What Is Protection?

Protection, in the context of fast restoration, is having procedures in place that, when applied to selected resources, ensures minimal loss of traffic upon failure. Protected resources could either be viewed as physical resources (link or nodes) or logical resources (the LSPs that cross a link or node). Regardless of how you look at it, network failures are always physical in origin—a link or a node goes down, taking some LSPs with it. What protection boils down to is protection of logical resources (LSPs) from physical failures (links or nodes).

For all subsequent discussions in this chapter (or, for that matter, anywhere in MPLS), the term protection should be associated with the fact that backup resources are preestablished and are not signalled after a failure has occurred, as you'd see in the case of headend LSP reroute. The preestablishment of protection resources is fundamental for any protection strategy. If protection resources weren't preestablished, they'd have to be set up after the failure was detected; by then, it's too late.

This chapter calls this preestablished LSP a backup tunnel or protection tunnel. They mean the same thing.