Cisco NCS 6000 Series Configuration Manual page 261

Implementing IS-IS
As a result, during link failures, extended LFA or Remote LFA (RLFA) is required to enable the router to use
a non-directly connected next hop as backup. RLFA ensures that this nearest non-connected neighbor does
not loop back to the source router.
RLFA uses an LDP tunnel between source and backup routers and is implemented in IPv4, IPv6, and MPLS
networks. When the network converges, micro-loops can form due to inconsistencies in the forwarding tables
of different routers. RLFA also provides a mechanism to avoid micro-loops.
Remote LFA is suitable for incremental deployment within a network, including a network that is already
deploying LFA.
Remote LFA Process
The remote LFA process on the source router involves:
1. For a protected link between the source router and the immediate next hop, the IGP (OSPF or IS-IS)
computes the remote LFA backup path by using the PQ algorithm.
2. The IGP updates the RIB table with the remote LFA path information.
3. LDP establishes a session with the remote router to exchange labels for prefixes.
4. LDP sets up MPLS forwarding for the protected prefix, and the corresponding backup path.
5. On link failure, and fast reroute trigger, the remote LFA backup path is activated with less than 50
millisecond convergence time.
6. The backup path is active until the IGP converges to the new primary path.
Remote LFA Topology
Consider the topology in the following figure. The best path from Source Router A to Destination Router E
is through Router D. The best backup path for this route would be: Router A -> B -> C -> D -> Router E.
Because Router B uses Router A as its primary next hop, this backup path cannot be used and LFA fails in
this topology.
As a result, by implementing RLFA, Router A calculates its nearest non-directly connected neighbor that does
not route back to it. In this example, Router A chooses Router C as its RLFA backup, and uses LDP to establish
a tunnel between them. Traffic from Router A is routed to Router C on fast reroute trigger.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Configure Remote Loop-Free Alternate Paths for IS-IS
239