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 1. 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.
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. In the figure above, the
green arrow between A and D shows the tunnel that is automatically created by
the remote LFA feature to bypass looping.