As seen in the application scenarios in the previous section, traffic engineering is accomplished in two steps: computing a path that satisfies a set of constraints and forwarding traffic along this path. These steps are discussed in detail in the following sections. However, it is first necessary to introduce the concept of LSP priorities.

2.4.1. LSP priorities and preemption

MPLS-TE uses LSP priorities to mark some LSPs as more important than others and to allow them to confiscate resources from less important LSPs (preempt the less important LSPs). Doing this guarantees that:

  1. In the absence of important LSPs, resources can be reserved by less important LSPs.

  2. An important LSP is always established along the most optimal (shortest) path that fits the constraints, regardless of existing reservations.

  3. When LSPs need to reroute (e.g. after a link failure), important LSPs have a better chance of finding an alternate path.

MPLS-TE defines eight priority levels, with 0 as the best and 7 as the worst priority. An LSP has two priorities associated with it: a setup priority and a hold priority. The setup priority controls access to the resources when the LSP is established and the hold priority controls access to the resources for an LSP that is already established. When an LSP is set up, if not enough resources are available, the setup priority of the new LSP is compared to the hold priority of the LSPs using the resources in order ...

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