1.0 ! . external metric is 46251776 Administrator tag is 1000 (0x000003E8) Notice that although the link between Routers One and Two has a bandwidth of 1.1.0 Router Two router eigrp 2000 redistribute igrp 1000 route−map to−eigrp2000 network 172.255.1.1 AS number of route is 1000 External protocol is EIGRP.0 255.1.

but is learning about this directly−connected interface through redistribution from IGRP.1. the topology table entry for 10.1 AS number of route is 1000 External protocol is IGRP.1. For example.255.1.0/24 State is Passive. EIGRP is not routing for this network. There is one caveat to redistribution between IGRP and EIGRP that should be noted. from 20. FD is 46763776 Routing Descriptor Blocks: 20. 1 Successor(s).0 The configuration for Router One is shown below: one# show ip eigrp topology 10.1.0/24 State is Passive.1.1.1.1. from 20. Send flag is 0x0 Composite metric is (2169856/1). On Router One. external metric is 180671 Administrator tag is 1000 (0x000003E8) IGRP metrics are preserved when routes are redistributed into EIGRP with a different autonomous system.0 255.1.1.1.0/24 is directly connected to Router Two.0. FD is 2169856 Routing Descriptor Blocks: 20. 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 10.1.0 255.0.2.0 IP−EIGRP topology entry for 10. the network 10. 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. but they are scaled by multiplying the IGRP metric by the constant 256. Query origin flag is 1.1 (Serial0).2.0/24 shows: one# show ip eigrp topology 10.1.1.1.1.255.255.1. 1 Successor(s).1 (Serial0). Send flag is 0x0 Composite metric is (46763776/46251776).1.1. Query origin flag is 1. and IGRP is routing for this network (there is a network statement under router IGRP that covers this interface).1. If the network is directly connected to the router doing the redistribution.0 IP−EIGRP topology entry for 10.1. it advertises the route with a metric of 1.255.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.

Send flag is 0x0 Composite metric is (46763776/46251776).1.1.1.1. FD is 46763776 Routing Descriptor Blocks: 20.1.1 (Serial0).2.1. 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 1 External data: . is 1.1. from 20.0.1. which is bolded.16.1 AS number of route is 1000 External protocol is IGRP.0 ! router igrp 2000 network 10.0 And Router One is configured as follows: one# show ip eigrp topology 10.0/24 State is Passive.255.0.External data: Originating router is 10." 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. 1 Successor(s).1.1.1. external metric is 0 Administrator tag is 1000 (0x000003E8) Note that the reported distance from Router Two.0 255.0.0 Router Three router igrp 2000 network 10.0 IP−EIGRP topology entry for 10.2. Query origin flag is 1.0 Router Two router eigrp 2000 network 172.255.16.0.

Originating router is 10.1.1.4.1 AS number of route is 2000 External protocol is IGRP. is redistributed from IGRP to EIGRP with a metric of 1 − the same metric we see when redistributing between two different autonomous systems.1. 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. which is directly connected to Router One. 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.1.1. The directly attached 10.1.1. 1 Successor(s). Router Two runs both EIGRP and IGRP in autonomous system .0/24 network is handled the same way in both scenarios: one# show ip eigrp topology 10. external metric is 0 Administrator tag is 0 (0x00000000) So this network. FD is 2169856 Routing Descriptor Blocks: 20.1.0/24 as an external in EIGRP autonomous system 100.1 AS number of route is 2000 External protocol is IGRP. from 20.255.1.1. Router Four advertises 10.1.0 IP−EIGRP topology entry for 10.1. Send flag is 0x0 Composite metric is (2169856/1).1.1.1.1. • External EIGRP route metrics are compared to scaled IGRP metrics (the administrative distance is ignored).0 255. 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.0/24 in IGRP autonomous system 100.0/24 State is Passive.255. Let us examine these caveats in Figure 11: Router One advertises 10.1 (Serial0).1.4.

