Path protection is not currently available on Cisco routers. It is provided here for the sake of completeness and is not discussed further in this chapter. It is, however, discussed again in Chapter 9, "Network Design with MPLS TE," where the scalability of path protection is compared with that of link and node protection.
Path protection is essentially the establishment of an additional LSP in parallel with an existing LSP, where the additional LSP is used only in case of failure. This LSP is sometimes called the backup, secondary, or standby LSP. The backup LSP is not used to carry traffic except during a failure condition—hence, the term standby.
The backup LSP is built along paths that are as diverse as possible from the LSP they're protecting. This ensures that a failure along the path of the primary LSP does not also affect the backup LSP. Path protection is simple in concept. Each primary LSP is backed up by a standby LSP. Both the primary and backup LSPs are configured at the headend. Both are signalled ahead of time in the control plane.
The primary and backup LSPs might have the same constraints. If the primary LSP has a bandwidth reservation of 100 Mbps, the backup LSP can also reserve 100 Mbps. This way, the end-to-end characteristics essentially remain the same, no matter whether the LSP used to carry traffic is the primary LSP or the protection LSP.
As mentioned in the preceding section, simply having a second path option under the tunnel interface does not make it path protection—it would be an LSP reroute. Path protection has better convergence than IGP convergence in an IP network or MPLS TE LSP reroute because it makes use of a presignalled LSP that is ready to go in case the primary LSP fails. With path protection, the relationship between the backup LSP and the number of primary LSPs it is protecting is 1:1. This makes the path protection scheme less scalable.
In other words, for every LSP you want to protect, you have to signal another LSP. If you want the primary and backup LSPs to share the same bandwidth characteristics, they need to reserve the same amount of bandwidth. Protection LSPs kick in only when there's a failure, and hopefully your network failure rate is far less than 50 percent, so you end up reserving backup bandwidth that you won't use most of the time and keeping other LSPs in the network from being able to use that bandwidth.
Figure 7-1 shows a primary tunnel going from 7200a to 7200c over 12008a and 12008c. The path protection tunnel is also from the headend 7200a to tail 7200c but goes over a diverse path
(7200a—* 7500a"* 12008b"*12008d->7500c—*7200c).
Figure 7-1. Path Protection
Figure 7-1. Path Protection
Path protection tunnel
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