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Study Guide CCNP - BSCI ( 642-901 )
EIGRP And Split Horizon
EIGRP is a major topic for your CCNA and CCNP studies, and one basic
skill you'll need to pass your Cisco certification exams is to
identify situations where you need to enable or disable split
horizon. EIGRP commands tend to be a little different than those
used with other protocols, so let's take a look at how EIGRP and
split horizon interoperate.
R1 is our hub router, with R2 and R3 as the spokes. There are no
subinterfaces, and each router is advertising a single loopback
network using its router number for each octet. R1 will see both R2
and R3's loopback network, but the spokes will not have a route to
the other spoke's loopback. "show ip route eigrp" verifies this.
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
EIGRP runs split horizon by default, making it impossible for R1 to
forward an advertisement to R2 regarding R3's loopback. Likewise, R1
cannot advertise R2's loopback address to R3. We could configure two
subinterfaces on R1 to resolve this issue, but here we're going to
disable split horizon instead.
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 down: split horizon changed
04:10:02: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3
(Serial0) is down: 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
Note that disabling split horizon resulted in the EIGRP adjacencies
being torn down. They came back up 20 - 35 seconds after being torn
down according to the timestamps, but that's a good detail to keep
in mind!
The routing tables of each spoke should now show the loopback
network configured on the remote spoke.
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#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
Be careful when disabling split horizon. In this scenario, R1 can
and will advertise routes out Serial0 that were learned about on
that interface in the first place, and that's not always desirable.
Split horizon is enabled by default for a reason, so be careful when
disabling it!