[rbridge] Distribution Tree and ECMP

[Ayan Banerjee (ayabaner)]

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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