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.
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)
criteria, which is subset of LFA, is met when:
Cost of path (N_1, D) < Cost of path (E, S) + Cost of path
Link Protecting LFA
Figure 16. Link Protecting
In the above
illustration, if router E fails, then both router S and router N detects a
failure and switch to their alternates, causing a forwarding loop between both
routers S and N. Thus, the Link Protecting LFA causes Loop on Node Failure;
however, this can be avoided by using a down-stream path, which can limit the
coverage of alternates. Router S will be able to use router N as a downstream
alternate, however, router N cannot use S. Therefore, N would have no alternate
and would discard the traffic, thus avoiding the micro-looping.
Node Protecting LFA
Link and node
protecting LFA guarantees protection against either link or node failure.
Depending on the protection available at the downstream node, the downstream
path provides protection against a link failure; however, it does not provide
protection against a node failure, thereby preventing micro looping.
The criteria for LFA
selection priority is that: the Link and Node protecting LFA is greater than
the Link Protecting Downstream is greater than the Link Protecting LFA.