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

Advanced EIGRP Concepts: Passive Interfaces + Propagating Default Routes

Passive Interfaces

An interface that is declared ‘passive’, is an interface that doesn’t participate in EIGRP. The interface will not send any Hello packets to any neighbours and therefore not form an adjacency.

  • Option 1: Use the passive-interface default command to make every EIGRP-enabled interface on the router passive, and then use the no passive-interface command to indicate the interfaces that should not be passive.
  • Option 2: Use the no passive-interface default command to disable this feature globally, and then use the passive-interface command to indicate interfaces that should be passive.

Option 1 makes a lot more sense and is a best practice to know intentionally configure the interfaces that SHOULD participate in your EIGRP process. When setting up a new EIGRP router, I would personally set every interface as passive and then nail up the interfaces you want for EIGRP with the no passive command. (We don’t want to accidentally form neighbour adjacencies on unknown interfaces)

This command is performed globally with the EIGRP process and not on an interface level.

EIGRP and the default route

There are several ways of propagating a default route into EIGRP:

  1. We can use the redistribute static command within EIGRP, this will redistribute any static routes such as the default route into the routing process. *We must also specify the K values (Bandwidth, Delay, Reliability, Load and MTU) as per standard redistribution INTO an EIGRP AS.
  2. Using the ‘ip default-network’ command, however we must be sure that the default route is within the routing table of the local router that we issue this command on. This command is issued in global config, not within EIGRP.
  3. Using a network statement with the EIGRP AS. Yes we can in fact use ‘network’ to advertise our default route to EIGRP neighbours. When we use this command the AD of the route will be internal with 90 and not 170 D EX as per a route learned by redistribution.