is through Router One. distance 100. Router Two prefers the EIGRP external route with the same metric (after scaling) and a higher administrative distance. . When we add the EIGRP route. an IGRP metric.1. metric 3072256. metric 12001 Redistributing via igrp 100.0/24 Known via "eigrp 100". It is always best to use the default metric when redistributing between protocols.1. 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.1. Hops 0 Note the administrative distance is 100.2. minimum bandwidth is 1000 Kbit Reliability 1/255. This is true whenever automatic redistribution occurs between EIGRP and IGRP within the same autonomous system.2 on Serial1.4.1.1. Redistribution To and From Other Protocols Redistribution between EIGRP and other protocols − RIP and OSPF. eigrp 100 Advertised by igrp 100 (self originated) eigrp 100 Last update from 10.4.0/24 Known via "igrp 100". minimum MTU 1 bytes Loading 1/255. an EIGRP metric. traffic share count is 1 Total delay is 20010 microseconds.100.2 on Serial0.1.2. eigrp 100 Last update from 10.0 Routing entry for 10. • External EIGRP routes have an administrative distance of 170. for instance). 00:53:59 ago.1.2.1. 00:53:59 ago Routing Descriptor Blocks: * 10. distance 170.2.2. and 3072256. for example − works in the same way as all redistribution.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. Router Two shows: two# show ip route 10. is through Router Four. traffic share count is 1 Total delay is 20010 microseconds. from 10.2.1.0 Routing entry for 10.4. If we ignore the EIGRP route advertised by Router Four (by shutting down the link between Routers Two and Four.1. 00:00:42 ago Routing Descriptor Blocks: * 10. type external Redistributing via igrp 100. via Serial1 Route metric is 12001.2. from 10.4.1. 00:00:42 ago. minimum MTU 1 bytes Loading 1/255.1. via Serial0 Route metric is 3072256. minimum bandwidth is 1000 Kbit Reliability 1/255. Router Two shows: two# show ip route 10. The router always prefers the path with the lowest cost metric and ignores the administrative distance.

1 subnets D 172.0/24 is directly connected.0.2. Router One has a static route to the network 172.0. For example..0/8 network to Router One. On Router Two. 10. in Figure 13.1. 2 masks C 10. because the interface Router Two uses to reach Router One is in a different major network.0 Serial0 And Router One also has a network statement for the destination of this static route: router eigrp 2000 network 10.0. because EIGRP considers this a directly attached network..255.0/8 is variably subnetted.16.1.1.1. EIGRP redistributes this route as if it were a directly connected interface. 00:00:47.1. this looks as follows: two# show ip route .16.16.1. Let us look at the network in Figure 12.0 network 172.0/24 appears as an internal EIGRP route on Router Two.0. which includes the static route.Redistribution of Static Routes to Interfaces When you install a static route to an interface. and configure a network statement using router eigrp. Serial0 172. 00:00:47.1. Serial0 Note the route to 172.0 [90/2169856] via 10. Serial0 D 10.16.0.0..0. Router Two advertises only the 10.0/24 [90/2169856] via 10.1.1. 2 subnets.1.1. even though it is not redistributing static routes.16.0/24 is subnetted.0. . Auto−Summarization EIGRP performs an auto−summarization each time it crosses a border between two different major networks. Summarization There are two forms of summarization in EIGRP: auto−summaries and manual summaries.0 no auto−summary Router One redistributes this route.0/24 configured through interface Serial 0: ip route 172.255.1.1.16.0 255.

0.2.0.0.1. On the router doing the summarization.0.0/8.0/8 is a summary.0. Query origin flag is 1. Serial1 10.0.0 Routing entry for 10. from 172. 4 known subnets Attached (2 connections) Variably subnetted with 2 masks Redistributing via eigrp 2000 C D D C 10.1.16.0.0.0.1.2.0 IP−EIGRP topology entry for 10.0.0 (Null0). The topology table entry for this summary route looks like the following: two# show ip eigrp topology 10.0/24 [90/10537472] via 10. The metric is the best metric from among the summarized routes.0/24 is directly connected.0.0.0.1.0 here means this route is originated by this router) Composite metric is (10511872/0).0. Send flag is 0x0 Composite metric is (11023872/10511872). Serial1 The route to 10. Send flag is 0x0 (note: the 0. a route is built to null0 for the summarized address: two# show ip route 10.On Router One.1 (Serial0).2.16.0/8 State is Passive.0/24 is directly connected.0. it looks like an internal route.0. Route is Internal Vector metric: Minimum bandwidth is 256 Kbit Total delay is 20000 microseconds Reliability is 255/255 Load is 1/255 .0.0.1. 1 Successor(s).0.0.0.0.0 network that have a bandwidth of 56k.0/8 State is Passive.1. Serial2 10. Query origin flag is 1. Note that the minimum bandwidth on this route is 256k. this looks like the following: one# show ip eigrp topology 10. FD is 10511872 Routing Descriptor Blocks: 0.0. Null0 10. 00:23:24. 1 Successor(s). 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.0. from 0.0 IP−EIGRP topology entry for 10. 00:23:20.3.0/8 is marked as a summary through Null0.1.1.0. although there are links in the 10. FD is 11023872 Routing Descriptor Blocks: 172.

