EIGRP by default uses multicast for neighbor discovery but it also allows you to configure EIGRP neighbors statically. Once you do this, EIGRP will only use unicast and disables EIGRP multicast on the selected interface.
This could be useful in certain scenarios where multicast is not supported or when you want to reduce the overhead of multicast traffic. Here’s an example:
Above we have a frame-relay hub and spoke network. The hub and spoke1 routers are the only two routers that are running EIGRP. When the hub router sends an EIGRP multicast packet, it will be replicated on all PVCs. All 4 spoke routers will receive this multicast traffic even though only spoke1 is interested in it.
In a scenario like this, it would make sense to configure the EIGRP neighbor statically so that multicast won’t be used.
Let’s see how we can configure EIGRP static neighbors…
For this demonstration I’ll use the following two routers:
R1 and R2 are connected through frame-relay. Here’s the configuration of the interfaces:
R1#show run | begin Serial0/0 interface Serial0/0 ip address 192.168.12.1 255.255.255.0 encapsulation frame-relay clock rate 2000000 frame-relay map ip 192.168.12.2 102 no frame-relay inverse-arp
R2#show run | begin Serial0/0 interface Serial0/0 ip address 192.168.12.2 255.255.255.0 encapsulation frame-relay clock rate 2000000 frame-relay map ip 192.168.12.1 201 no frame-relay inverse-arp
Above you can see that frame-relay Inverse ARP has been disabled, two static frame-relay maps are used for our mappings. This means that we are unable to send any broadcast or multicast traffic through this PVC. You can also verify this with the following command:
R1#show frame-relay map Serial0/0 (up): ip 192.168.12.2 dlci 102(0x66,0x1860), static, CISCO, status defined, active
R2#show frame-relay map Serial0/0 (up): ip 192.168.12.1 dlci 201(0xC9,0x3090), static, CISCO, status defined, active
As you can see the frame-relay mappings are there but the broadcast keyword is missing. Let’s configure EIGRP to use static neighbors:
R1(config)#router eigrp 12 R1(config-router)#network 192.168.12.0 R1(config-router)#neighbor 192.168.12.2 Serial 0/0
R2(config)#router eigrp 12 R2(config-router)#network 192.168.12.0 R2(config-router)#neighbor 192.168.12.1 Serial 0/0
You only have to use the neighbor command to specify the remote neighbor and the interface to reach it. After a few seconds the neighbor adjacency will appear:
R1# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.2 (Serial0/0) is up: new adjacency
R2# %DUAL-5-NBRCHANGE: IP-EIGRP(0) 12: Neighbor 192.168.12.1 (Serial0/0) is up: new adjacency
You you can also verify your static neighbors with the show ip eigrp neighbors command: