Cisco NCS 5500 Series

• Secondary-path—Eliminates candidates that are ECMPs.
Fast Reroute with Remote Loop-Free Alternate
Fast Reroute with Remote Loop-Free Alternate (FRR Remote LFA) feature enables you to tunnel a packet
around a failed link to a remote loop-free alternate that is more than one hop away.
When a link or a router fails, distributed routing algorithms compute new routes that take into account the
failure. The time taken for computation is called routing transition. Until the transition is complete and all
routers are converged on a common view of the network, the connectivity between the source and destination
pairs is interrupted. You can use the IP Loop-Free Alternate (LFA) Fast Reroute (FRR) to reduce the routing
transition time to less than 50 milliseconds using a precomputed alternate next hop. When a router is notified
of a link failure, the router immediately switches over to the repair path to reduce traffic loss. Note that the
routing transition in IGP/BGP convergence can take up to several hundreds of milliseconds.
IP Loop-Free Alternate (LFA) Fast Reroute (FRR) supports the precomputation of repair paths. Intermediate
System-to-Intermediate System (IS-IS) routing protocol enables the repair path computation. The resulting
repair paths are sent to the Routing Information Base (RIB). Cisco Express Forwarding (formerly known as
CEF) and Open Shortest Path First (OSPF) installs the repair path.
With IP local LFA FRR, IGPs only compute directly connected neighbor as an LFA backup path to protect
the given prefix's primary path. Label Distribution Protocol (LDP) sets up labeled backup LSP with the
next-hop for the protected prefix. Some topologies (for example the commonly used ring-based topology)
require protection that is not afforded by LFA FRR. In such cases, use the LDP-based FRR Remote LFA
feature where IGPs compute non-directly connected neighbor, which are more than one hop away, as LFA
backup path to protect the given prefix's primary path. The LDP sets up labeled backup LSP with the remote
next-hop for the protected prefix. LDP also sets up another transport LSP to tunnel traffic to remote next-hop
without exposing the LFA backup label as learnt from remote node.
Consider the topology shown in the figure below:
Figure 8: FRR with Remote LFA with Ring Topology
Device A tries to send traffic destined to F to next-hop B. Device B cannot be used as an LFA for prefixes
advertised by nodes C and F. The actual LFA is node D. However, node D is not directly connected to the
protecting node A. To protect prefixes advertised by C, node A must tunnel the packet around the failed link
A-C to node D, provided that the tunnel does not traverse the failing link.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
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Implementing Fast Reroute Loop-Free Alternate
Fast Reroute with Remote Loop-Free Alternate