How to force Loopack interface to be advertised with a /24 netmask
Calin | Nov 12, 2008 | Comments 1
Many of you already know that usually when you advertise a Loopback interface into OSPF it is advertised with /32 netmask (one IP address) even if the IP address under Loopback interface is with a /24 netmask. If you checked my previous post, you’ll see at the end that I’m right, and the Lo0 address is advertised with /32 netmask.
But what if you have to advertised as a /24 subnet (task require or some testing) ? There is “dirty little trick” to do it. I will use the same topology and design like in the post before. If you do no have the topology please download it here.
Watch the movie:
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Filed Under: Cisco-IGP
[...] R3: As well on R3, the IP subnet between R2 and R3 is present in OSPF area 0 (zero) and the other 2 subnets (10.10.1.0 /24 and 10.10.2.0 /24) are in OSPF area 1 (one). Now, if you remember from my older post, if you have a Loopback interface with a IP address (doesn’t matter what netmask) this will be always advertised in OSPF as /32. I did a little trick to be sure that they are still advertised in OSPF as presented under Loopack 1 (10.10.1.1 /24) and Loopback 2 (10.10.2.1 /24). You want to know how, check this post. [...]