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). We'd like all three routers to have IP connectivity to each remote loopback, but the EIGRP routing tables tell us that we've got a problem.

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 be used to.

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

R2 and R3 now have all the routes we expect them to.

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!

To your success,

Chris Bryant

CCIE #12933