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For CCNA exam success, you had better know what split horizon is, how
to turn it off, and when to turn it off. Knowing when to turn split
horizon off is also important in production networks, because it can
cause a hub-and-spoke network to have incomplete routing tables on the
spokes.
Split horizon exists for a very good reason - routing loop prevention.
The rule of split horizon states that a router cannot send an
advertisement for a route out the same interface that it came in on.
Split horizon is on by default on all interfaces running RIP, IGRP, and
EIGRP.
In this CCNA tutorial, R1 will serve, as the hub and R2 and R3 will be
the spokes. We'll first configure EIGRP over the 172.16.123.0 /24
network, the network connecting the three routers.
R1#conf t
R1(config)#router eigrp 100
R1(config-router)#no auto-summary
R1(config-router)#network 172.12.123.0 0.0.0.255
R2#conf t
R2(config)#router eigrp 100
R2(config-router)#no auto-summary
R2(config-router)#network 172.12.123.0 0.0.0.255
R3#conf t
R3(config)#router eigrp 100
R3(config-router)#no auto-summary
R3(config-router)#network 172.12.123.0 0.0.0.255
Running show ip eigrp neighbor on R1 shows that adjacencies to R2 and R3 are up.
R1#show ip eigrp neighbor
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
1 172.12.123.3 Se0/0 11 00:02:45 1 5000 0 1
0 172.12.123.2 Se0/0 161 00:03:01 1 5000 0 1
Each router will now advertise its loopback address via EIGRP.
R1#conf t
R1(config)#router eigrp 100
R1(config-router)#network 1.1.1.0 0.0.0.255
R2#conf t
R2(config)#router eigrp 100
R2(config-router)#network 2.2.2.0 0.0.0.255
R3#conf t
R3(config)#router eigrp 100
R3(config-router)#network 3.3.3.0 0.0.0.255
Running show ip eigrp route on each router shows that R1 has a route
for both R2’s and R3’s loopback. R2 and R3 will only see R1’s loopback
address, and not each other’s. Why?
R1#show ip route eigrp
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/2297856] via 172.12.123.2, 00:03:19, Serial0/0
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/2297856] via 172.12.123.3, 00:03:04, Serial0/0
R2#show ip route eigrp
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:03:40, Serial0/0.123
R3#show ip route eigrp
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:05:17, Serial0/0.31
EIGRP uses Split Horizon by default to prevent routing loops. In this
lab, though, it prevents full network reachability. R2 and R3 both form
neighbor relationships with R1’s Serial physical interface. R2
advertises its loopback address to R1’s Serial interface, as does R3.
Split Horizon does not allow a route to be advertised back out the same
interface it was received on. This prevents R1 from advertising R2’s
loopback to R3, or R3’s loopback to R2.
Split Horizon must be disabled to allow full network reachability in
this lab. To do so, run no ip split-horizon eigrp 100 on R1’s Serial
interface. When Split Horizon is disabled, that will cause the neighbor
relationships to fail, and then reestablish. Run show ip route eigrp
100 on both R2 and R3. The appropriate route to the remote loopback
address will now appear.
R1#conf t
R1(config)#int serial0
R1(config-if)#no ip split-horizon eigrp 100
10:02:23: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0/0) down: split horizon changed
10:02:23: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0/0) down: split horizon changed
10:02:27: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0/0) ip: new adjacency
10:02:54: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0/0) ip: new adjacency
R2#show ip route eigrp
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:00:06, Serial0/0.123
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/2809856] via 172.12.123.1, 00:00:06, Serial0/0.123
R3#show ip route eigrp
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:00:12, Serial0/0.31
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/2809856] via 172.12.123.1, 00:00:12, Serial0/0.31
Disabling split horizon should be done with care, but knowing when and
where to do so shows that you truly understand how this technology
works - and that's a big step on the way to earning your CCNA!
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