RSVP FastReroute

One-to-One Local Repair (Fast Reroute)

To protect against link failure and node failure, RSVP LSPs can create always-on backup LSPs. These backup paths start at the point of local repair and are only used for a very short duration (till the headend calculates the new path for the main LSP).

There are two ways of doing this.

1. One-to-one backup (fast-reroute)
2. Facility backup (link-protection or node-link-protection)

In one-to-one backup approach, each LSP creates a detour path at each hop. This causes RSVP state to increase by huge amount.

In facility backup approach, one single bypass LSP can protect many LSPs, scales much better in large networks.

Note, these local repair paths are only used temporarily. The device detecting the error, sends a PathErr to the ingress to calculates and signal a new path. Also, it indicates that the current LSP can continue to forward traffic unitl the new path is live.

One-to-One Backup paths:

for every one LSP, there is one detour, at every hop. Detours offer node protection, and fallback to link protection if the topology forces. Also, detours take the shortest path to the egress router (not mandatory to go back to the main LSP’s path).

Detours for the same LSP can merge together to reduce some state.

To enable this one-to-one backups, add fast-reroute to the LSP. No additional configuration required on other routers.

[edit]
root@juniper# set protocols mpls label-switched-path <lsp-name> to X.X.X.X fast-reroute

Also, add an additional load-balancing policy to preinstall detour paths into the FIB.

To verify, use show mpls lip ingress extensive command and look for FastReroute desired and in the logs, look for Fast-reroute Detour Up.

To see Detour information, use the show rsvp session detail command and look for Detour Explicit route

Note, the detour does not have a unique name, uses the same name as the main LSP.

On tranist routers, we see something like thisDetour branch from X.X.X.X, to skip Y.Y.Y.Y, Up

Detour count shows how many detours have been merged into one.

Default behaviour :

Admin groups are inherited but BW configuration is not.

Hop count has a default value of six.

We can change these defaults within the fast-reroute hierarchy of the LSP.