You are on page 1of 44

Enhanced Interior Gateway Routing Protocol

Document ID: 16406
Interactive: This document offers customized analysis of your Cisco
device.

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

Router One chooses the path with the lowest metric. you can simplify the formula as follows: metric = bandwidth + delay Cisco routers do not perform floating point math. which can cause your network to fail to converge. you need to round down to the nearest integer to properly calculate the metrics. and the other through Router Three. 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. on the route to the destination network. Note: If K5 = 0. 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. the total cost through Router Four is: In this example. EIGRP calculates the total metric by scaling the bandwidth and delay metrics. so you must divide by 10 before you use it in this formula. in tens of microseconds. 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: . In this example. with a minimum bandwidth of 128 and a delay of 1200. The delay as shown in the show ip eigrp topology or show interface commands is in microseconds. The default values for K are: • K1 = 1 • K2 = 0 • K3 = 1 • K4 = 0 • K5 = 0 For default behavior. the formula reduces to Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 − load) + k3 * delay]. Mismatched K values prevent a neighbor relationship from being built. Throughout this paper. 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.56 and a total delay of 2200. Let us compute the metrics. so at each stage in the calculation.

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

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

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

if it shows any path to Network A through Router One. EIGRP combines these two rules to help prevent routing loops. they exchange topology tables during startup mode. it advertises Network A as unreachable through its link to Routers Two and Three. When Router One learns about Network A from Router Two. For each table entry a router receives during startup mode.Poison reverse is another way of avoiding routing loops. 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. Topology Table Change In Figure 5. 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. removes that path because of the unreachable advertisement. Let us say the routers in Figure 4a have poison reverse enabled. . and the 128k link between Routers Three and Four (see the Load Balancing section for a discussion of variance). Router Three. 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.

For instance. However. Router Two turns off split horizon for this route. effectively restarting the neighbor session. it sends a query to each of its neighbors. in fact. Stuck In Active Routes In some circumstances. Router Three receives a query concerning 10. The first is to increase the amount of .Router Two sees the path through Router Three as a feasible successor. In this case. that the router that issued the query gives up and clears its connection to the router that is not answering. it takes a very long time for a query to be answered. and Four. Let us take a look at the network in Figure 6. If Three does not have a successor for this destination because a link flap or other temporary network condition. After some investigation. and advertises Network A as unreachable. This is known as a stuck in active (SIA) route.1. because Router One is its successor to this network. Router One is recording a large number of SIA routes from Router Two.0/24 (which it reaches through Router One) from Router Four. When a router changes its topology table in such a way that the interface through which the router reaches a network changes.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. Since the split horizon rule states that you should never advertise a route out the interface through which you learned about it. If the link between Routers Two and Four goes down. however. in this case. Instead. Routers One. it only sends queries to Routers Two and Four. the problem is narrowed down to the delay over the satellite link between Routers Two and Three. Two. this leaves Router One with an invalid topology table entry. it turns off split horizon and poison reverses the old route out all interfaces. Router Two simply re−converges on the path through Router Three. If. Router Three receives a query or update (such as a metric change) from Router One for the destination 10. in Figure 7. Router Two would not normally send an update. There are two possible solutions to this type of problem. Router One hears this advertisement and flushes its route to Network A through Router Two from its routing table. it does not send a query back to Router One.2. So long. 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.2.0/24.

they may appear among the other RDBs. Find the reason that router is not receiving or answering queries. generally two to three minutes. Examine this neighbor to see if it is consistently waiting for replies from any of its neighbors. A − Active. active 00:00:01. This setting can be changed using the timers active−time command. The second step is more difficult. The better solution. To avoid these problems.2 (Infinity/Infinity). such as below−optimal routing. 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. Serial1 1 replies.2 (Infinity/Infinity). Q − Query. The first step should be fairly easy. however. If you are logging console messages. a quick perusal of the log indicates which routes are most frequently marked SIA.1.1. 0 successors. or other problems with this neighbor.2. Redistribution This section examines different scenarios involving redistribution. r. r − Reply status A 10.4. Find the router that is consistently failing to answer queries for these routes. Repeat this process until you find the router that is consistently not answering queries. r. routing loops. If you run into a situation where it seems that the query range is the problem. More often.0/24. please see "Avoiding . U − Update. Serial3 Remaining replies: via 10. Redistribution can potentially cause problems. it is always best to reduce the query range rather than increasing the SIA timer. Pay particular attention to routes that have outstanding replies and have been active for some time. 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).time the router waits after sending a query before declaring the route SIA. The command to gather this information is show ip eigrp topology active: Codes: P − Passive. query−origin: Local origin via 10. or slow convergence.1. R − Reply. You can look for problems on the link to this neighbor.3. 2. memory or CPU utilization. Query range is covered in the Query Range section.1. is not a common reason for reported SIA routes. 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. Serial0 Any neighbors that show an R have yet to reply (the active timer shows how long the route has been active). 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. is to redesign the network to reduce the range of queries (so very few queries pass over the satellite link). Please note that the examples below show the minimum required to configure redistribution. however. Q 1 replies.2.2. Note that these neighbors may not show up in the Remaining replies section. FD is 512640000. Query range in itself. Find the routes that are consistently being reported as SIA. active 00:00:01. query−origin: Local origin via 10. 3.

