Configuring EIGRP Static Neighbors in Cisco

EIGRP (Enhanced Interior Gateway Routing Protocol) is an Enhanced-DVRP (Distance Vector Routing Protocol) that generally uses DUAL (Diffuse Update Algorithm) to find the best path to the destination. Since EIGRP is a Dynamic Routing Protocol, it sends the EIGRP messages (Hello, Update, Query, Reply, Acknowledgement) to a multicast address 224.0.0.10, and it uses this multicast address for discovering the neighbors across the Autonomous System (AS group of routers and networks working under a single administrative domain). However, EIGRP also allows us to change this multicast traffic to unicast traffic by adding Static Neighbors instead of Dynamically discovering them. Using Static Neighbors is useful in cases when the admin wants to limit the multicast traffic across a particular interface or if multicast is not supported. These Static Neighborship formed in EIGRP supersedes the Dynamically formed Neighborship. 

For example, if we enable EIGRP routing for two connected routers,

• To form Dynamic Neighborship, we can just simply add the connected network using the below command: –

router(config)#router eigrp <AS_number>
router(config)#network <network_IP> <wildcard_mask>
router(config)#exit

• To create Static Neighborship, we have to first add the network in the EIGRP topology, then configure the static neighbor using its IP and the exit interface of the router.

router(config)#router eigrp <AS_number>
router(config)#network <network_IP> <wildcard_mask>
router(config)#neighbor <ip_add_neighbor> <exit_interface_id>
router(config)#exit

This neighbor command stops sending and receiving any multicast traffic for EIGRP on that exit interface, i.e., if the router is sending unicast traffic to create a Static Neighborship it also wants a unicast reply in return on that interface to successfully create the neighborship.

Topology:

Configuring IP Addresses On Routers:

R1: 

R1(config)#int f0/0
R1(config-if)#ip add 10.1.1.1 255.255.255.0
R1(config-if)#no sh
R1(config-if)#exit
R1(config)#int f1/0                     
R1(config-if)#ip add 11.1.1.1 255.255.255.0
R1(config-if)#no sh 
R1(config-if)#exit

R2: 

R2(config)#int f0/0
R2(config-if)#ip add 10.1.1.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#exit
R2(config)#int f1/0                     
R2(config-if)#ip add 12.1.1.1 255.255.255.0
R2(config-if)#no sh 
R2(config-if)#exit

R3: 

R3(config)#int f0/0
R3(config-if)#ip add 12.1.1.2 255.255.255.0
R3(config-if)#no sh
R3(config-if)#exit

R4: 

R4(config)#int f0/0
R4(config-if)#ip add 11.1.1.2 255.255.255.0
R4(config-if)#no sh
R4(config-if)#exit

Configuring EIGRP on all Routers:

R1:

R1(config)#router eigrp 100
R1(config-router)#network 10.1.1.0 0.0.0.255
R1(config-router)#network 11.1.1.0 0.0.0.255
R1(config-router)#exit

R2: 

R2(config)#router eigrp 100
R2(config-router)#network 10.1.1.0 0.0.0.255
R2(config-router)#network 12.1.1.0 0.0.0.255
R2(config-router)#exit

R3: 

R3(config)#router eigrp 100
R3(config-router)#network 12.1.1.0 0.0.0.255
R3(config-router)#exit

R4: 

R4(config)#router eigrp 100
R4(config-router)#network 11.1.1.0 0.0.0.255
R4(config-router)#exit

Configure Static Neighborship Between R1 and R2:

Step 1: First, let’s just configure static neighbor on R1 only and see the difference using debugging.

R1(config)#router eigrp 100
R1(config-router)#neighbor 10.1.1.2 f0/0
R1(config-router)#exit

R1#debug eigrp packets

As you can see, R1 is receiving multicast hello messages of EIGRP from R2, but since we have now configured that interface as a static neighbor, that interface can only accept and send unicast hello messages of EIGRP and because of this behavior it is now ignoring the multicast hello messages received on that interface.

Let’s check the behavior on R2 after this configuration.

R2#debug eigrp packets

As you can see, R2 is also ignoring the unicast hello messages sent by R1.

Since both the routers are not accepting each other’s hello messages, the dynamic neighborship also gets compromised after the EIGRP hold time.

Step 2: Configure Static Neighbor on R2.

R2(config)#router eigrp 100
R2(config-router)#neighbor 10.1.1.1 f0/0
R2(config-router)#exit

Step 3: Verify the Neighborship

R1: 

R1#show ip eigrp neighbor detail

In the output, it is clearly visible that R1 has a static neighborship with 10.1.1.2 (R2) and a dynamic neighborship with 11.1.1.2 (R4).

R2: 

R2#show ip eigrp neighbor detail

In the output, it is clearly visible that R2 has a static neighborship with 10.1.1.1 (R1) and a dynamic neighborship with 12.1.1.2 (R3).

R3: 

R3#show ip eigrp neighbor detail

Since R3 has only one neighbor that is 12.1.1.1 (R2) with which it has a dynamic neighborship.

R4: 

R4#show ip eigrp neighbor detail

Since R4 has only one neighbor that is 11.1.1.1 (R1) with which it has a dynamic neighborship.