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OSPF And Passive Interfaces

Your BSCI exam may well be the most challenging of the four exams you must pass to become a CCNP, so you have to have the details of every protocol on the exam mastered! Today, we'll look at the passive-interface command as it relates to OSPF.

Passive interfaces accept routing updates, but do not send them. Regarding OSPF, even though OSPF does not sent "routing updates" in the form that RIP, IGRP, and EIGRP do, you can still configure an OSPF-enabled interface as passive in order to prevent OSPF traffic from exiting that interface. No OSPF adjacency can be formed if one of the interfaces involved is a passive interface, and if you configure an OSPF-enabled interface as passive where an adjacency already exists, the adjacency will drop almost immediately.

In the following example, R1 and R2 have an existing OSPF adjacency over their Ethernet interfaces. In an effort to reduce routing traffic, R1's e0 interface is configured as passive. The adjacency drops right away.

R1(config)#router ospf 1

R1(config-router)#passive-interface ethernet0


18:31:11: %OSPF-5-ADJCHG: Process 1, Nbr on Ethernet0 from FULL to DOWN, Neighbor Down: Interface down or detached

That's a pretty important detail to keep in mind when you're using the passive-interface command, wouldn't you say?

You may well have a router that you want to configure most interfaces as passive. There's no longer a need to configure each interface as passive in that case - As of IOS version 12.0, you can now set all interfaces on a router as passive for a given protocol with the passive-interface default command. You can then configure each interface that you do NOT want to be passive with the "no passive-interface" 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

The passive interface is a simple topic, but it can get a little tricky when you start changing the default and then start configuring interfaces on an individual level. Just be careful with this command on exam day and in the real world, and you'll succeed in the BSCI exam room and on real-world networks as well!

About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!