Open socket on OSPF Stub interfaces

Alex Bligh alex at
Tue Jul 5 15:30:04 CEST 2011


>> As all my routes are directly connected, my plan was simply to
>> redistribute into OSPF.
> You mean export them to OSPF as external routes? Yes, that is probably
> the best solution, having too many stub networks have problems (large
> LSAs) and limits (about several thousands stub networks by one router).

That was my initial plan, until I discovered bird did not support
NSSA, and...

>> However, the issue here is that some of the
>> routes are interface routes (i.e. routed to the interface rather
>> than the interface being numbered). For reasons I don't fully
>> understand, a numbered interface can form part of an OSPF
>> stubby area, but an interface route can't (wrong LSA type).
> You mean that you cannot export external LSAs to stub areas? Yes,
> that is true.

... that made me want to do the following:

>> In the absence of NSSA, is changing the LSA type of redistributed
>> connected and interface routes an option? (this also has the
>> advantage I don't need to reconfigure bird when adding/removing
>> routes).
> I do not understand how you want to change the LSA type.

OK, what I meant was this. Redistribution into an NSSA creates
(from memory) type 7 LSAs. I can't do that (or indeed any
redistribution) into a stub area, as I'm only permitted
LSA types 1, 2, 3 and 4 in a stub.

However, a normal "interface" route (i.e. an attached network)
would create a type 2 LSA in a stubby area. I don't
quite see why redistributed statics pointing at interfaces
should not also generate type 2 LSAs in a stubby area. IE
they would behave as if the interface was numbered. How
a route "at the interface" rather a numbered interface
and an accompanying kernel interface related route be any
different from the routing protocol's point of view?

I have hundreds (or perhaps thousands) of such routers,
each carrying tens (or perhaps hundreds) of interface routes,
and I'd really like it if they didn't have to see the LSA's
from eachother (i.e. they only heard a default route). I
think these days Cisco supports OSPF filtering which is
another way to do this I suppose.

Alex Bligh

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