Troubleshooting EIGRP

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jul 26, 2002.

Chapter Description

Learn how to quickly identify and fix the most common causes of EIGRP problems with these debugs, configurations, and useful show commands.

Troubleshooting EIGRP Route Summarization

Summarization is extremely important in a well-designed EIGRP network. Summarization is one of the few weapons to prevent stuck in active problems. Most summarization problems are the result of a misconfiguration of the router. Figure 7-29 shows a flowchart for troubleshooting an EIGRP summarization problem.

Figure 7-29Figure 7-29 Flowchart for Troubleshooting EIGRP Summarization Route Problem

EIGRP Summarization Route Problem—Cause: Subnetworks of Summary Route Don't Exist in Routing Table

Consider the case shown in Figure 7-30, in which Router A is configured to send out a summary route of on its Ethernet 0 interface to Router B. Example 7-52 shows the configuration of Router A. However, the next-hop router is not seeing the route, and the route is not in the router's topology table. Example 7-53 shows a snapshot of the router's routing table.

Figure 7-30Figure 7-30 Network Diagram for Case Study on EIGRP Summarization Route Problem

Example 7-52 Configuration of Router A in the Example Shown in Figure 7-30

Router_A#interface ethernet 0
ip address
ip summary-address EIGRP 1
interface Serial 0
ip address
interface Serial 1
ip address
router EIGRP 1

Example 7-53 Routing Table Snapshot

Router A# show ip route 

C is directly connected, Serial 0
C is directly connected, Serial 1
C is directly connected, Ethernet 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0

In the configuration shown in Example 7-52, the summary route is configured to be by using the command ip summary-address eigrp 1 This summary route covers the network address range from to From the routing table shown in Example 7-53, notice that no routes fit between the range of to Therefore, if no subnetworks of the configured summary route are present in the routing table, the router doesn't generate the summary route.

The solution to this problem is to configure an interface that falls in the range. You can configure a loopback interface with address to generate the summary route configured on Ethernet 0. Example 7-54 shows the changed configuration in Router A that will fix this manual-summarization problem.

Example 7-54 Changed Configuration of Router A to Fix the Manual-Summarization Problem

interface loopback 0
ip address
interface Ethernet 0
ip address
ip Summary-address EIGRP 1
interface Serial 0
ip address
Interface Serial 1
ip address
router EIGRP 1

After the configuration change, the routing table on Router A shows the manual-summarization route of, as shown in Example 7-55.

Example 7-55 Routing Table Snapshot of Router A After the Configuration Change to Verify the Fix

Router A# show ip route 

C is directly connected, Serial 0
C is directly connected, Serial 1
C is directly connected, Ethernet 0
C is directly connected, Loopback 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D [90/409600] via, Serial 0
D is a summary, 00:03:24, Null 0
D [90/409600] via, Serial 0

EIGRP Summarization Route Problem—Cause: Too Much Summarization

Another EIGRP summarization route problem stems from when the summary route covers more subnetworks than exist. Figure 7-31 shows the network diagram to refer to for this case study.

Figure 7-31Figure 7-31 EIGRP Network Diagram—Too Much IP Address Summarization

As shown in Figure 7-31, Router B is connected to the network cloud with network of through Router B is summarizing those networks into one big summary route of and sending it to Router A. Router A is connected to the core network, and Router A is sending Router B a default route of The problem arises when a device in the core network tries to reach a network of, which is nonexistent in the network. When the device in the core network is trying to ping or traceroute to the network, the packets are looping between Router A and Router B.

Example 7-56 shows Router A's routing table for

Example 7-56 Router A Routing Table for

Router A# show ip route
Routing entry for
   Known via "EIGRP 1", distance 90, metric 409600,  type internal
t update from on Serial0
, 00:20:25 ago
   Routing Descriptor Blocks:
* from192.168.2.2, 00:20:25 ago, via Serial 0
   Route metric is 409600, traffic share count is 1
   Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
   Reliability 255/255, minimum MTU 1500 bytes
   Loading 1/255, Hops 1

The routing entry in Router A shows the summary route of coming from Router B. Therefore, Router A forwards the packet to Router B. However, Router B sends the packet right back to Router A because Router B doesn't have the route for; it has only the default route pointing back to Router A. This causes the routing loop between Router A and Router B for any nonexistent network in the range.

This problem is more of a design issue. The main issue is that Router B's summary route is too broad and includes nonexistent subnets. Also, Router A is sending a more general summary route (default route) to Router B. The solution is to have Router B send out only the summary route that covers the through networks. In other words, instead of sending the summary route, Router B can send the summary route to Router A. Therefore, when Router A tries to look at the routing table for the entry, the routing table simply returns with % Network not in table message and drops the packet instead of sending it to Router B, which ends the loop.

6. Troubleshooting EIGRP Redistribution Problems | Next Section Previous Section

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