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IS-IS and IP FRR
When a local link fails in a network, IS-IS would be re-computing new primary next-hop routes for all affected prefixes. These prefixes would be updated in the RIB as well as the FIB (Forwarding Information Base). Until the primary prefixes that were being updated in the forwarding plane, traffic directed towards the affected prefixes is also discarded. This process might take hundreds of milliseconds.
In IP FRR, IS-IS is going to compute LFA next-hop routes for the forwarding plane so as to use in case of failures occurred in the primary path. LFA is computed per prefix.
When there are multiple LFAs present for a given primary path, IS-IS utilizes a tiebreaking rule so as to pick a single LFA for a primary path. In case of a primary path with multiple LFA paths, prefixes would be distributed equally among LFA paths.
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Repair Paths
The Repair paths would be forwarding traffic during a routing transition. When a link or a router fails, due to the loss of a physical layer signal, primarily, only the neighboring routers would be aware of the failure. All other routers in the network would be unaware of nature as well as the location of this failure until information about this failure would be propagated through a routing protocol, which might take several hundred milliseconds. It is, therefore, necessary so as to arrange for packets which would be affected by the network failure to be steered to their specific destinations.
A router adjacent to the failed link would employ a set of repair paths for packets that would have to use the failed link. These repair paths would be utilizing from the time the router detects the failure until the transition of routing is complete. By the time the routing transition is complete, all routers in the network would be revising their forwarding data and the failed link is eliminated from the routing computation.
Repair paths are considered to be pre-computed in anticipation of failures so that they could be activated the moment a failure is detected.
The LFA FRR feature utilizes the following repair paths:
• ECMP (Equal Cost Multipath) would be utilizing a link as a member of an equal cost path-split which would be set for a destination. The other members of the set could be provided an alternative path when the link fails.
• LFA is considered to be a next-hop route that would be delivering a packet to its destination without looping back. Downstream paths are believed to be a subset of LFAs.
LFA Overview:
LFA is believed to be the node other than the primary neighbor. Traffic would be redirected to an LFA after a network failure. An LFA would be making the forwarding decision without any knowledge of the failure.
An LFA must neither be utilized with a failed element nor use a protecting node to forward traffic. An LFA might not cause loops. By default, LFA would be enabled on all supported interfaces as long as the interface could utilize a primary path.
Advantages of utilizing per-prefix LFAs are as follows:
• The repair path would be forwarding traffic during the transition when the primary path link would be down.
• All destinations would be having a per-prefix LFA are protected. This leaves would be only a subset, a node at the far side of the failure, unprotected.
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