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Altering MPLS BFD Return Path behavior which is to follow IGP

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TitleAltering MPLS BFD Return Path behavior which is to follow IGP
MPLS BFDis configured, and there is an LSP that's UP, for which there exists a BFD session. From the LSP ingress router, BFD packets take the same path as that of LSP. For BFD packets sent by LSP egress to ingress for the same session, the path could be different. LSP egress router uses the shortest path as found by the IGP which may be different than the path traversed by the LSP. In such a scenario, any link breaks in the IGP shortest path from LSP egress to ingress will cause the BFD session to go down (as LSP ingress stops to receive BFD packets until IGP converges). The BFD session going down will bring the LSP down.

MPLS BFDwas intended to detect any failure on the LSP path, but when the BFD's return path is different than the LSP's, any fault on the return path will cause the LSP to unnecessarily go down.
Software Release: NI 3.9 and higher (XMR, MLX), NI 5.6 and higher (CER, CES)
Fixed in Version: N/A
Root cause is that BFD RFCs (5880, 5881, 5883) assume that "...the far-end BFD peer will use the best route, regardless of the route being used to send BFD control packets towards it." There is an IETF proposed draft to specify the return path, but it is in a discussion state. In such a scenario, it could end up with 'forward' and 'reverse' directions ofBFDpackets for the same session to take up a different path.
See workaround section.
Additional notes
1) IGPmetrics can be changed so that LSPegress router will take the same path as that of LSP
2) Alternatively, an LSP can be established from egress router to ingress with
a. Path which is in the same path as the LSPfrom the ingress router to egress router, but in reverse direction
b. IGPshortcuts enabled
c. Metric set to a lower value than that of IGProute

This will cause the egress router to use the LSPto send the BFD packets over to ingress. Note that for the LSP from egress router to ingress, if the BFD is enabled, the same has to be handled at ingress (if the IGP calculated shorted path from ingress router to egress is different from that of the LSP path).



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