Mldp Lfa Frr - Behavior - Cisco ASR 9000 Series Configuration Manual

Implementing Layer-3 Multicast Routing on Cisco IOS XR Software
MLDP Loop-Free Alternative Fast Reroute

MLDP LFA FRR - Behavior

In the following scenarios, S is source router, D is the destination router, E is primary next hop, and N_1 is
the alternative next hop.
Figure 14: LFA FRR Behavior - LFA Available
With LFA FRR, the source router S calculates an alternative next hop N_1 to forward traffic towards the
destination router D through N_1, and installs N_1 as a the alternative next hop. On detecting the link failure
between routers S and E, router S stops forwarding traffic destined for router D towards E through the failed
link; instead it forwards the traffic to a pre-computed alternate next hop N_1, until a new SPF is run and the
results are installed.
Figure 15: LFA FRR Behavior - LFA Not Available
In the above scenario, if the link cost between the next hop N_1 and the destination router D is increased to
30, then the next hop N_1 would no longer be a loop-free alternative. (The cost of the path, from the next hop
N_1 to the destination D through the source S, would be 17, while the cost from the next hop N_1 directly to
destination D would be 30). Thus, the existence of a LFA next hop is dependent on the topology and the nature
of the failure, for which the alternative is calculated.
LFA Criteria
In the above example, the LFA criteria of whether N is to be the LFA next-hop is met, when:
Cost of path (N_1, D) < Cost of path (N_1, S) + Cost of path (E, S) + Cost of path (D, E)
Downstream Path criteria, which is subset of LFA, is met when:
Cost of path (N_1, D) < Cost of path (E, S) + Cost of path (D, E)
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 6.0.x
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