MPLS-Enabled Applications: Emerging Developments and New Technologies

3.6. LINK PROTECTION

As discussed in the previous section, link protection refers to the ability to protect traffic being forwarded on an LSP when a link along the LSP fails. To protect against the failure of a link, a backup tunnel is set up around the link. This backup is called a detour in the case of one-to-one protection and bypass in the case of many-to-one protection.

Figure 3.10 shows one LSP, LSPxy from X to Y, along the path X-A-B-Z-Y. Link A-B is protected by a backup LSP taking the path A-C-D-B. When link A-B fails, traffic from LSPxy (the protected path) is forwarded on this backup around the broken link at A and delivered to B, from where it continues on its normal path to destination Y. Node A, where traffic is spliced from the protected path on to the backup, is the PLR, and node B, where traffic merges from the backup into the protected path again, is the MP.^[] Throughout this chapter, we will use the terms 'protected path' and 'main path' interchangeably to mean the LSP receiving protection.

^[] As noted earlier in this chapter, in the case of one-to-one protection, the MP does not necessarily need to be the router immediately downstream of the protected link. However, for the discussion in this section, we have chosen a topology such that the MP is the same for both the one-to-one protection and facility protection cases, so that the label operations can be compared more easily for the two cases.

Figure 3.10. Topology for detailed link protection discussion