ospf and (lack of) auto summarization support

smcclenaghan

Ok, just the process of making this post will probably be enough to help me remember, but just in case...

I keep getting burned on the practice questions about which routing protocols support auto summarization.

RIP, RIPv2 and EIGRP do.

OSPF and IS-IS don't.

*Why* doesn't OSPF support auto summarization? Has anyone got a good idea? Google-fu turns up plenty of pages that it doesn't, but not why it doesn't. I'm hoping that knowing why will help me remember.

NetworkVeteran

smcclenaghan wrote: »

I keep getting burned on the practice questions about which routing protocols support auto summarization.

RIP, RIPv2 and EIGRP do.

OSPF and IS-IS don't.

*Why* doesn't OSPF support auto summarization?

With a link-state protocol, you can't just summarize whenever you encounter a classful boundary, because the topology database on all the devices in the same vicinity needs to be identical. Cisco could've used the same "auto-summary" command to automatically summarize on only (for example) ABRs and ASBRs if they happened to fall on classful boundaries, but an argument against that is that using the same command with different meanings would be confusing.

smcclenaghan

Thanks for the response. Honestly I'll have to think about it awhile for it to sink in. But thanks.

Dieg0M

OSPF does not support auto-summarization because of how Dijsktra algorithm is calculated. The topology database (LSDB) needs to be identical in every router of the area for SPF to calculate the best path for prefixes. That is why OSPF allows summarization only on ABR and ASBR's routers.

smcclenaghan

Thanks, both of you.

Quick question though. Doesn't EIGRP also require that the topology database be identical on all routers in the same AS? It does support automatic summarization though.

NetworkVeteran

smcclenaghan wrote: »

Doesn't EIGRP also require that the topology database be identical on all routers in the same AS?

Nope. EIGRP is a distance-vector protocol.

smcclenaghan

Ah, that's right... or it wouldn't need DUAL.

Ok, thanks again for the help NetworkVeteran.

You folks on this board are amazing.