For example. r − Reply status P 10.1.3.0 network instead of a summary. 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 (11049472/10537472). 1 successors.1.0/24. 1 successors. FD is 46354176 via 20.1.0/24 into the CIDR block 192.1. configure no auto−summary on the EIGRP process on Router Two: On Router Two router eigrp 2000 network 172.0/24.0.0/24.1.0/24.0.0. 192.. Router Two is summarizing the 192. Serial0 P 172.1.2.0/22.1 (11023872/10511872). FD is 2169856 via Connected.0 network: one# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive. and 192.1.16.0. 1 successors. Serial0 P 10.Minimum MTU is 1500 Hop count is 0 To make Router Two advertise the components of the 10..0/24. A − Active. Router One now sees all of the components of the 10.1 (46354176/45842176).1.0.1.3.1.1. R − Reply. The configuration on Router Two is shown below: two# show run . in Figure 14.1.16.1.1. Manual Summarization EIGRP allows you to summarize internal and external routes on virtually any bit boundary using manual summarization.0 network 10.0.1. FD is 11049472 via 20. Serial0 P 10. FD is 11023872 via 20. Q − Query.2.0/24. 1 successors.0. U − Update.1.0 no auto−summary With auto−summary turned off. ..0.

1.50.255.1. 1 successors.255. as shown in the configurations below.1. U − Update.1 (10537472/281600).0/24.0/26 and 192.50. R − Reply. .1 (10639872/128256).0/24.0.2. Q − Query. and the summary route via Null0.1 (46354176/45842176).10. FD is 10511872 via Summary (10511872/0).1. R − Reply. Serial1 P 192. Router Three is injecting external routes to 192. 1 successors. FD is 10537472 via 192.1. 1 successors. we see this as an internal route: one# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive.1.0.1.. 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.0 255. FD is 46354176 via 10. 1 successors. FD is 10511872 via Connected.50.1.3.0 no ip mroute−cache ! .1 (11023872/10511872).0/24. Q − Query.1. FD is 45842176 via Connected. Loopback0 P 10. Serial0 P 192. r − Reply status P 10.1.1.0/24.50.1. 1 successors.1.1.252.0/22. let us look at Figure 15. Null0 P 192.1 255.1.0 ip summary−address eigrp 2000 192. two# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive. A − Active. Serial0 P 192.! interface Serial0 ip address 10. 1 successors. FD is 10639872 via 192.10..64/26 into EIGRP using the redistribute connected command.0/22. U − Update. FD is 11023872 via 10..1.1. A − Active. r − Reply status P 10.0/24.2.255.0/24. FD is 2169856 via Connected. 1 successors.1. Serial0 P 10. Serial1 Note the ip summary−address eigrp command under interface Serial0. 1 successors. FD is 2169856 via Connected.0/24.1. Serial1 P 192. On Router One.50. 1 successors.1.0. To illustrate.2.

