Unanswered Question
Mar 28th, 2008

As they say " its possible to use same ip address for loopback interfaces in different VRFs"

but even if ospf process running in different VRF they cannot use same router-id

but my question is "why this kind of behaviour?"

is this ospf which is not allowing this or cisco vrf implentation is the culprit ?



I have this problem too.
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Harold Ritter Mon, 03/31/2008 - 11:42


You are correct. Each ospf process (vrf or not) need to have a unique RID. This is indeed an implementation restriction.


rajatsetia Tue, 04/01/2008 - 04:24

Thanks Harrold, I agree with you on this.

I got one more input regarding this case which is based on observation and would like to share and discuss with you - correct me if i am wrong

This is how cisco implements :-

VRF (e.g. vrf lite)is basically virtual instance of routing table, so if we run ospf in an VRF its basically a differnt instance of OSPF.

so this is an implementation restriction because OSPF needs distuinguished router-id for all the instance running on a same router.

Now take the case of Redback Networks SE800, this is how they implement the Virtual routing concept:-

we can define differnt contexts (Virtual Router contexts) and OSPF process running in the context have nothig to do with OSPF process of other context so they can have same router-id.

Based on this observation can we say that OSPF has defined guidelines about the router-d and cisco is implementing its VRF concept as Virtual routing instance rather complete virtual router so have to be different router-id for each instance ..



Harold Ritter Tue, 04/01/2008 - 13:49


This is simply an implementation decision to avoid having issues if LSAs from one VRF were inadvertently leaked between separate VRFs.


rajatsetia Fri, 04/04/2008 - 03:43

Sorry for the late reply.

My doubts are now pretty much cleared

Thanks Harrold and Sam for this discussion.


cisco_lad2004 Mon, 03/31/2008 - 13:25

There has been new developments in BGP where by each address family may have its own RID.

hopefully OSPF will follow.


Harold Ritter Tue, 04/01/2008 - 11:16


ospf also allows you to configure the RID within the VRF context. But an implementation choice has been made not to allow the same RID to exist in different VRFs, just to avoid any issues if LSA are inadvertently being leaked from one VRF to the other.


cisco_lad2004 Tue, 04/01/2008 - 11:24

to clarify this, I read OSPF will only support a different ID under each VRF. which has not been the case for BGP until recently.

Is this right ?

Many thanks !


Harold Ritter Tue, 04/01/2008 - 13:46


Correct. BGP is a different story as you can have many VRF instances under the same BGP process. For ospf each process represents a separate VRF.



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