Disabling split horizon
Distance vector routing protocols like RIP and EIGRP rely on a number of measures for loop avoidance. One of these is split horizon, which prevents a route from being readvertised out the interface on which it was received. For example, if a router learns about 192.168.0.0/16 from a neighbor on its Serial1/0 interface, it can't advertise that route back out Serial1/0. This helps mitigate routing loops while working to optimize communication between neighbors.
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However, there are instances where split horizon is undesirable. One good example of such a scenario is a nonbroadcast multiaccess (NBMA) frame relay network lacking a full mesh. Such a topology is illustrated below:
The point-to-multipoint frame relay network is comprised of two virtual circuits, one between R1 and R2 and one between R2 and R3. All three routers are addressed within the 10.0.0.0/24 subnet. EIGRP adjacencies are formed along the virtual circuits (R1 peers with R2 but not with R3):
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R1# show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) ( ms) Cnt Num
0 10.0.0.2 Se1/0 153 02:26:57 16 200 0 11
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R2# show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 10.0.0.3 Se1/0 134 02:27:00 527 3162 0 7
0 10.0.0.1 Se1/0 169 02:27:00 547 3282 0 7
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Remember that the split horizon rule forbids R2 from relaying advertisements back out the interface on which they were received. As a result, R3 never receives advertisements from R1 and vice versa. Here we can verify R3 has no knowledge of R1's 192.168.1.0/24 network:
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R3# show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial1/0
D 192.168.2.0/24 [90/2195456] via 10.0.0.2, 02:29:48, Serial1/0
C 192.168.3.0/24 is directly connected, FastEthernet0/0
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One solution to this predicament is to disable split horizon for EIGRP on R2. This is accomplished with the command no ip split-horizon eigrp <AS> under interface configuration.
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R2(config)# interface s1/0
R2(config-if)# no ip split-horizon eigrp 1
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This command will prompt EIGRP to immediately tear down and reestablish all its adjacencies on the interface, as evidenced by this log message on R2:
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.0.0.3 (Serial1/0) is resync: split horizon changed
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When the adjacencies are reformed, we can see that R2 is now relaying advertisements between R1 and R3:
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R3# show ip route
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial1/0
D 192.168.1.0/24 [90/2707456] via 10.0.0.2, 00:02:39, Serial1/0
D 192.168.2.0/24 [90/2195456] via 10.0.0.2, 02:33:55, Serial1/0
C 192.168.3.0/24 is directly connected, FastEthernet0/0
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Additionally, if we were running RIP instead of EIGRP, the command to disable split horizon is simply no ip split-horizon at the interface.