255.0/26 is subnetted.2.0/24 auto−summary is then generated on Router Two.1.50..2.1.0 is directly connected.255.1. Serial0 D 192. 00:06:48..0 default−metric 10000 1 255 1 1500 With this configuration on Router Three.1. Serial0 192.1.. Serial0 D EX 192.64 [170/11049472] via 10.50. Router Three interface Ethernet0 ip address 192.1. Null0 .2.192 ! interface Ethernet1 ip address 192.1. Serial0 C 10.1.255. 10.255.1.0/8 is subnetted.1.0/24: two# show ip route .0/24 is a summary...1.0/8 is subnetted..2.0.1.0.1.255. 00:00:53.1.1 255.1..0 [170/11049472] via 10.2.50.64/26 routes into one major net destination (192.1.1.1.2.2. it does not do this because both routes are external.2.0/24 [90/11023872] via 10.255. 2 subnets D 10.0.2.1.2. and add network statements for this network on Routers Two and Three. Serial0 Although auto−summary normally causes Router Three to summarize the 192. 1 subnets D EX 192.2.2.1..0/24).1. And Router One shows only the summary route: one# show ip route .0.2..1 255.192 ! router eigrp 2000 network 192.50.0 !router eigrp 2000 redistribute connected network 10.1.1.2.2.2.192 ! interface Serial0 ip address 192.192 ! interface Ethernet2 ip address 10.255. the 192. if you reconfigure the link between Routers Two and Three to 192.128/26.1.50.0 is directly connected.255. 00:02:03.255.255.0/26 and 192.130 255. 1 subnets C 10.2. 10. 00:00:53..65 255.255. Serial0 .0.2. However. the routing table on Router One shows: one# show ip route ..0 [90/11023872] via 10.2..0.192 ! interface Ethernet1 ip address 192.0 Now Router Two generates the summary for 192.255.1.1.2.65 255. 00:00:36.2.Router Three interface Ethernet0 ip address 192.2. D 192.2.1 255.

reply with an unreachable if there is a good successor. To see how these rules affect the way queries are handled. if successful. reply with new information. if not successful. 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. . 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).Query Processing and Range When a router processes a query from a neighbor. 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. which is running under normal conditions. if successful. if not successful. reply with the current path information successor active attempt to find new successor. let us look at the network in Figure 16. reply with new information.

so it marks 192.0/24 fails.3.168. attempts to find a new feasible successor to this network.168.3.3. What activity can we expect to see on this network? Figures 16a through 16h illustrate the process.0/24 as unreachable.168.168.0/24 (far right side): • Router One has two paths to 192.168.168.3.3.3. upon receiving a query from its successor.0/24 as unreachable and query Routers Two and Three: .0/24 through Router Four Suppose that 192. Router Five marks 192.We can expect the following to happen regarding network 192.0/24: ♦ 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. It does not find one. and queries Router Four: Router Four.

they both send queries to Router One: For simplicity. and marks the route as unreachable. .168. see that they have lost their only feasible route to 192. Router One then receives the query from Router Two.3.Routers Two and Three. in turn.0/24. and mark it as unreachable. they will all have the same final result. let us assume that Router One receives the query from Router Three first. Although another order is possible.

0/24: . Router One is now passive for 192. Routers Two and Three are now passive for 192.0/24: Routers Two and Three reply to the query from Router Four.Router One replies to both queries with unreachables.168.3.168.3.

all routers in the network process a query for network 192. Router Five is now passive for network 192. if the queries were to reach the routers in a different order.0.3. • Router Three has a topology table entry for the 10. It is important to understand that although there may be other query paths or processing orders.1.0/8 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.1. How Summarization Points Affect the Query Range Now let us look at the paths to 10.1.0/24. removes network 192. In fact.0/24 network with a cost of 46251885 through Router One. • Router Four has a topology table entry for the 10.0/24 when that link goes down.0/24 in the same network: • Router Two has a topology table entry for the 10. so the path through Router Two is not a feasible successor).1. Router Five sends updates back to Router Four so the route is removed from the topology and routing tables of the remaining routers. Some routers may end up processing more than one query (Router One in this example).3.0/24 from its routing table. This is a good example of an unbounded query in an EIGRP network.168.1. some would end up processing three or four queries.3.0.0/24 network with a cost of 20281600 through Router One.Router Five. upon receiving the reply from Router Four.168. .1.168.

marks the route as unreachable (because the query is from its successor) and then queries Routers Four and Three: . Router One marks it as unreachable. and then queries each of its neighbors (Routers Two and Three) for a new path to that network: Router Two.1.If 10.1.0/24 goes down. on receiving the query from Router One.

when it receives the query from Router One.0/24 is unreachable (note that Router Four has no knowledge of the subnet in question. since it only has the 10. when it receives the queries from Routers Two and Three.0.0/8 route): . marks the destination as unreachable and queries Routers Two and Four: Router Four.Router Three.0. replies that 10.1.1.

