Subject: Re: [Idr] AS_SET versus AS_PATH

From: "john smith" <[email-masked]>
Date: Thu, 21 Oct 2004 18:19:56 +0000

hi Curtis,

>Tony's statement is 100% accurate.  Please go back and reread the
>parts of the BGP spec that deal with AS path and loop prevention.

yes.

>
>Search for the word loop in the spec.
>
>    If the AS_PATH attribute of a BGP route contains an AS loop, the BGP
>    route should be excluded from the Phase 2 decision function.  AS loop
>    detection is done by scanning the full AS path (as specified in the
>    AS_PATH attribute), and checking that the autonomous system number of
>    the local system does not appear in the AS path.  Operations of a BGP
>    speaker that is configured to accept routes with its own autonomous
>    system number in the AS path are outside the scope of this document.
>
>Regarding AS_SET and loop avoidance.
>
>       An AS_SET implies that the destinations listed in the NLRI can be
>       reached through paths that traverse at least some of the con-
>       stituent autonomous systems. AS_SETs provide sufficient informa-
>       tion to avoid routing information looping; however their use may
>       prune potentially feasible paths, since such paths are no longer
>       listed individually as in the form of AS_SEQUENCEs. In practice
>       this is not likely to be a problem, since once an IP packet
>       arrives at the edge of a group of autonomous systems,

the part I like:

"the BGP
>       speaker at that point is likely to have more detailed path infor-
>       mation and can distinguish individual paths to destinations."

...this was my question..... as you know :)

>
>I hope that answers your questions.  This was rather easy to find, so
>in the future could you please look at the BGP spec before posting a
>question about BGP basics.

I guess so! Sorry folks! Time to look at "Wren and Martin" and then read BGP 
specs ;) (I like the 'MAY" on top)

-JS

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From: Curtis Villamizar <[email-masked]>
Date: Sat, 23 Oct 2004 00:28:48 -0400

> ...this was my question..... as you know :)

Yeah.  Sure it was.

> specs ;) (I like the 'MAY" on top)

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.

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?

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.

Curtis