Re: [trill] Asymmetric link costs and D-Tree calculations

Donald Eastlake <> Fri, 01 March 2013 07:21 UTC

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From: Donald Eastlake <>
Date: Fri, 1 Mar 2013 02:21:14 -0500
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To: Olen Stokes <>
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Subject: Re: [trill] Asymmetric link costs and D-Tree calculations
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Hi Olen,

On Thu, Feb 28, 2013 at 1:21 PM, Olen Stokes
<> wrote:
> Appendix A.1 of RFC 6325 discusses the fact that “two RBridges may know they
> are connected but each sees the link as a different speed from how it is
> seen by the other.”

Yes, and asymmetry can also occur through manual cost configuration of costs.

> We have been looking at this and how it relates to D-Tree calculation as
> discussed in Section 4.5.1.  In that section, it states, “Each RBridge RBn
> independently calculates a tree rooted at RBi by performing the SPF
> (Shortest Path First) calculation with RBi as the root”.  So, it appears
> that the D-Tree is calculated from the root RBridge outward.
> The following three paragraphs describe how to determine the potential
> parents P for a node N.  There does not appear to be any mention of
> direction so it would seem that all calculations are done in the direction
> from the root RBi outward.

That was the intent. The tree is calculated through a Dijkstra SPF
process starting at the Root and working outwards.

> The last paragraph in Section 4.5.1 states, “In other words, the set of
> potential parents for N, for the tree rooted at R, consists of those that
> give equally minimal cost paths from N to R and that have distinct IS-IS
> IDs, based on what is reported in LSPs.”  In this case, costs are discussed
> as inward from node N towards root RBi.

That text in RFC 6325 is wrong, as far as I know.

> This change in direction of cost calculation could potentially be confusing
> when there are asymmetric link costs as described in Appendix A.1.  For any
> parent P found during the SPF calculation, it is possible that the cost from
> N to
> P is different than the cost from P to N.  It is also possible that none of
> the parents found during the SPF calculation are found along any of the
> “equally minimal cost paths from N to R”.


Although I think the asymmetric costs will be rarely encountered, it
is critical that all RBridges calculate the same tree so the method
must be unambiguous.

> Is the potential change of direction when calculating costs in the last
> paragraph unintended?  Would the paragraph have been more clear discussing
> “equally minimal cost paths from R to N”?

Yes, thanks for spotting this. It should have said "from R to N" or
something like that. I'll file an Errata against RFC 6325.

> Is there agreement in the presence of asymmetric link costs that it is
> possible that none of the parents found during the SPF calculation are found
> along any of the “equally minimal cost paths from N to R” calculated for
> known unicast traffic?

It seems to me that could happen.

 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 155 Beaver Street, Milford, MA 01757 USA

> Cheers,
> Olen