IP Split-Horizon EIGRP

IP Split-Horizon EIGRP

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Command

IP Split-Horizon EIGRP
Use

Split Horizon protects against routing loops by not accepting routes on the same interface that a router sends updates out.
This normally has to be turned off in hub and spoke topologies to ensure full reachability.
Syntax

Router(config-router)#network
OR
Router(config-router)#network
Example




In this example we will configure EIGRP 100 on all routers in the in topology.

R1(config)#router eigrp 100
R1(config-router)#no auto
R1(config-router)#network 10.4.4.0 0.0.0.255
R1(config-router)#network 1.0.0.0

Notice that EIGRP also stripes the network statements back to its classful network boundary.

3(config)#router eigrp 100
R3(config-router)#no auto
R3(config-router)#network 3.0.0.0
R3(config-router)#network 10.4.4.0 0.0.0.255

If we configure the network statement with a wildcard, we can control what interfaces are put into EIGRP.

R4(config)#router eigrp 100
R4(config-router)#no auto
R4(config-router)#network 4.0.0.0
R4(config-router)#network 10.4.4.0 0.0.0.255

Checking the EIGRP routing tables we see that R1 and R3 have not received any routes.

R1(config-router)#do show ip route eigrp
4.0.0.0/32 is subnetted, 1 subnets
D 4.4.4.4 [90/2297856] via 10.4.4.4, 00:02:34, Serial1/0
R1(config-router)#

R3(config)#do show ip route eigrp
4.0.0.0/32 is subnetted, 1 subnets
D 4.4.4.4 [90/2297856] via 10.4.4.4, 00:02:54, Serial1/0
R3(config)#

R4(config-router)#do show ip route eigrp
1.0.0.0/32 is subnetted, 3 subnets
D 1.1.1.1 [90/2297856] via 10.4.4.1, 00:00:20, Serial0/0
D 1.3.3.3 [90/2297856] via 10.4.4.1, 00:00:20, Serial0/0
D 1.2.2.2 [90/2297856] via 10.4.4.1, 00:00:20, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/2297856] via 10.4.4.3, 00:00:28, Serial0/0
R4(config-router)#


Now we will disable split-horizon on R4's frame-relay interface.
R4(config-router)#int s0/0
R4(config-if)#no ip split-horizon eigrp 100
R4(config-if)#
*Mar 1 00:14:05.915: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.4.4.1 (Serial0/0) is resync: split horizon changed
*Mar 1 00:14:05.919: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.4.4.3 (Serial0/0) is resync: split horizon changed
R4(config-if)#

Now we see all the routes on R1 and R3.
R1(config-router)#do show ip route eigrp
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/2809856] via 10.4.4.4, 00:00:58, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
D 4.4.4.4 [90/2297856] via 10.4.4.4, 00:00:07, Serial1/0
R1(config-router)#

R3(config-router)#do show ip route eigrp
1.0.0.0/32 is subnetted, 3 subnets
D 1.1.1.1 [90/2809856] via 10.4.4.4, 00:01:23, Serial1/0
D 1.3.3.3 [90/2809856] via 10.4.4.4, 00:01:23, Serial1/0
D 1.2.2.2 [90/2809856] via 10.4.4.4, 00:01:23, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
D 4.4.4.4 [90/2297856] via 10.4.4.4, 00:00:32, Serial1/0
R3(config-router)#

show ip eigrp topology

show ip eigrp topology
Command
Show IP EIGRP Topology
Use
This command shows EIGRP topology information on the router. This information includes the advertised and feasible distance for all paths as well as the successors. This is especially useful when calculating variance.
Syntax
Router#show ip eigrp topology
Example

In the below example we see the EIGRP topology information on R2
R2(config)#do show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(2.2.2.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 1.1.1.1/32, 1 successors, FD is 156160
via 10.1.1.1 (156160/128256), FastEthernet0/0
P 3.3.3.3/32, 1 successors, FD is 156160
via 10.2.2.3 (156160/128256), FastEthernet1/0
P 2.2.2.2/32, 1 successors, FD is 128256
via Connected, Loopback0
P 1.3.3.3/32, 1 successors, FD is 156160
via 10.1.1.1 (156160/128256), FastEthernet0/0
P 1.2.2.2/32, 1 successors, FD is 156160
via 10.1.1.1 (156160/128256), FastEthernet0/0
P 10.1.1.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/0
P 10.4.4.0/24, 2 successors, FD is 2172416
via 10.1.1.1 (2172416/2169856), FastEthernet0/0
via 10.2.2.3 (2172416/2169856), FastEthernet1/0
P 10.2.2.0/24, 1 successors, FD is 28160
via Connected, FastEthernet1/0