Redistribution: Passive Interfaces

Not really specifically a redistribution concept, but categorised by Chris Bryant as redistribution so what the hell.

Passive Interfaces

Passive interfaces can be a big help in controlling routing updates and or/ routing control traffic, depending on which protocol you’re dealing with:

RIP: Passive interfaces do not send routing updates, but will accept them. RIP adjacencies aren’t affected by passive interfaces since RIP doesn’t have adjacencies in the first place. A RIP passive interface will not send routing updates, but it will accept them.

EIGRP: Hellos aren’t sent, so no adjacency can be formed via a passive interface. If an adjacency exists on an interface that is then made passive, the adjacency is dropped. A subnet running a passive interface can be advertised.

EIGRP passive interfaces do not send Hellos, therefore the neighbour relationship on this specific interface is torn down.

EIGRP passive interfaces do not send Hellos, but the subnet running on that passive interface can still be advertised via the network command.

OSPF: Passive interfaces do not send OSPF Hellos, so again no adjacency can be formed, and existing adjacencies are lost on interfaces that are then configured as passive. Additionally, the subnet running on the passive interface will be advertised as a stub network.

Just as with EIGRP, the adjacency through the now passive interface is lost, but the subnet is still advertised via the network command.

You can set all interfaces on a router as passive for a given protocol with the passive-interface default command.

R3(config)#router ospf 1
R3(config-router)#passive-interface default

To set the interfaces back to their default, just use the no passive-interface default command.

R3(config-router)#no passive-interface default