Subject: area-border router's behaviour

From: "Kostiner, Assaf (Assaf)" <[email-masked]>
Date: Wed, 23 Oct 2002 09:29:42 +0200

Hi,
There are couple of issues i can't understand:
1. In RFC an area-border router (ABR) is defined as one that is attached to multiple areas, while Cisco and IBM defined later ABR as one that is physically or virtually connected to the backbone. I understand theire reason for changing the RFC definition. What i don't understand, is why does the RFC allows a router which is connected for example only to areas 1 and 2, to be an ABR, and to advertise summaries from one to the other. This router is allowed to use only summaries received from area 0, when running SPF, so what's the point?
2. Overlapping area ranges - in RFC 1583, an ABR which aggregates all subnets of 10.0.0.0/8 ignores summaries which fall into this range, like 8.3.0.0/16. That's why, any such subnet outside the aggregated area will become unreachable. Why does the ABR ignores all such summaries. If it didn't ignore them, all subnets could be reachable...

I would appreciate any help, Assaf.

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From: Acee Lindem <[email-masked]>
Date: Wed, 23 Oct 2002 09:48:44 -0400

Kostiner, Assaf (Assaf) wrote:

> 1. In RFC an area-border router (ABR) is defined as one that is attached
> to multiple areas, while Cisco and IBM defined later ABR as one that is
> physically or virtually connected to the backbone. I understand theire
> reason for changing the RFC definition. What i don't understand, is why
> does the RFC allows a router which is connected for example only to
> areas 1 and 2, to be an ABR, and to advertise summaries from one to the
> other. This router is allowed to use only summaries received from area
> 0, when running SPF, so what's the point?

Hi Assaf,

Routers internal to area 1 and 2 will use the summaries advertised into
their respective areas. They will be able to reach other even though
their ABR is not connected to the backbone. However, you would not want
to design a network without a backbone area or ABRs without backbone
interfaces.

>
> 2. Overlapping area ranges - in RFC 1583, an ABR which aggregates all
> subnets of 10.0.0.0/8 ignores summaries which fall into this range, like
> 8.3.0.0/16. That's why, any such subnet outside the aggregated area will
> become unreachable. Why does the ABR ignores all such summaries. If it
> didn't ignore them, all subnets could be reachable...

In RFC 2328 (the current OSPF specification), overlapping subnet ranges
are supported. If you have an ABR that ignores overlapping summaries
then it doesn't properly support RFC 2328.

--
Acee