Re: [trill] Asymmetric link costs and D-Tree calculations
Donald Eastlake <d3e3e3@gmail.com> Fri, 01 March 2013 07:21 UTC
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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Fri, 01 Mar 2013 02:21:14 -0500
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To: Olen Stokes <ostokes@extremenetworks.com>
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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 <ostokes@extremenetworks.com> 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”. Yes. 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. Thanks, Donald ============================= Donald E. Eastlake 3rd +1-508-333-2270 (cell) 155 Beaver Street, Milford, MA 01757 USA d3e3e3@gmail.com > Cheers, > Olen
- [trill] Asymmetric link costs and D-Tree calculat… Olen Stokes
- Re: [trill] Asymmetric link costs and D-Tree calc… Donald Eastlake
- Re: [trill] Asymmetric link costs and D-Tree calc… Olen Stokes
- Re: [trill] Asymmetric link costs and D-Tree calc… Donald Eastlake