Cisco CCNA, CCENT, And CompTIA Network+ Troubleshooting Exercise:

"Why Aren't We Seeing Our Subnets?"

By Chris Bryant, CCIE #12933

A big part of being a world-class network troubleshooter - as well as passing your certification exams - is developing your troubleshooting ability. Today's Networking Troubleshooting Challenge will help you do just that!

Today, our client has a hub-and-spoke network where both spokes have subnets of 20.0.0.0/8. The Serial interfaces are all on the 172.12.123.0 /24 network, with the router number serving as the final octet.  All interfaces are in EIGRP AS 100. Each router's EIGRP configuration contains only the necessary network commands to advertise each connected network.

The client reports severe IP connectivity issues, and after running some pings, you see the following:

R1#ping 20.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.1.1.1, timeout is 2 seconds:
!U!.!
Success rate is 60 percent (3/5), round-trip min/avg/max = 68/68/68 ms
R1#ping 20.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.2.2.2, timeout is 2 seconds:
U!.!U
Success rate is 40 percent (2/5), round-trip min/avg/max = 68/68/68 ms
R1#ping 20.3.3.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.3.3.3, timeout is 2 seconds:
U!.!U
Success rate is 40 percent (2/5), round-trip min/avg/max = 68/68/68 ms
R1#ping 20.4.4.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.4.4.4, timeout is 2 seconds:
!U!.!
Success rate is 60 percent (3/5), round-trip min/avg/max = 68/68/68 ms

The EIGRP route table looks like this:

R1#show ip route eigrp
D    20.0.0.0/8 [90/2297856] via 172.12.123.3, 00:11:19, Serial0
                [90/2297856] via 172.12.123.2, 00:11:19, Serial0

The routing table may look like you thought it would - or maybe it doesn't. It is safe to say that those pings are ugly with a capital U. What do we need to do about this - and exactly where should the default configuration be changed, if that is indeed the case?

Network 20.0.0.0 is discontiguous – there is no single path to all subnets of the major network number.  That's a problem for routing protocols such as RIPv1 that do not carry subnet mask information.  

EIGRP and RIPv2 do carry subnet mask information, but the default autosummarization causes trouble with this network.  R1 is now receiving the exact same update from both R2 and R3, and it's for the classful network 20.0.0.0 /8.

Here's R1's EIGRP route table.  None of the subnets are present in the routing table.

R1#show ip route eigrp
D    20.0.0.0/8 [90/2297856] via 172.12.123.3, 00:11:19, Serial0
                [90/2297856] via 172.12.123.2, 00:11:19, Serial0

Since the metrics for both paths are exactly the same,  equal-cost load balancing for the classful network 20.0.0.0 will be performed, ensuring that at least half of the packets destined for any particular subnet of 20.0.0.0 will be going to the wrong router. 

This default behavior is easily removed with the no auto-summary command.   When both of the routers sending updates add this command to their EIGRP configuration, the routes will no longer be summarized at the network boundary. 

One often-ignored side effect of adding no auto-summary to an existing EIGRP configuration - the adjacencies will drop.

R3(config)#router eigrp 100
R3(config-router)#no auto-summary

00:26:09: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.1 (Serial0) is down: summary configured

After configuring no auto-summary on both R2 and R3 and waiting for the adjacencies to reform, R1 now has a much more accurate routing table. Note that no auto-summary is configured on the spoke routers in this network - this command should be configured on the router(s) performing the autosummarization.

R1#show ip route eigrp
     20.0.0.0/16 is subnetted, 4 subnets
D       20.4.0.0 [90/2297856] via 172.12.123.3, 00:00:11, Serial0
D       20.1.0.0 [90/2297856] via 172.12.123.2, 00:03:47, Serial0
D       20.2.0.0 [90/2297856] via 172.12.123.2, 00:03:47, Serial0
D       20.3.0.0 [90/2297856] via 172.12.123.3, 00:00:20, Serial0

Bottom line: If you're running EIGRP and you're not seeing the subnets or routes you expect, the first thing I'd check is to see if the no auto-summary command is in the configuration. If it's not, I'd put it there.

router eigrp 100
 network 20.3.0.0 0.0.255.255
 network 20.4.0.0 0.0.255.255
 network 172.12.0.0
 auto-summary     <<<< BEWARE!

Ultimate CCNA Study Package | The Ultimate CCNA Study Guide