Disabling EIGRP on an Interface

Problem

You want to disable an interface from participating in EIGRP.

Solution

You can prevent an interface from participating in EIGRP by simply designating it as passive:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router eigrp 55
Router1(config-router)#passive-interface Serial0/1
Router1(config-router)#exit
Router1(config)#end
Router1#

Discussion

The passive-interface command in EIGRP prevents directly connected routers from establishing an EIGRP neighbor relationship. Since they can't become neighbors, they will never exchange routing information. This is critically different from the way RIP behaves, as we saw in Chapter 6. In RIP, making an interface passive means that it will still accept routes; it just won't send them. But with EIGRP, a passive interface will not send or receive any routing information.

Furthermore, configuring one router to be passive means that it can't form an EIGRP adjacency relationship with any other routers through this interface. So if there are only two routers on a link, you can disable EIGRP on that link by simply configuring one of the routers with a passive interface.

You can see the neighbor relationships with the following command:

Router1#show ip eigrp neighbors
IP-EIGRP neighbors for process 55
H Address Interface Hold Uptime SRTT RTO Q Seq Type
 (sec) (ms) Cnt Num
0 172.25.2.2 Se0/0.2 11 00:07:03 1563 5000 0 81 
3 172.25.1.7 Fa0/0.1 77 00:18:17 11 200 0 348 
2 172.22.1.4 Fa0/1 12 00:18:42 4 200 0 197 
1 10.1.1.1 Se0/1 14 00:18:43 7 200 0 196 
Router1#

If we then implement the passive-interface command on this router, as shown above, you can see that the neighbor disappears from the table:

Router1#show ip eigrp neighbors
IP-EIGRP neighbors for process 55
H Address Interface Hold Uptime SRTT RTO Q Seq Type
 (sec) (ms) Cnt Num
0 172.25.2.2 Se0/0.2 14 00:08:56 1563 5000 0 81 
3 172.25.1.7 Fa0/0.1 69 00:20:10 11 200 0 348 
2 172.22.1.4 Fa0/1 12 00:20:35 4 200 0 197 
Router1#

The show ip protocols command lists all of the passive interfaces that are configured on this router:

Router1#show ip protocols 
Routing Protocol is "eigrp 55"
 Outgoing update filter list for all interfaces is not set
 Redistributed static filtered by 7
 Incoming update filter list for all interfaces is not set
 Default networks flagged in outgoing updates
 Default networks accepted from incoming updates
 EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
 EIGRP maximum hopcount 100
 EIGRP maximum metric variance 1
 Redistributing: static, eigrp 55
 Automatic network summarization is in effect
 Automatic address summarization:
 172.25.0.0/16 for FastEthernet0/1
 Summarizing with metric 28160
 172.22.0.0/16 for FastEthernet0/0.1, Serial0/0.2, Loopback0
 Summarizing with metric 28160
 10.0.0.0/8 for FastEthernet0/0.1, Serial0/0.2, Loopback0
 FastEthernet0/1
 Summarizing with metric 3845120
 Maximum path: 4
 Routing for Networks:
 10.0.0.0
 172.22.0.0
 172.25.0.0
 Passive Interface(s):
 Serial0/1
 Routing Information Sources:
 Gateway Distance Last Update
 172.25.1.7 90 00:09:57
 172.25.2.2 90 00:09:57
 172.22.1.4 90 00:09:57
 Distance: internal 90 external 170
Router1#

A useful variant of the passive-interface command is passive-interface default:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router eigrp 55
Router1(config-router)#passive-interface default
Router1(config-router)#no passive-interface Serial0/1
Router1(config-router)#exit
Router1(config)#end
Router1#

This command makes all of the router's interfaces passive by default so that they will not take part in EIGRP. Then you can specifically enable only those interfaces that you want to take part by using a no passive-interface command. This is particularly useful when there are a lot of interfaces on the router and only a few of them will be running the routing protocol.