Discussion

Solution
Fast reroute reduces packet loss when a link in the LSP fails. You configure fast
reroute on the ingress router only:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R5 to 10.0.0.5
aviva@R1# set label-switched-path R1-to-R5 fast-reroute
Discussion
A basic function of IP routing protocols is to reroute traffic changes that occur in the
network, such as a link or router node failure. Because the IP routing protocols are
distributed across the network devices and because all routers must have a consistent
view of the network, it can take some time for the routes to converge after a
topology change. In a large network, convergence times can be on the order of several
seconds, which may be unacceptable for your service-level agreements (SLAs).
MPLS fast reroute provides a solution to the convergence problem by rerouting traffic
around a point of failure in an LSP. Fast reroute sets up a protection LSP around a
point of failure in advance to protect an individual link between two routers. Each
router in the LSP sets up protection LSPs when the ingress router signals the initial
setup of the LSP. When a link along an LSP fails, the router upstream of the failure
switches to the protection LSP as soon as it detects the failure. No route calculations
need to be done because the protection LSP is signaled and set up in advance, and
the routing protocols don’t need to converge, so the move to a path that circumvents
the point of failure can happen quickly. Following a failure, the ingress router
is notified and can compute a new path at its leisure. Traffic is protected in the
meantime.
Fast reroute does not eliminate packet loss; it merely minimizes it. When a path fails
and MPLS switches to the protection LSP, the MPLS routers still need some small
amount of time to detect the failure and switch to the alternate path. The ingress
router can then recalculate the LSP if necessary. During the switchover, the LSP will
continue forwarding traffic while a new LSP is established.
You configure fast reroute only on the ingress router. You do not need to configure it
on the LSP’s transit and egress routers. As this recipe shows, the configuration is
straightforward: just include the fast-reroute statement in the LSP’s configuration.
Once the LPS is running fast reroute, the ingress router signals all downstream routers
that fast reroute has been requested and indicate which link requires protection,
and each downstream router does its best to set up detours for the LSP. If a downstream
router does not support fast reroute, it ignores the request to set up detours
but continues to support the LSP. A router that does not support fast reroute will
cause some of the detours to fail but otherwise has no impact on the LSP.
Using Fast Reroute to Reduce Packet Loss Following a Link Failure | 529
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