When routes from EIGRP 2000 are redistributed back to EIGRP 1000.0.1. the routers are configured as follows: Router One router eigrp 2000 !−−− The "2000" is the autonomous system network 172.0 0.0.0 0.255 Router Two router eigrp 2000 redistribute eigrp 1000 route−map to−eigrp2000 network 172.16.0.2.1. the routes with 1000 tags are denied to ensure a loop−free topology.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. Note: The routes from EIGRP 1000 are tagged 1000 before redistributing them to EIGRP 2000.0 0.255 ! router eigrp 1000 redistribute eigrp 2000 route−map to−eigrp1000 network 10. Redistribution Between Two EIGRP Autonomous Systems In Figure 8.1.0.255 Router Three is advertising the network 10.0/24 to Router Two through autonomous system 1000.0.0.1.1.Problems Due to Redistribution" in Redistributing Routing Protocols. For more information on redistribution among routing protocols. please see .0 0. Router Two is redistributing this route into autonomous system 2000 and advertising it to Router One.255.255.0.16.0.

Redistribution Between EIGRP and IGRP in Two Different Autonomous Systems In Figure 9. On Router One.1.Redistributing Routing Protocols. we see: one# show ip eigrp topology 10.0 IP−EIGRP topology entry for 10.2.0 255. from 20. 1 Successor(s). FD is 46763776 Routing Descriptor Blocks: 20. external metric is 46251776 Administrator tag is 1000 (0x000003E8) Notice that although the link between Routers One and Two has a bandwidth of 1.544Mb.1. Send flag is 0x0 Composite metric is (46763776/46251776).2.1 AS number of route is 1000 External protocol is EIGRP. This means that EIGRP preserves all metrics when redistributing between two EIGRP autonomous systems. the minimum bandwidth shown in this topology table entry is 56k.0.16.0/24 State is Passive.1.0 ! router igrp 1000 redistribute eigrp 2000 route−map to−igrp1000 network 10.2.1 (Serial0).0 ! .1. Query origin flag is 1.0 Router Two router eigrp 2000 redistribute igrp 1000 route−map to−eigrp2000 network 172.1.255.1.1.0.1. we have changed the configurations as follows: Router One router eigrp 2000 network 172.16.1. 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 10.255.1.

2. the topology table entry for 10.255. 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 10.0/24 is directly connected to Router Two. On Router One. If the network is directly connected to the router doing the redistribution.1.1.0 IP−EIGRP topology entry for 10.1.1. EIGRP is not routing for this network.1 (Serial0). Send flag is 0x0 Composite metric is (46763776/46251776).1.1.255.0/24 State is Passive.0/24 State is Passive.1 (Serial0).0 The configuration for Router One is shown below: one# show ip eigrp topology 10.2.1. 1 Successor(s).1.1 AS number of route is 1000 External protocol is IGRP.1. There is one caveat to redistribution between IGRP and EIGRP that should be noted. but they are scaled by multiplying the IGRP metric by the constant 256.1. from 20.1. FD is 2169856 Routing Descriptor Blocks: 20. from 20. and IGRP is routing for this network (there is a network statement under router IGRP that covers this interface).1.1. Send flag is 0x0 Composite metric is (2169856/1). but is learning about this directly−connected interface through redistribution from IGRP. Query origin flag is 1.0 IP−EIGRP topology entry for 10.0.0 255. it advertises the route with a metric of 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. For example.0/24 shows: one# show ip eigrp topology 10.1. Query origin flag is 1.1.1.1.255.1. 1 Successor(s). FD is 46763776 Routing Descriptor Blocks: 20.0 255.0.1.255.1.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. 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 . the network 10.1.

0.16. 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 Two router eigrp 2000 network 172. 1 Successor(s).0 255.0 IP−EIGRP topology entry for 10.1. which is bolded.2.1.1 AS number of route is 1000 External protocol is IGRP.0.0/24 State is Passive.1.1.255. from 20.0 And Router One is configured as follows: one# show ip eigrp topology 10.1.1 (Serial0). Send flag is 0x0 Composite metric is (46763776/46251776).16.1. external metric is 0 Administrator tag is 1000 (0x000003E8) Note that the reported distance from Router Two.1.1.255. FD is 46763776 Routing Descriptor Blocks: 20.0 Router Three router igrp 2000 network 10.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 172.0 ! router igrp 2000 network 10. is 1.1.2.0. Query origin flag is 1.1.0.

4.0/24 in IGRP autonomous system 100.0/24 State is Passive.1 (Serial0).1. Send flag is 0x0 Composite metric is (2169856/1). Router Two runs both EIGRP and IGRP in autonomous system .1. 1 Successor(s).1.1.Originating router is 10. which is directly connected to Router One.1.1.4. 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 10.255.1. Query origin flag is 1.0 IP−EIGRP topology entry for 10. is redistributed from IGRP to EIGRP with a metric of 1 − the same metric we see when redistributing between two different autonomous systems. Router Four advertises 10.1.1. from 20. The directly attached 10.255.1. 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.0 255.1.1.1.1. FD is 2169856 Routing Descriptor Blocks: 20.1 AS number of route is 2000 External protocol is IGRP.1. Let us examine these caveats in Figure 11: Router One advertises 10. external metric is 0 Administrator tag is 0 (0x00000000) So this network.1 AS number of route is 2000 External protocol is IGRP. 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. • External EIGRP route metrics are compared to scaled IGRP metrics (the administrative distance is ignored).0/24 as an external in EIGRP autonomous system 100.1.1.0/24 network is handled the same way in both scenarios: one# show ip eigrp topology 10.

an IGRP metric.1.1.2. minimum bandwidth is 1000 Kbit Reliability 1/255.4. When we add the EIGRP route.4. Router Two prefers the EIGRP external route with the same metric (after scaling) and a higher administrative distance. and 3072256. for instance). Router Two shows: two# show ip route 10.0 Routing entry for 10.1. Redistribution To and From Other Protocols Redistribution between EIGRP and other protocols − RIP and OSPF. traffic share count is 1 Total delay is 20010 microseconds.1.2.2. It is always best to use the default metric when redistributing between protocols. via Serial1 Route metric is 12001. 00:00:42 ago. • External EIGRP routes have an administrative distance of 170.2.1. Hops 0 Note the administrative distance is 100.4. 00:53:59 ago.1. is through Router One.2 on Serial0. 00:53:59 ago Routing Descriptor Blocks: * 10. The router always prefers the path with the lowest cost metric and ignores the administrative distance.1.0/24 Known via "igrp 100".1. via Serial0 Route metric is 3072256. metric 3072256. Router Two shows: two# show ip route 10.1. is through Router Four. minimum bandwidth is 1000 Kbit Reliability 1/255. from 10. traffic share count is 1 Total delay is 20010 microseconds. distance 170. .100.4. eigrp 100 Last update from 10.2 on Serial1. minimum MTU 1 bytes Loading 1/255. from 10. metric 12001 Redistributing via igrp 100.2.1.0 Routing entry for 10. an EIGRP metric.1.0/24 Known via "eigrp 100". 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. 00:00:42 ago Routing Descriptor Blocks: * 10.2.2.1. This is true whenever automatic redistribution occurs between EIGRP and IGRP within the same autonomous system. 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. type external Redistributing via igrp 100. for example − works in the same way as all redistribution.1. distance 100. minimum MTU 1 bytes Loading 1/255. If we ignore the EIGRP route advertised by Router Four (by shutting down the link between Routers Two and Four. eigrp 100 Advertised by igrp 100 (self originated) eigrp 100 Last update from 10.

