Subject: Multiple ABR summarization

From: Andrew Smith <[email-masked]>
Date: Fri, 11 Mar 2005 14:21:24 -0500

Group,
If two ABRs for a regular sub-tending area are summarizing the same
intra-area routes into the backbone area, do both ABRs send the same LSA-3
advertisement or does one ABR defer the announcement to the other ABR?

I believe for NSSA areas one of the ABRs will not send the LSA-3
summarization (might be only for external routes) into the backbone area. I
was wondering if this behavior also occurred on regular areas.

Regards,
Andy Smith

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From: Erblichs <[email-masked]>
Date: Fri, 11 Mar 2005 11:48:54 -0800

Group,

	Don't both ABRs summarize?

	Why would they even want to defer?

	Mitchell Erblich
	--------------------

> Andrew Smith wrote:
> intra-area routes into the backbone area, do both ABRs send the same
> LSA-3 advertisement or does one ABR defer the announcement to the
> other ABR?
> 
> summarization (might be only for external routes) into the backbone
> area. I was wondering if this behavior also occurred on regular areas.
> 
> Regards,
> Andy Smith

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From: Roch Guerin <[email-masked]>
Date: Fri, 11 Mar 2005 14:50:38 -0500

Andy,

As far as I know, this rule only applies to external LSAs. See section 
12.4.4.1 of RFC 2328, which states:

                    The following rule is
                    thereby established: if two routers, both reachable
                    from one another, originate functionally equivalent
                    AS-external-LSAs (i.e., same destination, cost and
                    non-zero forwarding address), then the LSA
                    originated by the router having the highest OSPF
                    Router ID is used.  The router having the lower OSPF
                    Router ID can then flush its LSA.

There is no similar definition for "functionally equivalent" summary 
LSAs, and even if the two ABRs advertised the same summary route with 
the same cost, this still represents different connectivity options, 
e.g., one may be much closer that the other for some routers in the 
area.  This is unlike the example of section 12.4.4.1 where routing is 
unaffected whether one or two ASBRs advertise the (functionally 
equivalent) external LSA.

Hope this helps.

Roch

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From: Acee Lindem <[email-masked]>
Date: Fri, 11 Mar 2005 15:48:27 -0500

> equivalent) external LSA.

Roch is correct. AS external LSAs may include a forwarding addresses and 
the advertised
route can be exactly the same with a non-zero forwarding address.

Thanks,
Acee