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Link Protection


Link protection offers per-link traffic protection. It supports fast rerouting of user traffic over one mission-critical link. It does this on a per-LSP basis, much like the fast reroute option. However, it can also aggregate several protected LSPs over a single bypass LSP.

This flexible approach to single-link, rapid reroute does not require any new protocol modification beyond the RSVP-TE specification. Bypass LSPs efficiently aggregate traffic from multiple LSPs when the reroute occurs.

When link protection is enabled on a router interface and a protected LSP traverses this protected interface, Junos OS creates a trunk-like, bypass LSP to provide an alternate path to the RSVP neighbor. Each bypass LSP keeps track of all protected LSPs that are associated with the neighbor. In case of a neighbor failure, the protected LSPs are rerouted over the bypass LSP. Bypass LSPs use label stacking to protect user traffic.

At the interface level, the router keeps track of bypass LSP characteristics. Whenever an interface enables or disables link protection, the changes are saved at the interface level and then propagated to the RSVP neighbor. When a neighbor requires link protection, the router checks the associated interface structure to determine how to create a bypass LSP.

On a per-RSVP neighbor basis, the router keeps track of all the LSP sessions passing through a neighbor as well as the bypass LSP status. For the bypass LSP, the router maintains information about protected neighbors. For regular LSPs, the router monitors all threads containing the LSP. When a regular LSP is lost, the bypass LSP reroutes user traffic by using information about the next hop, egress Explicit Route Object (ERO), interface, and peer address.


Fast reroute, link protection, and node-link protection all rely on Constrained Shortest Path First (CSPF) to select bypass LSPs. The CSPF computation attempts to find an LSP that bypasses an affected node first, but can select an alternate link through the affected node if a node bypass LSP is not available.