Queries can also be bound by manual summarization. it replies to the query within the normal processing rules and launches a new query into the other autonomous system. How Autonomous System Boundaries Affect the Query Range If a router is redistributing routes between two EIGRP autonomous systems. in this case. is bounded by the autosummarization at Routers Two and Three.1.0/24 is unreachable: The query.1.1.0/24 is unreachable: Since Routers Two and Three now have no outstanding queries. Router Three marks the route unreachable and queries Router Two for a new path: .Routers Two and Three reply to each other that 10. and is not involved in the re−convergence of the network. and distribution lists. For example.1. autonomous system borders. if the link to the network attached to Router Three goes down. they both reply to Router One that 10. Router Five does not participate in the query process.

In fact. Router Three is now passive for this network: Router One replies to Router Two. .Router Two replies that this network is unreachable and launches a query into autonomous system 200 toward Router One. but it does not solve the overall problem that each router must process the query. 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. How Distribution Lists Affect the Query Range Rather than block the propagation of a query. 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. distribution lists in EIGRP mark any query reply as unreachable. Let us use Figure 19 as an example. the original query leaks into the second autonomous system in the form of a new query. Once Router Three receives the reply to its original query. and the route goes passive: While the original query did not propagate throughout the network (it was bound by the autonomous system border).

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. When Router One loses its connection to Network A. • 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). Router Three does not advertise a path to Network A because of the distribution list on its serial ports. then queries Router Two: . and does not use Router Two as a feasible successor. Router Three marks the route as unreachable. • Router Three uses Router One as its preferred path to Network A.

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

There is a field in show ip eigrp interface that displays the pacing timer. the largest packet that can be sent over the interface. and is typically expressed in milliseconds. thereby using only a portion of the available bandwidth. each time EIGRP queues a packet to be transmitted on an interface. EIGRP avoids this congestion by pacing the speed at which packets are transmitted on a network. The pacing timer determines when the packet is sent. 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). EIGRP waits: • (8 * 100 * 512 bytes) / (56000 bits per second * 50% bandwidth) (8 * 100 * 512) / (56000 * 50) 409600 / 2800000 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. 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. and the packet is 512 bytes.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. .1463 seconds. 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. The pacing time for the packet in the above example is 0.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).

0/0 route). Similarly.0.0/0 overrides a default route learned from any other routing protocol. and the metrics for these paths are: • path 1: 1100 • path 2: 1100 • path 3: 2000 • path 4: 4000 The router. you can also configure an administrative distance on the end of the ip summary−address eigrp command.0. can also load−balance over unequal cost links.0/0. EIGRP. you must use the ip default−network command to mark the network as a default network. you can configure EIGRP to use up to six routes of equal cost. because 1100 x 2 = 2200.0 x. issue variance 4 under the router eigrp command.0.0.0. which is greater than the metric through path 3.0.0. The type of load balancing (per packet or per destination) depends on the type of switching being done in the router. Note that a summary to 0. Summarizing to a default route is effective only when you want to provide remote sites with a default route.x. To load balance over paths 1. Use the first method when you want to draw all traffic to unknown destinations to a default route at the core of the network.0 Load Balancing EIGRP puts up to four routes of equal cost in the routing table. Using EIGRP.0/0.0.1.0. (Beginning in Cisco IOS Software 12.0. router eigrp 100 network 10.1 frame−relay interface−dlci 10 ip summary−address eigrp 100 0.0.1.0(4)T. which the router then load−balances.1 point−to−point ip address 10.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. 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.Default Routing There are two ways to inject a default route into EIGRP: redistribute a static route or summarize to 0. 2. Refer to Configuring a Gateway of Last Resort for further information. Let us say there are four paths to a given destination.0. This method is effective for advertising connections to the Internet. use variance 2. so the local summary does not override the 0. however. and 3. For example: ip route 0.0.0.0.0 0. places traffic on both path 1 and 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. Since summaries are configured per interface.x. Note: Using max−paths.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 ! interface serial 0 encapsulation frame−relay no ip address ! interface serial 0.0 0. you can use the variance command to instruct the router to also place traffic on paths 3 and 4. If you use another network. by default.0.0.0. Refer to How Does Unequal Cost Path . to also add path 4.0.