0.2.0.0/24 configured through interface Serial 0: ip route 172.16. Serial0 D 10.0 no auto−summary Router One redistributes this route. because EIGRP considers this a directly attached network. 00:00:47. even though it is not redistributing static routes. this looks as follows: two# show ip route . because the interface Router Two uses to reach Router One is in a different major network. On Router Two.0. Summarization There are two forms of summarization in EIGRP: auto−summaries and manual summaries.1.16. 1 subnets D 172. EIGRP redistributes this route as if it were a directly connected interface. 00:00:47.16.1.0/8 network to Router One.0/24 is subnetted..0.1.0 255.1.1.255..0.1. in Figure 13. which includes the static route. For example.0.1.1. 10.1. Router Two advertises only the 10. Let us look at the network in Figure 12. .16.0 [90/2169856] via 10. Auto−Summarization EIGRP performs an auto−summarization each time it crosses a border between two different major networks.1.16.Redistribution of Static Routes to Interfaces When you install a static route to an interface.0.16.1.1.0 network 172.255. 2 masks C 10..0/24 is directly connected. 2 subnets. Serial0 Note the route to 172.0/24 appears as an internal EIGRP route on Router Two.0/24 [90/2169856] via 10. and configure a network statement using router eigrp.0/8 is variably subnetted.0 Serial0 And Router One also has a network statement for the destination of this static route: router eigrp 2000 network 10. Serial0 172.1. Router One has a static route to the network 172.0.

2. On the router doing the summarization.0.0 IP−EIGRP topology entry for 10. The topology table entry for this summary route looks like the following: two# show ip eigrp topology 10.0.1. 00:23:20.0 here means this route is originated by this router) Composite metric is (10511872/0). Null0 10.0.0/24 is directly connected.1.1.0. FD is 10511872 Routing Descriptor Blocks: 0.0. Serial2 10.0.2.0.On Router One.0 network that have a bandwidth of 56k.0/8 is marked as a summary through Null0. Route is Internal Vector metric: Minimum bandwidth is 256 Kbit Total delay is 20000 microseconds Reliability is 255/255 Load is 1/255 .0.0/24 is directly connected.1. Send flag is 0x0 (note: the 0.0. 1 Successor(s).0.0/8 State is Passive.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. Query origin flag is 1.0.0 Routing entry for 10. from 172.0.0.0. 1 Successor(s). Send flag is 0x0 Composite metric is (11023872/10511872).0. a route is built to null0 for the summarized address: two# show ip route 10.16. although there are links in the 10.2.0.1. Note that the minimum bandwidth on this route is 256k.0. The metric is the best metric from among the summarized routes.1.16. FD is 11023872 Routing Descriptor Blocks: 172.1 (Serial0). Serial1 10.3. Serial1 The route to 10.0. Query origin flag is 1.0.0/24 [90/10537472] via 10.0.0.0/8 is a summary.0 IP−EIGRP topology entry for 10.0/8 State is Passive. 4 known subnets Attached (2 connections) Variably subnetted with 2 masks Redistributing via eigrp 2000 C D D C 10.1.1.0.0. 00:23:24. this looks like the following: one# show ip eigrp topology 10.0/8.0 (Null0). from 0. it looks like an internal route.0.

0.16.1. R − Reply.0.0/24.1. 1 successors.2.1 (11023872/10511872).. The configuration on Router Two is shown below: two# show run .1. 1 successors. FD is 2169856 via Connected.0 network instead of a summary. FD is 46354176 via 20.1.0/24.0.0/22.1. Serial0 P 172.1.3.3.Minimum MTU is 1500 Hop count is 0 To make Router Two advertise the components of the 10. 192.2. FD is 11049472 via 20.1 (46354176/45842176).1. r − Reply status P 10. . in Figure 14. Serial0 P 10. 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.1.0. 1 successors.0.0/24. FD is 11023872 via 20. Router One now sees all of the components of the 10.1 (11049472/10537472).0 network 10.0/24.1.0 no auto−summary With auto−summary turned off.0.1.0 network: one# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive.1.1.1. Router Two is summarizing the 192. 1 successors.1. U − Update.0/24 into the CIDR block 192. For example. Manual Summarization EIGRP allows you to summarize internal and external routes on virtually any bit boundary using manual summarization.0/24.16.1.. Serial0 P 10. Q − Query.. configure no auto−summary on the EIGRP process on Router Two: On Router Two router eigrp 2000 network 172.0.0.0/24. A − Active.1. and 192.

