Label Distribution Protocol
7450 ESS MPLS Guide Page 559
Figure 52: Topology with Primary and LFA Routes
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 Figure 53
5
al_0215
R1
5
5
5
5
5 (8)
R3
R5
Primary Route
LFA Link-Protect Route
LFA Node-Protect Route
R4
R2
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