LAB: EIGRP Split Horizon

R5:
interface Serial0/0
no ip split-horizon eigrp 100


Once split-horizon for EIGRP is disabled on R5’s link to the Frame Relay network
updates can be exchanged to R2 as they come from other neighbors on the
Frame Relay segment, such as R1. The result of this can be seen in the routing
table of R2.

Rack1R2#show ip route eigrp
155.1.0.0/24 is subnetted, 15 subnets
D 155.1.146.0 [90/2684416] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.10.0 [90/2175232] via 155.1.0.5, 00:03:16, Serial0/0.1
D 155.1.8.0 [90/2172672] via 155.1.0.5, 00:03:16, Serial0/0.1
D 155.1.9.0 [90/2684928] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.13.0 [90/3193856] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.7.0 [90/2684672] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.5.0 [90/2172416] via 155.1.0.5, 00:17:53, Serial0/0.1
D 155.1.58.0 [90/2172416] via 155.1.0.5, 00:17:53, Serial0/0.1
D 155.1.45.0 [90/2681856] via 155.1.0.5, 00:17:53, Serial0/0.1
D 155.1.37.0 [90/2684416] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.79.0 [90/2684672] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.67.0 [90/2684672] via 155.1.0.5, 00:00:24, Serial0/0.1
D 155.1.108.0 [90/2174976] via 155.1.0.5, 00:03:16, Serial0/0.1
150.1.0.0/24 is subnetted, 10 subnets
D 150.1.7.0 [90/2812416] via 155.1.0.5, 00:00:24, Serial0/0.1
D 150.1.6.0 [90/2812416] via 155.1.0.5, 00:00:24, Serial0/0.1
D 150.1.5.0 [90/2297856] via 155.1.0.5, 00:07:18, Serial0/0.1
D 150.1.4.0 [90/2809856] via 155.1.0.5, 00:03:18, Serial0/0.1
D 150.1.3.0 [90/2809856] via 155.1.0.5, 00:00:24, Serial0/0.1
D 150.1.1.0 [90/2809856] via 155.1.0.5, 00:00:23, Serial0/0.1
D 150.1.10.0 [90/2302976] via 155.1.0.5, 00:03:16, Serial0/0.1
D 150.1.9.0 [90/2812672] via 155.1.0.5, 00:00:24, Serial0/0.1
D 150.1.8.0 [90/2300416] via 155.1.0.5, 00:03:17, Serial0/0.1

Although disabling split-horizon on R5 does not cause a routing loop, it does add additional route replication into the topology. For example if we look at the EIGRP topology table on R2 for the prefix 150.1.2.0/24, which is directly connected, we can see that this prefix is originated from the connected route, but is also learned back in from R5.

Rack1R2#show ip eigrp topology 150.1.2.0 255.255.255.0
IP-EIGRP (AS 100): Topology entry for 150.1.2.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256
Routing Descriptor Blocks:
0.0.0.0 (Loopback0), from Connected, Send flag is 0x0
Composite metric is (128256/0), Route is Internal
Vector metric:
Minimum bandwidth is 10000000 Kbit
Total delay is 5000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1514
Hop count is 0
155.1.0.5 (Serial0/0.1), from 155.1.0.5, Send flag is 0x0
Composite metric is (2809856/2297856), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 45000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2

Even though there are multiple paths to the same destination a loop cannot occur based on the EIGRP feasibility condition. The feasibility condition determines which routes from the EIGRP topology will actually be used for forwarding in the IP routing table.

First off the complete end-to-end composite metric is compared between routes. In this case R2’s local route has a metric of 128,256, while R5’s route has a metric of 2,809,856. This value is seen as the first value in parenthesis before the slash. The lower of these values, 128,256, is considered the Feasible Distance, and is the end-to-end metric of the active forwarding path. This active forwarding path is called the Successor.

Next, R2 compares the metric that the upstream neighbor is advertising for the destination. In this case R5 is advertising a metric of 2,297,856. This value seen as the second number inside parenthesis, the Advertised Distance, is used to find alternate loop-free forwarding paths. If the Advertised Distance of a neighbor’s route is lower than the current Feasible Distance of the Successor, the route is considered an alternate path, or a Feasible Successor. In this case R5’s Advertised Distance is higher than R2’s Feasible Distance, so the route is discarded and not considered a valid path.