Tuesday, March 07, 2006


Converting an ABR to a ABR+ASBR. (Question #30)

Consider a router running OSPF that is an ABR. The configuration for the OSPF router process on this router is

router ospf 100
network area 0
network area 0
network area 1

There are no other dynamic routing protocols running on the box. The output of show ip ospf indicates that this is an ABR. Can you type just one additional command under the router ospf process in the configuration to cause show ip ospf to report this system as an ASBR in addition to being an ABR?

I believe as soon as you configure the router to redistribute it will become an ASBR. Since we don't have other routing protocols, or really want to configure anything just for this purpose, and easy way should be to redistribute connected routes. These should be advertized anyhow, so it shouldn't affect anything.

And if it is an issue, there are various ways to have it not actually redistribute anything (i.e. ACL that is just 'deny any').

Is this correct?
What I had in mind was to just type in

default-information originate always

under the router ospf configuration.

That would cause the router to start generating type-5 LSAs for the default
route as an external route and so the system gets reported as an ASBR
in addition to being an ABR.

However, as you say configuring any redistribution should also do the trick as the ABR would then become an ASBR.
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