2..2.1 (10639872/128256). let us look at Figure 15. FD is 10639872 via 192. Router Three is injecting external routes to 192.0/24. r − Reply status P 10. we see this as an internal route: one# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive.255.50.1. U − Update.1.1. Serial0 P 192. FD is 46354176 via 10. Serial1 Note the ip summary−address eigrp command under interface Serial0.0/22.1. 1 successors. On Router One.0/24.50. Q − Query.1 (10537472/281600).1.0/24. as shown in the configurations below. 1 successors.1. Serial0 P 10. Null0 P 192. FD is 10537472 via 192. FD is 2169856 via Connected.255.0 no ip mroute−cache ! . Q − Query. Serial1 P 192. FD is 2169856 via Connected.1.1.1.0. 1 successors..2. To illustrate.50.1.1.3. 1 successors.1. 1 successors.0/24. 1 successors. FD is 10511872 via Connected.. R − Reply.0/24.0 ip summary−address eigrp 2000 192.0/24. Loopback0 P 10.252.1.0/22.50.1.0/26 and 192.0. 1 successors. . A − Active.255. 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 (11023872/10511872).! interface Serial0 ip address 10.1.1 255.10.0.1.1.10.50. FD is 45842176 via Connected.1.0 255. Serial1 P 192. 1 successors. R − Reply.0/24. A − Active. FD is 10511872 via Summary (10511872/0).1. 1 successors. two# show ip eigrp topology IP−EIGRP Topology Table for process 2000 Codes: P − Passive.64/26 into EIGRP using the redistribute connected command. r − Reply status P 10. FD is 11023872 via 10.1 (46354176/45842176). Serial0 P 192. U − Update. and the summary route via Null0.1.

Serial0 . 2 subnets D 10.1 255.255.128/26.1. 00:00:53.1..255.0 !router eigrp 2000 redistribute connected network 10.1.2.2.0/26 is subnetted.192 ! interface Serial0 ip address 192. Serial0 192.0. However.192 ! router eigrp 2000 network 192. Null0 .1.1.1.2. and add network statements for this network on Routers Two and Three.50.2.1.1. Serial0 D 192.1. 00:00:53.1. 00:02:03.0.0/26 and 192.50..0 is directly connected. And Router One shows only the summary route: one# show ip route .0/24 is a summary.2.1.2.Router Three interface Ethernet0 ip address 192. Router Three interface Ethernet0 ip address 192..1.192 ! interface Ethernet1 ip address 192.1.0/24 auto−summary is then generated on Router Two.1.2.50. Serial0 C 10. it does not do this because both routes are external.2.255.0.1.2. the routing table on Router One shows: one# show ip route .2.64/26 routes into one major net destination (192.65 255.0 [90/11023872] via 10.0/24: two# show ip route .1.0 Now Router Two generates the summary for 192.1. if you reconfigure the link between Routers Two and Three to 192.1 255. 1 subnets D EX 192..65 255..2.1.2..1..1. D 192.192 ! interface Ethernet2 ip address 10.255.. 1 subnets C 10.2..192 ! interface Ethernet1 ip address 192.1.2.255.0/8 is subnetted.1.2. the 192. Serial0 D EX 192.50.2.1.0/8 is subnetted.1 255. 10.130 255.255.2. Serial0 Although auto−summary normally causes Router Three to summarize the 192.2.255.0/24).2.. 00:06:48.1.2.255.64 [170/11049472] via 10.2.0 default−metric 10000 1 255 1 1500 With this configuration on Router Three.0/24 [90/11023872] via 10...255. 00:00:36.255.0 [170/11049472] via 10.0.1.2.255. 10.1.0 is directly connected.50.0.0.255.2.

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

We can expect the following to happen regarding network 192. so it marks 192.3.168.168.3. attempts to find a new feasible successor to this network.0/24 through Router Four Suppose that 192. upon receiving a query from its successor. Router Five marks 192.168. What activity can we expect to see on this network? Figures 16a through 16h illustrate the process.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 192.3.3.0/24 (far right side): • Router One has two paths to 192.3. It does not find one. and queries Router Four: Router Four.168.3.0/24 fails.168.168.0/24 as unreachable and query Routers Two and Three: .0/24 as unreachable.

and marks the route as unreachable.Routers Two and Three. Although another order is possible. see that they have lost their only feasible route to 192. Router One then receives the query from Router Two. and mark it as unreachable. . in turn.3.0/24. let us assume that Router One receives the query from Router Three first. they both send queries to Router One: For simplicity. they will all have the same final result.168.

