Subject: Summarization but still unnecessary new LSAs

From: Juergen Arlt <[email-masked]>
Date: Wed, 16 Jul 2003 09:27:07 +0200

In a very basic config a router is ABR with one interface in Area 0.0.0.0
and two interfaces in area 0.0.0.1. For those two interfaces I manually
configured a summary to get only one LSA Type 3 into the backbone area. (the
interfaces are 192.168.1.1/24 and 192.168.2.1/24 with a summarization of
192.168.0.0/16).

Now I found that whenever there is a change on those two interfaces (one
going down or up) a new summary LSA is generated into the backbone area. It
does not contain new information, only the sequence number is incremented.

In the worst case (like link flapping) the router would create new LSAs
permanently that would be flooded into the backbone area. Although no SPF
would be started the number of LSAs to process would force unnecessary CPU
load on the backbone devices.

I did not find an RFC stating how this summarization should be implemented
but the way I found this router working is probably not the best.

Can someone point me to a document if there is one?

Regards
Juergen

---

From: Acee Lindem <[email-masked]>
Date: Wed, 16 Jul 2003 09:35:24 -0400

Juergen Arlt wrote:
> In a very basic config a router is ABR with one interface in Area
> 0.0.0.0 and two interfaces in area 0.0.0.1. For those two interfaces I
> manually configured a summary to get only one LSA Type 3 into the
> backbone area. (the interfaces are 192.168.1.1/24 and 192.168.2.1/24
> with a summarization of 192.168.0.0/16).
>
> going down or up) a new summary LSA is generated into the backbone area.
> It does not contain new information, only the sequence number is
> incremented.
>
> permanently that would be flooded into the backbone area. Although no
> SPF would be started the number of LSAs to process would force
> unnecessary CPU load on the backbone devices.
>
> I did not find an RFC stating how this summarization should be
> implemented but the way I found this router working is probably not the
> best.
>

Juergen,

While an intra-area route change is one of the 10 events listed in
section 12.4, a router shouldn't re-originate the summary LSA
corresponding to the range unless the contents have changed.
Unfortunately, I can't find any place where this is stated explicitly
but, IMHO, it is common sense.

Thanks,
Acee

--
Acee