Subject: Re: Re: [Isis-wg] Summary Address

From: "Purushothaman NandaKumaran" <[email-masked]>
Date: 7 Jun 2001 04:32:37 -0000

Hi all,
Thanks for replying.
My doubt got extended.
What happens if some external route with external metric falls on that summary route?
Can we make metric and metric type configurable in the summary address table of the mib and leave the problem to the administrator?
thanks
with best regards,
Purush

------------- Original Message --------------
Henk Smit <[email-masked]> wrote:
To:[email-masked] (Purushothaman NandaKumaran)
From:Henk Smit <[email-masked]>
Date:Wed, 6 Jun 2001 15:25:42 -0700 (PDT)
CC:[email-masked] ([email-masked])
Subject:Re: [Isis-wg] Summary Address

> I have some doubts in advertising summary routes.
> 
> 1) Whether the configured summary address will go as IP internal or
>     external reachability information in the generated LSP's?

  Not defined anywhere, AFAIK.

> 2) If some redistributed routes fall under the summary route
>     (redistributed routes will be sent as IP external reachability
>     information) and if some level 1 routes also > fall under the same
>     summary route (level 1 to level 2 which should go as IP internal
>     reachability information) > then how should we advertise that summary
>     route?

  Not defined anywhere, AFAIK.

> Please clarify my doubts.

  See RFC1195.
 We quote the relevant section in rfc2966.

   RFC 1195 states this quite clearly in the note in paragraph 3.10.2,
   item 2c).  This document does not alter this rule of preference.

       NOTE: Internal routes (routes to destinations announced in the
       "IP Internal Reachability Information" field), and external
       routes using internal metrics (routes to destinations announced
       in the "IP External Reachability Information" field, with a
       metric of type "internal") are treated identically for the
       purpose of the order of preference of routes, and the Dijkstra
       calculation.

  So for route computation, it doesn't matter if you advertise your
 summaries as TVL128 or TVL130. So if it doesn't create routing loops,
 doesn't create suboptimal routing, and doesn't create interoperability
 problems, this problem is non-relevant. Some routers can advertise their
 summaries in TVL128, some might advertise summaries in TVL130. No problem.

  Pick whatever you like best .....
 Hope this helps.

       henk.

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From: Henk Smit <[email-masked]>
Date: Wed, 6 Jun 2001 21:40:17 -0700 (PDT)

> What happens if some external route with external metric falls on

  Summary-addresses are advertised in combination with a metric.
 If you have 10 routes that are more specific to that summary-address,
 each route with a different metric, what metric will you choose to be
 advertised for the summary ?
  Answer is simple: you take the lowest metric of all metrics associated
 with all the more specifics.

  Apply that rule to the internal/external metric-type situation .....

  You can view internal metrics as metrics that are always lower (better)
 than any external metrics. So if you apply the rule above to the
 internal/external metric-type situation, it is easy to see that if
 a summary has some specifics with internal metric and some specifics
 with external metric, the logic forces you to advertise the summary
 with an internal metric.

> Can we make metric and metric type configurable in the summary
> address table of the mib and leave the problem to the administrator?

  Ugh. A very cowardly solution. :-) ;-)
 As I explain above, there is no need to leave this unsolved. 

     henk.