168.168. Router One is now passive for 192.3.3.0/24: . 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.

.3. • Router Three has a topology table entry for the 10. so the path through Router Two is not a feasible successor).0. all routers in the network process a query for network 192.3. upon receiving the reply from Router Four.0/24 network with a cost of 46251885 through Router One.168. removes network 192.1. This is a good example of an unbounded query in an EIGRP network.0/24.1.168.1.Router Five. In fact.0/24 when that link goes down.0.0/24 from its routing table. How Summarization Points Affect the Query Range Now let us look at the paths to 10.1.1.0/24 in the same network: • Router Two has a topology table entry for the 10. if the queries were to reach the routers in a different order. Router Five is now passive for network 192. some would end up processing three or four queries.0/24 network with a cost of 20281600 through Router One. Some routers may end up processing more than one query (Router One in this example).3.1. Router Five sends updates back to Router Four so the route is removed from the topology and routing tables of the remaining routers. • Router Four 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.168. It is important to understand that although there may be other query paths or processing orders.

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

when it receives the queries from Routers Two and Three.1.0/24 is unreachable (note that Router Four has no knowledge of the subnet in question.0. marks the destination as unreachable and queries Routers Two and Four: Router Four. since it only has the 10.1.0. replies that 10.Router Three. when it receives the query from Router One.0/8 route): .

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

it removes the route from its table. but it does not solve the overall problem that each router must process the query. Router Three is now passive for this network: Router One replies to Router Two. 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. How Distribution Lists Affect the Query Range Rather than block the propagation of a query. In fact. distribution lists in EIGRP mark any query reply as unreachable. 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).Router Two replies that this network is unreachable and launches a query into autonomous system 200 toward Router One. 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). . the original query leaks into the second autonomous system in the form of a new query.

• 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). then queries Router Two: . it marks the route as unreachable and sends a query to Router Three. 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. Router Three marks the route as unreachable.In the figure above: • Router Three has a distribute−list applied against its serial interfaces that only permits it to advertise Network B. Router Three does not advertise a path to Network A because of the distribution list on its serial ports.

Note the query was not affected by the distribution list in Router Three: Router Two replies that Network A is reachable. Router Three now has a valid route: Router Three builds the reply to the query from Router One. even though Router Three has a valid route to Network A: . 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.

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. and the packet is 512 bytes. The pacing time for the packet in the above example is 0. 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.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). There is a field in show ip eigrp interface that displays the pacing timer.1463 seconds. The default configuration for EIGRP is to use up to 50 percent of the available bandwidth. 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. thereby using only a portion of the available bandwidth. . The pacing timer determines when the packet is sent. 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 largest packet that can be sent over the interface. 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 is typically expressed in milliseconds.

0.1 point−to−point ip address 10.0(4)T.0/0. If you use another network.0. To load balance over paths 1. so the local summary does not override the 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.0. Summarizing to a default route is effective only when you want to provide remote sites with a default route.0. Refer to Configuring a Gateway of Last Resort for further information. Refer to How Does Unequal Cost Path . Let us say there are four paths to a given destination. (Beginning in Cisco IOS Software 12. 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. issue variance 4 under the router eigrp command. you can use the variance command to instruct the router to also place traffic on paths 3 and 4.0.0. by default. Since summaries are configured per interface. EIGRP.0.0 0.0. and 3. however.0/0 overrides a default route learned from any other routing protocol. Note: Using max−paths. The type of load balancing (per packet or per destination) depends on the type of switching being done in the router.x.0. which the router then load−balances.1. because 1100 x 2 = 2200. you can configure EIGRP to use up to six routes of equal cost.0. you can also configure an administrative distance on the end of the ip summary−address eigrp command.0.1 frame−relay interface−dlci 10 ip summary−address eigrp 100 0. and the metrics for these paths are: • path 1: 1100 • path 2: 1100 • path 3: 2000 • path 4: 4000 The router.1. you must use the ip default−network command to mark the network as a default network.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 0.0. Use the first method when you want to draw all traffic to unknown destinations to a default route at the core of the network. which is greater than the metric through path 3.0.0.0.0.0. can also load−balance over unequal cost links. For example: ip route 0.x.0/0 route).0. This method is effective for advertising connections to the Internet. Using EIGRP. Note that a summary to 0. Similarly.0 ! interface serial 0 encapsulation frame−relay no ip address ! interface serial 0.0/0. 2.0.0.0.0 Load Balancing EIGRP puts up to four routes of equal cost in the routing table.0 x. to also add path 4. router eigrp 100 network 10.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 only way to configure a default route on a router using this method is to configure a static route to 0.