Reported Distance. via Serial1 Route metric is 12001. the next two packets over path 3. you can block redistribution from EIGRP into the external protocol.2 on Serial1.4. from 10. Because EIGRP uses the interface bandwidth to determine the rate at which to send packets. Using the Metrics When you initially configure EIGRP. the next three packets over path 2. it is important that these be set correctly. always use delay to do so. do not change the . the bandwidth has more influence over the total metric.1. Using Administrative Tags in Redistribution External administrative tags are useful for breaking redistribution routing loops between EIGRP and other protocols. In order to raise the metric. eigrp 100 Advertised by igrp 100 (self originated) eigrp 100 Last update from 10. and the next packet over path 4. use a route−map with prefix−list. By tagging the route when it is redistributed into EIGRP.1. Note: Even with variance configured. If it is necessary to influence the path EIGRP chooses. traffic share count is 1 Total delay is 20010 microseconds. and uses this number as the traffic share count. and Feasible Successors section for more information.2.2. minimum MTU 1 bytes Loading 1/255.4. and so on. router# show ip route 10. minimum bandwidth is 1000 Kbit Reliability 1/255. multipoint serial links and other mismatched media speed situations are the exceptions to this rule. Hops 0 For this example. 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. How does the router divide the traffic between these paths? It divides the metric through each path into the largest metric.1. • The delay should always be used to influence EIGRP routing decisions. metric 12001 Redistributing via igrp 100. at higher bandwidths.0 Routing entry for 10. distance 100.Load Balancing (Variance) Work in IGRP and EIGRP? for more information. the modification of the administrative distance only applies for internal routes. 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 1.1. The router then restarts by sending the next three packets over path 1.2.2.2. 00:00:42 ago. EIGRP will not send traffic over an unequal cost path if the reported distance is greater than the feasible distance for that particular route.1. It is not possible to modify the administrative distance for a default gateway that was learned from an external route because. the delay has more influence over the total metric. rounds down to the nearest integer. in EIGRP.0/24 Known via "igrp 100". At lower bandwidths. 00:00:42 ago Routing Descriptor Blocks: * 10. Refer to the Feasible Distance.

.1 255.0 loopback no keepalive ! interface Serial0 ip address 172.255. shows: three# show run .1.17.0.1.0 0.17.. 1 successors.0. A − Active.17...administrative distance.255. 1 successors.0.0 ! interface Ethernet0 ip address 172..1.. but this example does not show the entire configuration used for breaking redistribution loops.0/24..16.1. three# show ip eigrp topo IP−EIGRP Topology Table for process 444 Codes: P − Passive.19.0/24.19. r − Reply status P 172. which is redistributing routes connected into EIGRP. interface Loopback0 ip address 172. access−list 10 permit 172.1. tag is 1 via Redistributed (128256/0) ..0 default−metric 10000 1 255 1 1500 ...255 route−map foo permit 10 match ip address 10 set tag 1 . router eigrp 444 redistribute connected route−map foo network 172.1.1 255.1 255. R − Reply..19. A basic example of configuring these tags follows. Serial0 via Redistributed (2169856/0) P 172. Q − Query.255. Router Three.16.255. FD is 281600 via Redistributed (281600/0) P 172. FD is 2169856 via Connected. FD is 128256.255.255. 1 successors.0 . U − Update.255..0/24.

255.1.255.. A − Active..16. which is receiving the RIP routes redistributed by Router 2.0.17. Serial0 P 172.0/24.1. two# show ip eigrp topo IP−EIGRP Topology Table for process 444 Codes: P − Passive.255.0 ! router rip redistribute eigrp 444 route−map foo network 10.1 255. Serial0 P 172..1..18.1..2 255.255 Serial0 route−map foo deny 10 match tag 1 ! route−map foo permit 20 .1...19. tag is 1 via 172. interface Serial0 ip address 172.3 255.1.0.0/24.0 ! interface Serial1 ip address 172. U − Update.18.0.0 network 172. Serial0 Note the tag 1 on 172..1. R − Reply. FD is 2169856 via Connected.0 . 1 successors. FD is 2195456 via 172.0 .17.2 255.0 default−metric 1 ! no ip classless ip route 1.255. r − Reply status P 172.19.255. Q − Query..1.255. FD is 2297856.17.. which is redistributing routes from EIGRP into RIP.255.1.1 (2195456/281600). Router One..1 (2297856/128256).Router Two. shows: one# show run .. 1 successors.0 no fair−queue clockrate 1000000 router rip network 172. shows: two# show run .1.0.0/24..0.17.1. one# show ip route .17.18. 1 successors.18.255. interface Serial0 ip address 172...0/24. router eigrp 444 network 172.

