Figure 52: Topology With Primary And Lfa Routes - Alcatel-Lucent 7450 Manual

Ethernet service switch

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The primary route is by way of R3. The LFA route by way of R2 has two equal cost paths to reach

R5. The path by way of R3 protects against failure of link R1-R3. This route is computed by R1 by

checking that the cost for R2 to reach R5 by way of R3 is lower than the cost by way of routes R1

and R3. This condition is referred to as the loop-free criterion. R2 must be loop-free with respect

to source node R1.

The path by way of R2 and R4 can be used to protect against the failure of router R3. However,

with the link R2-R3 metric set to 5, R2 sees the same cost to forward a packet to R5 by way of R3

and R4. Thus R1 cannot guarantee that enabling the LFA next-hop R2 will protect against R3 node

failure. This means that the LFA next-hop R2 provides link-protection only for prefix R5. If the

metric of link R2-R3 is changed to 8, then the LFA next-hop R2 provides node protection since a

packet to R5 will always go over R4. In other words it is required that R2 becomes loop-free with

respect to both the source node R1 and the protected node R3.

Consider the case where the primary next-hop uses a broadcast interface as illustrated in

7450 ESS MPLS Guide

Primary Route

LFA Link-Protect Route

LFA Node-Protect Route

Figure 52: Topology with Primary and LFA Routes

Label Distribution Protocol