eigrp 100 Advertised by igrp 100 (self originated) eigrp 100 Last update from 10. it is important that these be set correctly. and Feasible Successors section for more information. traffic share count is 1 Total delay is 20010 microseconds.2. distance 100. • The delay should always be used to influence EIGRP routing decisions. 00:00:42 ago.1. in EIGRP. you can block redistribution from EIGRP into the external protocol. minimum bandwidth is 1000 Kbit Reliability 1/255. the delay has more influence over the total metric. 00:00:42 ago Routing Descriptor Blocks: * 10.0/24 Known via "igrp 100". It is not possible to modify the administrative distance for a default gateway that was learned from an external route because. 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.2 on Serial1. 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 1.2. Hops 0 For this example. At lower bandwidths.1. Because EIGRP uses the interface bandwidth to determine the rate at which to send packets. and the next packet over path 4. Using the Metrics When you initially configure EIGRP.4. from 10. rounds down to the nearest integer.2.1.2. Refer to the Feasible Distance. use a route−map with prefix−list. By tagging the route when it is redistributed into EIGRP. In order to raise the metric. EIGRP will not send traffic over an unequal cost path if the reported distance is greater than the feasible distance for that particular route. always use delay to do so.1. and so on. the next three packets over path 2.Load Balancing (Variance) Work in IGRP and EIGRP? for more information. The router then restarts by sending the next three packets over path 1. via Serial1 Route metric is 12001. router# show ip route 10. and uses this number as the traffic share count.2. minimum MTU 1 bytes Loading 1/255. the bandwidth has more influence over the total metric. How does the router divide the traffic between these paths? It divides the metric through each path into the largest metric. If it is necessary to influence the path EIGRP chooses. multipoint serial links and other mismatched media speed situations are the exceptions to this rule. at higher bandwidths. do not change the . metric 12001 Redistributing via igrp 100. the modification of the administrative distance only applies for internal routes. Note: Even with variance configured.0 Routing entry for 10.1. Using Administrative Tags in Redistribution External administrative tags are useful for breaking redistribution routing loops between EIGRP and other protocols.4. Reported Distance. the next two packets over path 3.

0. r − Reply status P 172.0.1.255. 1 successors. but this example does not show the entire configuration used for breaking redistribution loops..1. Serial0 via Redistributed (2169856/0) P 172.19. tag is 1 via Redistributed (128256/0) ....255. R − Reply. FD is 2169856 via Connected..255.1.0 loopback no keepalive ! interface Serial0 ip address 172.1 255. FD is 128256.1. 1 successors. A − Active. 1 successors. Q − Query..0/24..255. Router Three.. A basic example of configuring these tags follows.17.16.0 ! interface Ethernet0 ip address 172.0 0..19.0 .17.1 255.255 route−map foo permit 10 match ip address 10 set tag 1 .0. interface Loopback0 ip address 172.0/24.1 255.1. which is redistributing routes connected into EIGRP.administrative distance.1.0 default−metric 10000 1 255 1 1500 .0/24. shows: three# show run . U − Update. router eigrp 444 redistribute connected route−map foo network 172.255..16. FD is 281600 via Redistributed (281600/0) P 172..19.255. three# show ip eigrp topo IP−EIGRP Topology Table for process 444 Codes: P − Passive..255. access−list 10 permit 172.17.

