[rbridge] Distribution Tree and ECMP

[Radia Perlman]

Yup. I might have been the culprit who did that wording. At any rate, 
yeah -- it should be the "ID of
the parent node", so as you said, it should be the IS-IS 7-byte ID of
the node, whether the "parent" is a pseudonode or a regular node. 
"System ID" implies the 6 byte ID of a router,
and that is incorrect in this case, as you point out.

Thanks for catching that.

Radia

Ayan Banerjee (ayabaner) wrote:
> Suresh,
>
> I agree with your observation and in the event these links are P2P we
> should use the extended circuit-id to break the tie.
>
> Thanks,
> Ayan 
>
> -----Original Message-----
> From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
> Behalf Of Suresh Boddapati
> Sent: Monday, June 23, 2008 10:10 PM
> To: rbridge at postel.org
> Subject: [rbridge] Distribution Tree and ECMP
>
> 4.3.1 of the base protocol spec says: 
>
>    "When a node RBn has two or more minimal equal cost paths toward the
> Root RBi a deterministic tie-breaker is needed to guarantee that all
> Rbridges calculate the same distribution tree. This is obtained by
> selecting the path that goes to the parent that has the lower IS-IS
> System ID." 
>
> This however does not cover the case when the ECMP paths are through the
> same RBridge. For example, 
>
>           
> A ----- B ---------- C
>           ----------
>
> If B is the designated IS for both the links connected to C for the tree
> rooted at A, it is possible that C might compute the top link to be on
> the distribution tree whereas B might compute the bottom one.
>
> It seems to me that the algorithm will be more deterministic if the LAN
> ID were to be used for tie-breaking, when System IDs are the same. In
> this case if the top link's LAN ID were to be B.01 and the bottom link's
> were to be B.02, both could deterministically decide that B.01 should be
> the link on the distribution tree. 
>
> Comments?
>
> Suresh
>
>           
>
>
>
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