IS-IS and OSPF Support for Loop-Free Alternate Calculation
Page 560 7450 ESS MPLS Guide
Figure 53: Example Topology with Broadcast Interfaces
In order for next-hop R2 to be a link-protect LFA for route R5 from R1, it must be loop-free with
respect to the R1-R3 link’s Pseudo-Node (PN). However, since R2 has also a link to that PN, its
cost to reach R5 by way of the PN or router R4 are the same. Thus R1 cannot guarantee that
enabling the LFA next-hop R2 will protect against a failure impacting link R1-PN since this may
cause the entire subnet represented by the PN to go down. If the metric of link R2-PN is changed
to 8, then R2 next-hop will be an LFA providing link protection.
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.
5
5
al_0216
R1
5
5
5
5
5 (8)
R3
PN
R5
Primary Route
LFA Link-Protect Route
R4
R2
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