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.