Subject: [Isis-wg] attached bit

From: "Christian E. Hopps" <[email-masked]>
Date: Mon, 5 Jun 2000 01:56:50 -0400

I was looking at ``Technical Corrigendum 1'' to 10589.. and I noticed
that it changes when an IS should consider itself attached.

It adds the condition that if the IS has at least one active
reachable address prefix it should consider itself attached.

What is the intention of the change to 10589?  My initial thought
is since the reachability won't be advertised into level 1
the level 1-2 router sets ATT to attract traffic to itself.
I suspect I may be missing something though since this change
would seem to blackhole things.

In any case what is the analogous situation in IP?
Would it be:

	if the IS is announcing any external IP reachability.

?

If so thats easy enough but things get more interesting in light of
the domain wide draft.  For example, domainwide mentions how people
have allowed external reachability to be announced into level 1,
in which case I think that you would not set the ATT bit.  Also, if
you are leaking level 2 reachability into level 1 there too I don't
think setting ATT would be appropriate.

I would appreciate any clarification people can offer.

Thanks,
Chris.

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From: Henk Smit <[email-masked]>
Date: Mon, 5 Jun 2000 15:34:05 +0200 (MEST)

Hi Chris,

> the level 1-2 router sets ATT to attract traffic to itself.

  As usual, attracting traffic is the reason to set the ATT bit.

> would seem to blackhole things.

  The paragraph in the corrigendum seems to explain it.
 The old rule was: set ATT if you see other areas in your domain.
 The problem is, what do you do if your area is the only area in
 your domain ? If there is only one area in the domain, the routers
 in that area will never see other areas, and thus were never allowed
 to set the ATT bit.

  If your domain is connected to other domains, at least one L1L2 router
 should forward traffic from L1-only routers to the other domain. With
 the old spec that wouldn't happen, because the L1L2 router wasn't allowed
 to set the ATT bit. The corrigendum adds explanation that a L1L2 router
 is allowed to set ATT if it sees either: 1) other areas in its own
 domain, or 2) sees other domains.

> 	if the IS is announcing any external IP reachability.

  I think the rule is the same:
 set ATT if you are connected to either 1) other areas, or 2) other domains.
 How to determine if you are connected to another area or domain
 is kinda implementation specific, I am afraid. Esp for IP, as IP
 addressing is not strictly hierarchical.

> in which case I think that you would not set the ATT bit.

  Agreed. If you leak specific prefixes into L1, you attact traffic
 for for those destinations. IMHO there is no need to also set the ATT bit.
 Unless you leak only a few external prefixes, and don't leak all external
 prefixes.

>  Also, if

  Agreed again.

> I would appreciate any clarification people can offer.

  I think a L1L2 router should set the ATT bit if it can forward traffic
 to destinations outside of the L1 area, and those destinations are not
 explicitely known by L1-only routers.

  Hope this helps,

         Henk.