Why are summarized updates not sent out interfaces on different major classful networks

[wardwolfram]

Hello Team,

What is the reason that summarized updates are only sent out on interfaces with different major classful networks?  

Does this prevent loops - similar to split horizon?  Or prevent routing inconsistencies?

Thanks!

[Richard Burts · Accepted solution]

Your question would seem to relate to classful routing protocols (the older approach to routing protocols) where the summary advertisement is generated by the routing protocol and not relate to classless routing protocols (the more modern approach to routing protocols) where summary advertisements can be manually configured (and where summary advertisements can be sent out interfaces in the same major classful network).

In the classful routing protocols (such as RIPv1 or IGRP) summarization at major network boundaries is automatic. This is not related to preventing loops or preventing routing inconsistencies but relates to being efficient in the routing update. Perhaps an example will help to explain this. Let us think about some router that has 2 active interfaces. The first interface has IP address of 10.10.10.2 255.255.255.0 and connects to routerA. routerA has advertised 20 subnets within the major network of 10.0.0.0. The second interface has IP address of 172.16.1.2 255.255.255.0 and connects to routerB. routerB has advertised 15 subnets within the major network of 172.16.0.0. So the routing table in this router has 21 subnets in 10.0.0.0 and has 16 subnets in 172.16.0.0.

The basic logic here is that if you participate in a major classful network then you need full detail about all of its subnets so that you can make appropriate routing decisions. But if you do not participate in that major classful network then you only need a summary. It would be inefficient to advertise all 21 subnets of 10.0.0.0 to routerB or all 16 subnets of 172.16.0.0 to routerA. You only need to advertise 1 summary to the neighbor.

HTH

Rick

[Richard Burts]

I am glad that my explanation was helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick