Re: [trill] Asymmetric link costs and D-Tree calculations
Donald Eastlake <d3e3e3@gmail.com> Sat, 09 March 2013 04:26 UTC
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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Fri, 08 Mar 2013 23:26:01 -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, As I said I would below, I did file an Errata against RFC 6325. Although the notice I got saying it had been filled appeared to cc the TRILL WG mailing list, I don't see that mail in the archive. So I'm posting a pointer to it here: http://www.rfc-editor.org/errata_search.php?rfc=6325&eid=3508 Thanks, Donald ============================= Donald E. Eastlake 3rd +1-508-333-2270 (cell) 155 Beaver Street, Milford, MA 01757 USA d3e3e3@gmail.com On Fri, Mar 1, 2013 at 2:21 AM, Donald Eastlake <d3e3e3@gmail.com> wrote: > 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