EIGRP Basics (Passive Interfaces)

When EIGRP is up and operational, unicast and multicast EIGRP packets or constantly flooding the network to make sure EIGRP routes are up and are using the best path. These packets are important but not always necessary. There are situations when disabling an interfaces ability to spam the network with EIGRP packets is appropriate and will still allow for a fully operational EIGRP convergence. The way to achieve this is by configuring an interface to passive mode. EIGRP will still advertise the network/subnet configured on that interface, however, the interface will not participate in sending or processing of EIGPR packets.

Here is an example:

By setting the Lo1 interface to passive mode, it won’t be busy with EIGRP packets but EIGRP will still advertise the 172.18.1.0/24 network. The primary purpose for this would be to alleviate that interface from processing all those unneeded packets. The reason they are not needed is because there are no other networks beyond that interface that might need to be advertised, thus, that interface will never have a neighbor and for that reason there is no need to send send unicast/multicast EIGPR packets.

Config:

Router3#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router3(config)#router eigrp 1
Router3(config-router)#passive-interface loopback 1
Router3(config-router)#end

Verifying that the interface is in passive mode:

Router3#show ip protocols | b eigrp

You can see we have 1 passive interface which is Loopback1 and EIGRP is still advertising the 172.18.1.0/24 network.

You can verify that the route for 172.18.1.0/24 is still seen by other routers by checking a neighbors routing table.

Router1#show ip route eigrp

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