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Troubleshooting EIGRP And Disabling Split Horizon

A CCNA And CCNP Certification Exam Tutorial

By Chris Bryant, CCIE #12933

The network we're using in this tutorial is a hub-and-spoke network, with R1 as the hub and R2 and R3 as the spokes. Each router is advertising its loopback via EIGRP, and all routers are using physical interfaces (no subinterfaces).

The hub-and-spoke network is 172.12.123.0 /24; each router uses its own number for each octet of its single loopback interface as well as for the last octet of its serial interface on the 172.12.123.0 /24 network.

In other words, it looks a little something like this!

EIGRP Network For CCNA Tutorial

EIGRP has been correctly configured on each interface, but something odd occurs when we check the EIGRP tables - neither of the spoke routers can see the other spoke router's loopback network. (Remember, the 172.12.123.0 network doesn't appear in this table since it's a directly connected network on every router.)

R1#show ip route eigrp
     2.0.0.0/32 is subnetted, 1 subnets
D       2.2.2.2 [90/2297856] via 172.12.123.2, 00:03:29, Serial0
     3.0.0.0/32 is subnetted, 1 subnets
D       3.3.3.3 [90/2297856] via 172.12.123.3, 00:03:29, Serial0

R2#show ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/2297856] via 172.12.123.1, 00:05:20, Serial0

R3#show ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/2297856] via 172.12.123.1, 00:07:54, Serial0

Because EIGRP runs split horizon by default, R1 cannot send a route advertisement out the same interface the advertisement was received upon to begin with. R1 is learning about R2's loopback and R3's loopback on Serial0, so R1 cannot advertise those routes right back out the same interface.

We've got two choices - we can set up subinterfaces on R1 or we can disable split horizon on R1's Serial0 interface. Let's disable SH with EIGRP, since the command is a little different than what you may have seen in your studies.

R1(config)#int serial0
R1(config-if)#no ip split-horizon ?
eigrp Enhanced Interior Gateway Routing Protocol (EIGRP)
<cr>

R1(config-if)#no ip split-horizon eigrp ?
<1-65535> Autonomous system number

R1(config-if)#no ip split-horizon eigrp 100

04:10:02: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0) is do
wn: split horizon changed
04:10:02: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0) is do
wn: split horizon changed

04:10:25: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0) is up
: new adjacency

04:10:37: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0) is up
: new adjacency

Speaking of things many CCNA study guides leave out ... note that the adjacencies drop when you enabled split horizon. The adjacencies came back up 23 seconds later (check those timestamps!), but this still isn't the kind of thing you want to do during a busy time on a production network.

Let's clear our IP route tables on both R2 and R3:

R2#clear ip route *
R2#show ip route eigrp
     1.0.0.0/32 is subnetted, 1 subnets
D       1.1.1.1 [90/2297856] via 172.12.123.1, 00:00:02, Serial0
     3.0.0.0/32 is subnetted, 1 subnets
D       3.3.3.3 [90/2809856] via 172.12.123.1, 00:00:02, Serial0

R3#clear ip route *
R3#show ip route eigrp
     1.0.0.0/32 is subnetted, 1 subnets
D       1.1.1.1 [90/2297856] via 172.12.123.1, 00:00:02, Serial0
     2.0.0.0/32 is subnetted, 1 subnets
D       2.2.2.2 [90/2809856] via 172.12.123.1, 00:00:02, Serial0

You should be very careful when disabling split horizon for any reason; make sure that you're not getting any undesirable routing advertisments as a result!

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