Label Distribution Protocol
7450 ESS MPLS Guide Page 561
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.
C) If there is more than one LFA next-hop within the selected type, then it will pick one
based on the least cost.
D) If more than one LFA next-hop with equal cost results from Step C, SPF will select the
first one from the remaining set. This is not a deterministic selection and will vary
following each SPF calculation.
E) If no LFA next-hop results from Step D, SPF will rerun Steps B-D using the second set.
Note this algorithm is more flexible than strictly applying Rule 3 above; the link protect rule in the
presence of a PN and specified in RFC 5286. A node-protect LFA which does not avoid the PN;
does not guarantee link protection, can still be selected as a last resort. The same thing, a link-
protect LFA which does not avoid the PN may still be selected as a last resort.Both the computed
primary next-hop and LFA next-hop for a given prefix are programmed into RTM.
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