Subject: Re: [Idr] AS_SET versus AS_PATH

From: "john smith" <[email-masked]>
Date: Sun, 24 Oct 2004 19:31:32 +0000

Hi Curtis,
>
>
>
>John,
>
>The above text was written with aggregation in mind which is why the
>sentence that you pointed out refers to "more detailed path
>information".  Having your own AS in the AS_SET means you can't use a
>given aggregate.  In that case, the "more detailed path information"
>is the unaggregated routes which must come to you from other paths.
>
>For the case of multipath, there are no "unaggregated routes" and so
>you can't use this route with your own AS in the AS_SET because it
>truly does loop but hopefully you have alternate.
>
>Given the following topology:
>
>
>    A ---- B
>    |      |
>    |      |
>    C ---- D ---- E
>    |             |
>     \-----F------/
>
>If a destination is learned from F by C and E, then learned from D, D
>may use C and E as a multipath.  D can advertise to B and AS_PATH of D
>[C E F].  B can advertise this to A as A B [C E F].  C cannot use that
>route.  That makes sense anyway since it is advertising the direct
>route to D.
>
>With aggregation, something meaningful could be done with more
>information.  For example, If D aggregated prefixes from C and D
>together (given that they could be aggregated), but F only sent a
>subset of prefixes in that range from C, then C could send traffic for
>the larger aggregate to D since D will either black hole it or send it
>to E.  This would not normally be done, but the aggregate could be
>accepted by C knowing that D did the aggregation before receiving
>routes from C and E.

Curtis , my point was not releated to aggregation or anything
What I was trying to point out, both in this case and in case of the 
LocaL_pref thread is that arc lengths cannot be -ve. In other words there 
has to be a <path, metric> and not just <path> else we would enter 
convergence loops.
We can increment the path/arc lenght between 2 nodes though.

>
>Can you give an example where a multipath route would have an AS_SET
>where the router receiving the route could do something useful with
>that route and more information if it had its own AS in the AS_SET?

none , but I can think of cases where , if there are 2 ECMP AS_Seq paths to 
a destination, it make make sense to send both as one could specfically 
choose to override one path. My belief is that path_length/metric cannot be 
overridden and has to exist...

>
>I don't think there is anything useful that could be done beyond
>forming the AS_SET.  If there is, then let a provider speak up and
>describe the real world situation where a solution is needed.

for that matter is AS_SET needed in the real world too?

>Curtis

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