When a local link fails in a network, IS-IS recomputes new primary next-hop paths for all affected prefixes. These prefixes are updated in the RIB and the Forwarding Information Base (FIB). Until the primary path prefixes are updated in the forwarding plane, traffic directed towards the affected prefixes are discarded. This process can take hundreds of milliseconds.

In IPv6 FRR, IS-IS computes LFA next-hop routes for the forwarding plane to use in case of primary path failures. LFA is computed per prefix.

When there are multiple LFAs for a given primary path, IS-IS uses a tiebreaking rule to pick a single LFA for a primary path. In case of a primary path with multiple LFA paths, prefixes are distributed equally among LFA paths.

Repair paths forward traffic during a routing transition. When a link or a router fails, due to the loss of a physical layer signal, initially, only the neighboring routers are aware of the failure. All other routers in the networkare unaware of the nature and location of this failure until information about this failure is propagated through a routing protocol, which may take several hundred milliseconds. It is, therefore, necessary to arrange for packets affected by the network failure to be steered to their destinations.

A router adjacent to the failed link employs a set of repair paths for packets that would have used the failed link. These repair paths are used from the time the router detects the failure until the routing transition is complete. By the time the routing transition is complete, all routers in the network revise their forwarding data and the failed link is eliminated from the routing computation.

Repair paths are precomputed in anticipation of failures so that they can be activated the moment a failure is detected.

The IPv6 LFA FRR feature uses the following repair paths:

• Equal Cost Multipath (ECMP) uses a link as a member of an equal cost path-split set for a destination. The other members of the set can be used as a repair path when the link fails.

• LFA is a next-hop that delivers a packet to its destination without looping back. Downstream paths are a subset of LFAs.

LFA is a node other than the primary neighbor. Traffic is redirected to an LFA after a network failure. An LFA makes the forwarding decision without any knowledge of the failure.

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 of the failure) unprotected.

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.

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 of the protecting node to the destination.

• 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.

• Secondary-path—Eliminates candidates that are ECMPs.

From Cisco IOS XE 17.15.1a, you can use the following optional commands to further fine-tune LFA FRR configurations:

Router IS-IS / Address-family IPv6 Mode Commands

fast-reroute tie-break {level-1 | level-2}

Configures the following tie-breakers that impact backup path calculation and selection:

downstream                        Prefer repair path via downstream node
linecard-disjoint                 Prefer line card disjoint repair path
lowest-backup-path-metric         Prefer repair path with lowest total metric
node-protecting                   Prefer node protecting repair path
primary-path                      Prefer repair path from ECMP set
secondary-path                    Prefer non-ECMP repair path
srlg-disjoint                     Prefer SRLG disjoint repair path

fast-reroute interface disable <level>

Disables FRR protection on all interfaces by default. Interfaces where FRR is required can be configured explicitly using the interface level command.

fast-reroute load-sharing <level> disable

Disables load sharing between equal backup paths.

fast-reroute use-candidate-only <level>

Use as candidate interface only these allowed by the interface configuration.

Interface IS-IS IPv6 FRR Commands

isis ipv6 fast-reroute candidate <level> {disable}

Configures the interface for fast-reroute backup path.

isis ipv6 fast-reroute exclude <level> <interface>

Excludes another interface from being used for fast-reroute backup.

isis ipv6 fast-reroute protection <level> {disable}

Enables or disables fast-reroute protection on an interface.

isis ipv6 fast-reroute tie-break <level>

Creates the following set of tie-breakers specific for the interface:

default                            Use default tiebreakers set
downstream                         Prefer repair path via downstream node
linecard-disjoint                  Prefer line card disjoint repair path
lowest-backup-path-metric          Prefer repair path with lowest total metric
node-protecting                    Prefer node protecting repair path
primary-path                       Prefer repair path from ECMP set
secondary-path                     Prefer non-ECMP repair path
srlg-disjoint                      Prefer SRLG disjoint repair path