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4.03.2013

OSPF Network Types Part 3 - Loopback

Well since I rambled on and on about P2P network types I had to give loopback network types their own post. I hope you feel special LOOPBACK!

OSPF loopback network types are pretty straight forward. Any hardware or software loopback interface defaults to LOOPBACK for the OSPF network type. 

Routes learned off a loopback interface are considered to be stub networks and propagated throughout the OSPF network as /32 subnets. This makes sense as loopback interfaces are most commonly used to identify the router for management purposes most commonly for RIDs. The loopback does not share a segment with other devices.


To demonstrate we will use our previous network from my P2P post.
On R3 we will add a two loopback interfaces to demonstrate. Note how I'm giving the loopback IP's a mask of /24. Usually in the real world when working with loopback interfaces you would assign a /32 but I want to demonstrate how OSPF advertises these as /32s



First lets see what OSPF thinks about the loopback interfaces.



Notice how OSPF even tells you it is treating loopbacks as stub host.

Lets check R1 to make sure the loopbacks are being advertised properly.



What the crap!! Where the hell are my loopback routes???

Show ip route is showing them as /32s but why are they not showing up in the database?



In the show ip ospf database command they are hidden under the LSA for the segment. Look at the link count at the far right. Notice how R3 and subsequently R2 is now advertising 4 links?

To see your loopback you need to dig deeper into the OSPF database. Show ip ospf database router [link-id] to the rescue!!



WOW look at all that OSPF LSDB goodness!! You can see the show ip ospf database router [link--id] command gives you a great view of the Type 1 LSAs coming from a specific OSPF router.

For whatever reason, if you ever wanted to advertise the loopbacks with the original mask then you can just change the OSPF network type to point-to-point and your all set.

Now that the simple network types are out of the way lets dig into the others! This is were things start getting interesting.