Cisco CRS Configuration Manual page 38

IP LDP Fast Reroute Loop Free Alternate
This feature targets to address the fast convergence ability by detecting, computing, updating or enabling
prefix independent pre-computed alternate loop-free paths at the time of failure.
IGP pre-computes a backup path per IGP prefix. IGP selects one and only one backup path per primary path.
RIB installs the best path and download path protection information to FIB by providing correct annotation
for protected and protecting paths. FIB pre-installs the backup path in dataplane. Upon the link or node failure,
the routing protocol detects the failure, all the backup paths of the impacted prefixes are enabled in a
prefix-independent manner.
Prerequisites
The Label Distribution Protocol (LDP) can use the loop-free alternates as long as these prerequisites are met:
The Label Switching Router (LSR) running LDP must distribute its labels for the Forwarding Equivalence
Classes (FECs) it can provide to all its neighbors, regardless of whether they are upstream, or not.
There are two approaches in computing LFAs:
• Link-based (per-link)--In link-based LFAs, all prefixes reachable through the primary (protected) link
share the same backup information. This means that the whole set of prefixes, sharing the same primary,
also share the repair or fast reroute (FRR) ability. The per-link approach protects only the next hop
address. The per-link approach is suboptimal and not the best for capacity planning. This is because all
traffic is redirected to the next hop instead of being spread over multiple paths, which may lead to
potential congestion on link to the next hop. The per-link approach does not provide support for node
protection.
• Prefix-based (per-prefix)--Prefix-based LFAs allow computing backup information per prefix. It
protects the destination address. The per-prefix approach is the preferred approach due to its greater
applicability, and the greater protection and better bandwidth utilization that it offers.
Note
The per-prefix LFA approach is preferred for LDP IP Fast Reroute LFA for these reasons:
• Better node failure resistance
• Better capacity planning and coverage
Features Not Supported
These interfaces and features are not supported for the IP LDP Fast Reroute Loop Free Alternate feature:
• BVI interface (IRB) is not supported either as primary or backup path.
• GRE tunnel is not supported either as primary or backup path.
• In a multi-topology scenerio, the route in topology T can only use LFA within topology T. Hence, the
availability of a backup path depends on the topology.
For more information about configuring the IP Fast Reroute Loop-free alternate , see Implementing IS-IS on
Cisco IOS XR Software module of the Cisco IOS XR Routing Configuration Guide for the Cisco CRS Router.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
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The repair or backup information computed for a given prefix using prefix-based LFA
may be different from the computed by link-based LFA.
Implementing MPLS Label Distribution Protocol