17.0 default−metric 1 ! no ip classless ip route 1.19.. one# show ip route .0. FD is 2195456 via 172. Router One.. Serial0 P 172.17. 1 successors.1.255.0 network 172.. two# show ip eigrp topo IP−EIGRP Topology Table for process 444 Codes: P − Passive. FD is 2169856 via Connected.17.255 Serial0 route−map foo deny 10 match tag 1 ! route−map foo permit 20 .1..255..1.1.18.1..16. tag is 1 via 172... interface Serial0 ip address 172.1 255. U − Update..0/24.17. interface Serial0 ip address 172.0 ! router rip redistribute eigrp 444 route−map foo network 10.1 (2297856/128256).1 (2195456/281600). 1 successors.1.1. which is receiving the RIP routes redistributed by Router 2.255.0/24.0.1..2 255.Router Two.3 255. FD is 2297856. Q − Query.. router eigrp 444 network 172..1. shows: one# show run .255. Serial0 Note the tag 1 on 172. r − Reply status P 172.18.0. R − Reply.0/24.0.255.0.1. which is redistributing routes from EIGRP into RIP. Serial0 P 172.. 1 successors.255.1.0 ! interface Serial1 ip address 172.0 .0 no fair−queue clockrate 1000000 router rip network 172.2 255. shows: two# show run . A − Active.0/24..0 .19.255.255.17.18.18..

1. • Socket Queue displays the IP to EIGRP Hello Process socket queue counters (current−0/max−2000/highest−1/drops−0).16.Gateway of last resort is not set R R C 172. 1 subnets is directly connected.17.0 [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).0.1. • SIA−Replies sent/received displays the number of stuck in active reply packets sent and received (sent−0/received−0).3. An explanation of each output field follows the table. • Updates sent/received displays the number of update packets sent and received (sent−20/received−39).0/16 172.0/24 172.18.18. 00:00:15. Serial0 [120/1] via 172. The output of this command shows the information that has been exchanged between the neighboring EIGRP router.3.18. Serial0 is subnetted.1.1. 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.18.19.0/16 172.0/24 is missing. • PDM Process ID stands for protocol−dependant module IOS process identifier (251). • Acks sent/received stands for the number of acknowledgment packets sent and received (sent−66/received−41). • SIA−Queries sent/received means number of stuck in active query packets sent and received (sent−0/received−0). • Input Queue shows the EIGRP Hello Process to EIGRP PDM socket queue counters (current−0/max−2000/highest−1/drops−0).0. • Hello Process ID is the hello process identifier (270). 00:00:15. • Queries sent/received means the number of query packets sent and received (sent−10/received−18). .0. • Replies sent/received shows the number of reply packets sent and received (sent−18/received−16). Serial0 Note that 172.

• Via 10. or R.2. use the show ip eigrp topology all−links command. • Serial0 is the interface through which this neighbor is reachable. • r shows that we have queried this neighbor about the route and have not yet received a reply. • 1 replies shows the number of outstanding replies. show ip eigrp topology Configuration Explanations • A means active. Serial1 shows we are using this route (indicates which path the next path/destination will take when there are multiple routes of equal cost). the route is in transition.1. • Via 10. • active 00:00:01 shows how long this route has been active. if this is a summary route generated on this router. This field can also be: Multiple origins. • 0 successors shows how many successors (or paths) are available for this destination. • Serial1 is the interface through which this neighbor is reachable. • 10. or Successor origin. • Q means a query is pending.2 shows the neighbor from which we are waiting for a reply.2 (512640000/128256). if this route is being redistributed into EIGRP on this router. meaning passive.show ip eigrp topology This command only displays feasible successors.2.1.1.2 shows that we learned of this route from a neighbor whose IP address is 10. • Q is the send flag for this route. meaning that multiple neighbors have sent queries on this destination. meaning there is an update pending. which is the best metric to reach this destination or the best metric known when the route went active. meaning there is a query pending. • r shows that we have queried this neighbor and are waiting for a reply. This field can also be: U. meaning the successor originated the query. meaning there is a reply pending.2. or R.0/24 is the destination or mask. • FD is 512640000 shows the feasible distance. and the reported distance through this neighbor in the second field. or Summary. for update pending. This field can also be: U. This field can also be: Connected. To display all entries in the topology table.4. Redistributed. but not the successor. • tag is 0x0 can be set and/or filtered using route maps with the set tag and match tag commands.1. if successors is capitalized.1. if the network is directly connected to this router. . for reply pending.2.2. An explanation of each output field follows the table. • (Infinity/Infinity) shows the metric to reach this path through this neighbor in the first field. This could also show a P. • query origin: Local origin shows this route originated the query. • via 10.

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

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

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