About the Author: Calin is a network engineer, with more than 8 years of experience in designing, installing, troubleshooting, and maintaining large enterprise WAN and LAN networks. Currently he's Cisco (CCNA, CCNP, CCIP) and Linux (LPIC) certified and working to obtain CCIE certification.

How to force Loopack interface to be advertised with a /24 netmask


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

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  1. [...] 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. [...]

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