Hi Mike, My apologies.. I take back the comment below… SPT rooted on N does NOT have S-E link for paths to E and D1.. Sorry for the confusion once again…
I will see if I can come with any example where E and D1 can be excluded from PQ-space… If not I will modify the text to include E and D1 as well… Regards, -Pushpasis From: rtgwg on behalf of Pushpasis Sarkar Date: Monday, October 5, 2015 at 9:56 PM To: Mike Shand Cc: "Stewart Bryant (stbryant)", 'Jon Hudson', "[email protected]<mailto:[email protected]>" Subject: Re: Routing directorate QA review for draft-ietf-rtgwg-rlfa-node-protection Hi Mike, I forgot to mention.. Even if we follow the below definition of Extended P-Space from section 2 and 5.1.1.2 in RFC7490… “Extended P-space: Consider the set of neighbors of a router protecting a link. Exclude from that set of routers the router reachable over the protected link. The extended P-space of the protecting router with respect to the protected link is the union of the P-spaces of the neighbors in that set of neighbors with respect to the protected link (see Section 5.2.1.2). “ And “The description in Section 5.2.1.1 calculated router S's P-space rooted at S itself. However, since router S will only use a repair path when it has detected the failure of the link S-E, the initial hop of the repair path need not be subject to S's normal forwarding decision process. Thus, the concept of extended P-space is introduced. Router S's extended P-space is the union of the P-spaces of each of S's neighbors (N). This may be calculated by computing an SPT at each of S's neighbors (excluding E) and excising the subtree reached via the path N->S->E. Note this will excise those routers that are reachable through all ECMPs that include link S-E. The use of extended P-space may allow router S to reach potential repair tunnel endpoints that were otherwise unreachable. In cost terms, a router (P) is in extended P-space if the shortest path cost N->P is strictly less than the shortest path cost N->S->E->P. In other words, once the packet is forced to N by S, it is a lower cost for it to continue on to P by any path except one that takes it back to S and then across the S->E link. " So if we apply the above definitions in the diagram below from the current draft… D1 / S-x-E / / \ N---+ R3--D2 \ / R1---R2 Then Ext-P-Space of S w.r.t S-E link cannot include E and D1 as SPT rooted at N has ECMPs paths traversing the S-E link for destinations E and D1. Basically if S forces a packet destined for E or D1 to N, N can send it back over the path N->S-E. So once again E and D1 cannot be in PQ-space of S wrt S-E link. Hope this resolves your comment :) Thanks -Pushpasis
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