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MPLS Guide
3.7 RSVP-TE Fast Reroute (FRR)
FRR is a mechanism to protect against RSVP-TE signaled LSP failures by reacting
to these failures as soon as possible. FRR is set up from the iLER, which signals the
transit routers to precompute their backup LSPs. FRR creates a precomputed
backup LSP from each node in the LSP path. If a link or LSP between two routers
fails, traffic is rerouted immediately onto the precomputed backup LSP.
Note: In order for FRR to work, CSPF must be enabled.
The 7705 SAR supports FRR facility backup and one-to-one backup.
Facility backup mode allows FRR to be enabled on an aggregate basis and protects
a whole node or a whole link, regardless of the number of LSPs using that link. In
other words, facility backup mode creates a common bypass tunnel to protect all
LSP-paths traversing a common facility path. It provides flexibility, faster
provisioning, and faster convergence times compared with one-to-one backup or
LSP redundancy. One-to-one backup allows FRR to be enabled on a per-LSP basis.
With both methods, MPLS switches build many possible detour routes on the nodes
between the ingress and egress nodes of an LSP. The facility backup method
creates a detour route between two nodes, called a bypass tunnel, which is a single
tunnel that follows the primary LSP path except where the link or node has failed.
Traffic then switches to the bypass tunnel. The bypass tunnel merges with the
original LSP path at the merge point (MP) as soon as possible. The one-to-one
backup method creates a detour route, called a detour LSP, for each LSP that needs
to be rerouted. Unlike the bypass tunnel, the detour LSP takes the best path to the
termination point, and does not merge with the original LSP as soon as possible. The
detour LSPs of a one-to-one backup LSP can merge at a detour merge point (DMP),
which can either be at the termination point or at a point along the primary LSP.
One of the major differences between facility and one-to-one backup is the scalability
offered by the protection method. In facility backup mode, all LSPs of the same type
are rerouted over the bypass tunnel. Hence they are all protected against the failure
of a node or link in the network. In facility backup mode, each LSR along the path
verifies that it has a bypass tunnel available to meet its requirements; otherwise, if it
can, it signals a new bypass tunnel based on the requirements. If a new LSP is
configured for FRR facility backup, the existing backup tunnels are scanned and if
any one of them can be used for recovery, it is preferred. If there are no common
links, then a new bypass tunnel will be signaled, assuming that the LSP requirements
can be met. One-to-one backup mode uses similar reroute and protection methods
except a detour route is applied on a per-LSP basis.
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MPLS and RSVP-TE
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