Lfa Calculation; Interaction Between Rib And Routing Protocols - Cisco NCS 5500 Series Configuration Manuals

Implementing Fast Reroute Loop-Free Alternate
An LFA must neither use a failed element nor use a protecting node to forward traffic. An LFA must not
cause loops. By default, LFA is enabled on all supported interfaces as long as the interface can be used as a
primary path.
Advantages of using per-prefix LFAs are as follows:
• The repair path forwards traffic during transition when the primary path link is down.
• All destinations having a per-prefix LFA are protected. This leaves only a subset (a node at the far side

LFA Calculation

The general algorithms to compute per-prefix LFAs can be found in RFC 5286. IS-IS implements RFC 5286
with a small change to reduce memory usage. Instead of performing a Shortest Path First (SPF) calculation
for all neighbors before examining prefixes for protection, IS-IS examines prefixes after SPF calculation is
performed for each neighbor. Because IS-IS examines prefixes after SPF calculation is performed, IS-IS
retains the best repair path after SPF calculation is performed for each neighbor. IS-IS does not have to save
SPF results for all neighbors.

Interaction Between RIB and Routing Protocols

A routing protocol computes repair paths for prefixes by implementing tiebreaking algorithms. The end result
of the computation is a set of prefixes with primary paths, where some primary paths are associated with repair
paths.
A tiebreaking algorithm considers LFAs that satisfy certain conditions or have certain attributes. When there
is more than one LFA, configure the fast-reroute per-prefix command with the tie-break keyword. If a rule
eliminates all candidate LFAs, then the rule is skipped.
A primary path can have multiple LFAs. A routing protocol is required to implement default tiebreaking rules
and to allow you to modify these rules. The objective of the tiebreaking algorithm is to eliminate multiple
candidate LFAs, select one LFA per primary path per prefix, and distribute the traffic over multiple candidate
LFAs when the primary path fails.
Tiebreaking rules cannot eliminate all candidates.
The following attributes are used for tiebreaking:
• Downstream—Eliminates candidates whose metric to the protected destination is lower than the metric
• Linecard-disjoint—Eliminates candidates sharing the same linecard with the protected path.
• Shared Risk Link Group (SRLG)—Eliminates candidates that belong to one of the protected path SRLGs.
• Load-sharing—Distributes remaining candidates among prefixes sharing the protected path.
• Lowest-repair-path-metric—Eliminates candidates whose metric to the protected prefix is higher.
• Node protecting—Eliminates candidates that are not node protected.
• Primary-path—Eliminates candidates that are not ECMPs.
of the failure) unprotected.
of the protecting node to the destination.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x

LFA Calculation

251