Subject: Re: Multiple ABR summarization

From: Andrew Smith <[email-masked]>
Date: Mon, 14 Mar 2005 09:42:30 -0500

Thanks very much for the clarification. I thought this was the case but it
always helps to be sure.
 
Regards,
Andy Smith

-----Original Message-----
From: Mailing List [mailto:[email-masked]] On Behalf Of Roch
Guerin
Sent: Friday, March 11, 2005 2:51 PM
To: [email-masked]
Subject: Re: Multiple ABR summarization

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

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