Label Distribution Protocol
7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide Page 249
The following are the detailed rules for this criterion as provided in RFC 5286:
• Rule 1: Link-protect LFA backup next-hop (primary next-hop R1-R3 is a P2P interface):
Distance_opt(R2, R5) < Distance_opt(R2, R1) + Distance_opt(R1, R5)
and,
Distance_opt(R2, R5) >= Distance_opt(R2, R3) + Distance_opt(R3, R5)
• Rule 2: Node-protect LFA backup next-hop (primary next-hop R1-R3 is a P2P interface):
Distance_opt(R2, R5) < Distance_opt(R2, R1) + Distance_opt(R1, R5)
and,
Distance_opt(R2, R5) < Distance_opt(R2, R3) + Distance_opt(R3, R5)
• Rule 3: Link-protect LFA backup next-hop (primary next-hop R1-R3 is a broadcast
interface):
Distance_opt(R2, R5) < Distance_opt(R2, R1) + Distance_opt(R1, R5)
and,
Distance_opt(R2, R5) < Distance_opt(R2, PN) + Distance_opt(PN, R5)
where; PN stands for the R1-R3 link Pseudo-Node.
For the case of P2P interface, if SPF finds multiple LFA next-hops for a given primary next-hop, it
follows the following selection algorithm:
A) It will pick the node-protect type in favor of the link-protect type.
B) If there is more than one LFA next-hop within the selected type, then it will pick one
based on the least cost.
C) If more than one LFA next-hop with the same cost results from Step B, then SPF will
select the first one. This is not a deterministic selection and will vary following each SPF
calculation.
For the case of a broadcast interface, a node-protect LFA is not necessarily a link protect LFA if
the path to the LFA next-hop goes over the same PN as the primary next-hop. Similarly, a link
protect LFA may not guarantee link protection if it goes over the same PN as the primary next-
hop.
The selection algorithm when SPF finds multiple LFA next-hops for a given primary next-hop is
modified as follows:
A) The algorithm splits the LFA next-hops into two sets:
→ The first set consists of LFA next-hops which do not go over the PN used by primary
next-hop.
→ The second set consists of LFA next-hops which do go over the PN used by the
primary next-hop.
B) If there is more than one LFA next-hop in the first set, it will pick the node-protect type in
favor of the link-protect type.
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