OSPF Stub without no-autosummary

[Amine M.]

Hello guys,

This is my first post here. I created a multi-area OSPF topology with one particular area at the edge which I defined as "stub".

The stub router received a default route pointing to its neighbour but still its routing table showed all OSPF routes (including IA) a non-stub router with receive. 

Only after entering a "stub no-autosummary" that these OSPF routes were dropped from the routing table which became much smaller with only Connected routes.

Question: does it make any sense to use OSPF Stub without no-autosummary? 

[chrihussey]

It all depends on the network and requirements, so there isn't a yes / no answer.

For example, if the stub router had a single point of exit, or had a lot of E1/E2 routes and you wanted to keep the routing table as small as possible then yes.

However, if the router had links to multiple routers, or the stub area was larger with multiple routers and you wanted it to optimally route inside the OSPF domain then you would leave it off.

Hope this helps

[Amine M.]

Yes, it helps a lot :)

I am preparing the CCNP Route. I I have been working on the OSPF 3 weeks solid. 

[Georg Pauwen]

Hello,

when you define a stub area, you still receive all routes from other areas, including IA routes. The stub area creates a default route for external routes only, that is, routes redistributed into OSPF from another protocol.

By adding 'no-summary', you create a so-called totally stubby area. A totally stubby area keeps only the intra-area routes (the O routes), and for any inter-area routing, it has a default route.

A totally stubby area (no-summary) would actually only make sense if your OSPF network is very large and has a large amount of areas.

Does that make sense ?

This is I think a pretty good link that explains it:

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-8.html

[Amine M.]

Thank you! Yes, my area was totally stubby. I would have almost gotten away with a 0.0.0.0/0 route towards the single ABR but I needed to advertise connected networks.

I am going to read the document your shared.

[Rolf Fischer]

Hello,

if a [totally] stub area has only one ABR, there is no benefit to have inter-area routes in the routing-tables of the internal stub-routers pointing to the very same next-hop that the default-route uses.

However, if a totally stub area has more than one ABR, a internal stub-router will always choose the best default-route (originated from one of the ABRs) in order to reach inter-area destinations; and that's not necessarily always the best path to reach certain destinations. So you have a smaller routing-table and LSDB on the one hand, but possibly suboptimal paths on the other hand.

As a compromise, you could do manual summarization on the (multiple) ABRs (less routing-information but better forwarding paths).

HTH
Rolf

[Amine M.]

Thank you Rolf.

It makes more sense for me now. For some reason I always imagined a Stub area as connect to one single ABR. I am going to create an other test topology with links going to multiple routers.