• Acks sent/received stands for the number of acknowledgment packets sent and received (sent−66/received−41). The output of this command shows the information that has been exchanged between the neighboring EIGRP router. • Input Queue shows the EIGRP Hello Process to EIGRP PDM socket queue counters (current−0/max−2000/highest−1/drops−0).0/24 is missing.18. 00:00:15.16.18.17.1. • Replies sent/received shows the number of reply packets sent and received (sent−18/received−16).3.Gateway of last resort is not set R R C 172. show ip eigrp traffic Configuration Explanations • Hellos sent/received shows the number of hello packets sent and received (sent −1927/received − 1930). • Updates sent/received displays the number of update packets sent and received (sent−20/received−39).0/16 172. • SIA−Queries sent/received means number of stuck in active query packets sent and received (sent−0/received−0). • Queries sent/received means the number of query packets sent and received (sent−10/received−18). • Socket Queue displays the IP to EIGRP Hello Process socket queue counters (current−0/max−2000/highest−1/drops−0).19.18.18. • PDM Process ID stands for protocol−dependant module IOS process identifier (251).1. 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. • SIA−Replies sent/received displays the number of stuck in active reply packets sent and received (sent−0/received−0). . 1 subnets is directly connected. • Hello Process ID is the hello process identifier (270). 00:00:15.1. An explanation of each output field follows the table.3. Serial0 [120/1] via 172.0.1. Serial0 Note that 172.0.0 [120/1] via 172.0/24 172.0/16 172. Serial0 is subnetted.

• FD is 512640000 shows the feasible distance. which is the best metric to reach this destination or the best metric known when the route went active. use the show ip eigrp topology all−links command. This field can also be: Multiple origins. meaning passive. • active 00:00:01 shows how long this route has been active. This field can also be: U. meaning that multiple neighbors have sent queries on this destination. • tag is 0x0 can be set and/or filtered using route maps with the set tag and match tag commands. meaning there is a reply pending. if successors is capitalized. meaning the successor originated the query.1.2. • Serial1 is the interface through which this neighbor is reachable. the route is in transition. To display all entries in the topology table. for update pending. if the network is directly connected to this router. • Via 10. This field can also be: U.2 shows that we learned of this route from a neighbor whose IP address is 10. meaning there is a query pending.1. or R. This field can also be: Connected. • Q means a query is pending. • Serial0 is the interface through which this neighbor is reachable. An explanation of each output field follows the table. Redistributed. • 0 successors shows how many successors (or paths) are available for this destination.0/24 is the destination or mask.1. • via 10. . and the reported distance through this neighbor in the second field.1. • Via 10.2 shows the neighbor from which we are waiting for a reply.2. show ip eigrp topology Configuration Explanations • A means active.2. • 10. or R. but not the successor. • r shows that we have queried this neighbor and are waiting for a reply. • Q is the send flag for this route. meaning there is an update pending. This could also show a P. • 1 replies shows the number of outstanding replies. if this route is being redistributed into EIGRP on this router.1. for reply pending.show ip eigrp topology This command only displays feasible successors. or Summary. • r shows that we have queried this neighbor about the route and have not yet received a reply.4.2.2. Serial1 shows we are using this route (indicates which path the next path/destination will take when there are multiple routes of equal cost). • query origin: Local origin shows this route originated the query.2 (512640000/128256). or Successor origin. if this is a summary route generated on this router. • (Infinity/Infinity) shows the metric to reach this path through this neighbor in the first field.

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

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

Updated: Sep 09. but it also shows all links in the topology table. rather than just feasible successors. Terms & Conditions | Privacy Statement | Cookie Policy | Trademarks of Cisco Systems. 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.show ip eigrp topology all−links Same output format as show ip eigrp topology . Inc. Inc. 2005 Document ID: 16406 . All rights reserved.