Previously, you learned that split horizon is a Distance Vector protocol feature mandating that routing information cannot be sent back out of the same interface through which it was received. This prevents the re-advertising of information back to the source from which it was learned. While this characteristic is a great loop prevention mechanism, it is also a significant drawback, especially in hub-and-spoke networks. To better understand the drawbacks of this feature, refer to the EIGRP hub-and-spoke network in Figure 36.14 below:
Figure 36.14 – EIGRP Split Horizon
The topology in Figure 36.14 illustrates a classic hub-and-spoke network, with router HQ as the hub router and routers S1 and S2 as the two spoke routers. On the Frame Relay WAN, each spoke router has a single DLCI provisioned between itself and the HQ router in a partial-mesh topology. The Frame Relay configuration on the routers is verified as follows:
| HQ#show frame-relay mapSerial0/0 (up): ip 172.16.1.2 dlci 102(0x66,0x1860), static,broadcast,CISCO, status defined, activeSerial0/0 (up): ip 172.16.1.1 dlci 103(0x67,0x1870), static,
broadcast, CISCO, status defined, active
S1#show frame-relay map Serial0/0 (up): ip 172.16.1.2 dlci 301(0x12D,0x48D0), static, broadcast, CISCO, status defined, active Serial0/0 (up): ip 172.16.1.3 dlci 301(0x12D,0x48D0), static, broadcast, CISCO, status defined, active
S2#show frame-relay map Serial0/0 (up): ip 172.16.1.1 dlci 201(0xC9,0x3090), static, broadcast, CISCO, status defined, active Serial0/0 (up): ip 172.16.1.3 dlci 201(0xC9,0x3090), static, broadcast, CISCO, status defined, active |
We will cover Frame Relay later on in the WAN section. Enhanced IGRP has been enabled on all three routers, using AS 150. The following output illustrates the EIGRP neighbour relationships between the HQ router and the spoke routers:
|
HQ#show ip eigrp neighbors IP-EIGRP neighbors for process 150 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 1 172.16.1.1 Se0/0 165 00:01:07 24 200 0 2 0 172.16.1.2 Se0/0 153 00:01:25 124 744 0 2 |
The following output illustrates the EIGRP neighbour relationship between the first spoke router, S1, and the HQ router:
|
S1#show ip eigrp neighbors IP-EIGRP neighbors for process 150 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 172.16.1.3 Se0/0 128 00:00:53 911 5000 0 4 |
The following output illustrates the EIGRP neighbour relationship between the second spoke router, S2, and the HQ router:
|
S2#show ip eigrp neighbors IP-EIGRP neighbors for process 150 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 172.16.1.3 Se0/0 156 00:02:20 8 200 0 4 |
By default, EIGRP split horizon is enabled, which is undesirable in partial-mesh NBMA networks. This means that the HQ router will not advertise routing information learned on Serial0/0 out of the same interface. The effect of this default behaviour is that the HQ router will not advertise the 10.1.1.0/24 prefix received from S1 to S2 because the route is received via the Serial0/0 interface, and the split horizon feature prevents the router from advertising information learned on that interface back out onto the same interface. The same is also applicable for the 10.2.2.0/24 prefix the HQ router receives from S2.
This default behaviour means that while the HQ router is aware of both prefixes, the spoke routers have only partial routing tables. The routing table on the HQ router is as follows:
| HQ#show ip route eigrp10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksD 10.1.1.0/24 [90/2195456] via 172.16.1.1, 00:12:04, Serial0/0D 10.2.2.0/24 [90/2195456] via 172.16.1.2, 00:12:06, Serial0/0 |
The routing table on spoke S1 is as follows:
|
S1#show ip route eigrp 192.168.1.0/26 is subnetted, 1 subnets D 192.168.1.0 [90/2195456] via 172.16.1.3, 00:10:53, Serial0/0 |
The routing table on spoke S2 is as follows:
| S2#show ip route eigrp192.168.1.0/26 is subnetted, 1 subnetsD 192.168.1.0 [90/2195456] via 172.16.1.3, 00:10:55, Serial0/0 |
The result of this default behaviour is that while the HQ router will be able to reach both of the spoke router networks, neither spoke router will be able to reach the network of the other. There are several ways such a situation can be addressed and they are as follows:
- Disabling split horizon on the HQ (hub) router
- Advertising a default route from the HQ router to the spoke routers
- Manually configuring EIGRP neighbours on the routers
Disabling split horizon is performed at the interface level using the no ip split-horizon eigrp [AS] interface configuration command on the hub router. The command show ip split-horizon interface_name does not show the state of EIGRP split horizon as it does for RIP. To see if it is disabled, you have to examine the interface configuration section (i.e, show run interface_name). Referencing the network topology illustrated in Figure 36.14 above, this interface configuration command would be applied to the Serial0/0 interface on the HQ router. This is performed as follows:
| HQ(config)#interface Serial0/0HQ(config-if)#no ip split-horizon eigrp 150 |
After split horizon is disabled, the HQ router can advertise information back out onto the same interface on which it was received. For example, the routing table on spoke S2 now shows a routing entry for the 10.1.1.0/24 prefix advertised by spoke S1 to the HQ router:
| S2#show ip route eigrp10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksD 10.1.1.0/24 [90/2707456] via 172.16.1.3, 00:00:47, Serial0/0192.168.1.0/26 is subnetted, 1 subnetsD 192.168.1.0 [90/2195456] via 172.16.1.3, 00:00:47, Serial0/0 |
A simple ping test from spoke router S2 to the 10.1.1.0/24 subnet can be used to verify connectivity, as illustrated below:
| S2#ping 10.1.1.2Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-trip min/avg/max = 24/27/32 ms |
The second method of disabling split horizon is simply to advertise a default route from the HQ router to the spoke routers. In this situation, the ip summary-address eigrp 150 0.0.0.0 0.0.0.0 interface configuration command could be applied to the Serial0/0 interface of the HQ router. This would allow the spoke routers to reach each other through the HQ router, which contains the full routing table, negating the need to disable split horizon.
The final alternative method of disabling split horizon is to configure manually EIGRP neighbour statements on all routers using the neighbor router configuration command. Because updates between neighbours are Unicast when this configuration is used, the split horizon limitation is removed. This option works well in small networks; however, as the network grows and the number of spoke routers increases, so does the configuration overhead.
Given that the configuration of both EIGRP default routing and static neighbours was described in detail in earlier sections in this module, the configuration of these features is omitted for brevity.