Admission Control, Bandwidth Allocation, and LSP Preemption

When a Resv message is received on an LSR, admission control determines whether the LSP can be established, based on its configured priority. When an LSP passes admission control, bandwidth is allocated to the new LSP, possibly preempting existing LSPs that have lower priority.

An LSPs priority consists of a setup priority and a hold priority. The setup priority is the priority for taking resources; the hold priority is the priority for holding resources. An LSPs setup priority is considered during admission control, and its hold priority is considered when bandwidth is allocated to the LSP. The setup and hold priorities are expressed as numbers between zero (0) (highest priority level) and seven (7)(lowest priority level). An LSPs setup priority must be lower than or equal to its hold priority. The user can configure either of these values for an LSP; by default, an LSPs setup priority is seven and its hold priority is zero.

On an MPLS-enabled interface, a certain amount of bandwidth is allocated for usage by LSPs; this amount can be either the maximum available bandwidth on the interface (the default) or a user-specified portion. The amount of bandwidth an individual LSP can reserve from this pool of allocated bandwidth depends on two user-configured attributes of the LSP: the LSPs priority and the LSPs mean-rate (the average rate of packets that can go through the LSP). The following conditions also apply:

When setting up an LSP, the device actually performs admission control twice: when the Path message is received and when the Resv message is received. when the LSP passes admission control after the Resv message is received, bandwidth allocation and LSP preemption take place.

The sections that follow include examples of how admission control, bandwidth allocation, and preemption work.