Cisco NCS 5500 Series

• The remote LFA backup path for MPLS traffic can be setup only using LDP. Only per-prefix protection
is supported.
• Border Gateway Protocol (BGP) Prefix-Independent Convergence (PIC) and FRR can be configured on
the same interface as long as they are not used for the same prefix.
IS-IS and FRR
When a local link fails in a network, IS-IS recomputes new primary next-hop routes for all affected prefixes.
These prefixes are updated in the RIB and the Forwarding Information Base (FIB). Until the primary prefixes
are updated in the forwarding plane, traffic directed towards the affected prefixes are discarded. This process
can take hundreds of milliseconds.
In FRR, IS-IS computes LFA next-hop routes for the forwarding plane to use in case of primary path failures.
LFA is computed per prefix.
When there are multiple LFAs for a given primary path, IS-IS uses a tiebreaking rule to pick a single LFA
for a primary path. In case of a primary path with multiple LFA paths, prefixes are distributed equally among
LFA paths.
Repair Paths
Repair paths forward traffic during a routing transition. When a link or a router fails, due to the loss of a
physical layer signal, initially, only the neighboring routers are aware of the failure. All other routers in the
network are unaware of the nature and location of this failure until information about this failure is propagated
through a routing protocol, which may take several hundred milliseconds. It is, therefore, necessary to arrange
for packets affected by the network failure to be steered to their destinations.
A router adjacent to the failed link employs a set of repair paths for packets that would have used the failed
link. These repair paths are used from the time the router detects the failure until the routing transition is
complete. By the time the routing transition is complete, all routers in the network revise their forwarding
data and the failed link is eliminated from the routing computation.
Repair paths are precomputed in anticipation of failures so that they can be activated the moment a failure is
detected.
The LFA FRR feature uses the following repair paths:
• Equal Cost Multipath (ECMP) uses a link as a member of an equal cost path-split set for a destination.
The other members of the set can provide an alternative path when the link fails.
• LFA is a next-hop route that delivers a packet to its destination without looping back. Downstream paths
are a subset of LFAs.
LFA Overview
LFA is a node other than the primary neighbor. Traffic is redirected to an LFA after a network failure. An
LFA makes the forwarding decision without any knowledge of the failure.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
250
Implementing Fast Reroute Loop-Free Alternate
IS-IS and FRR