Vyatta NOS documentation

Learn how to install, configure, and operate Vyatta Network Operating System (Vyatta NOS), which helps to drive our virtual networking and physical platforms portfolio.

LSP preemption

When there is not enough unallocated bandwidth on an interface to fulfill the requirements of a new LSP that has passed admission control, existing LSPs that have a lower priority may be preempted.

When preemption occurs, bandwidth allocated to lower-priority LSPs is reallocated to the higher-priority LSP. LSP preemption depends on the bandwidth requirements and priority of the new LSP, compared to the bandwidth allocation and priority of already existing LSPs.

In the example above, bandwidth has been allocated to an LSP that has a hold priority of three and a mean-rate of 1,000 Kbps. When a new LSP with a setup priority of two, hold priority of one, and mean-rate of 10,000 Kbps is established, admission control, bandwidth allocation, and LSP preemption work as described below.

  1. Admission control: On the interface, there is 10,000 Kbps available to priority two. The mean-rate for the new LSP is 10,000, so the LSP passes admission control; bandwidth can be allocated to it.
  2. Bandwidth allocation: The hold priority for the new LSP is one. On the interface, 10,000 Kbps is available to priority one. This entire amount is allocated to the LSP.
  3. LSP preemption: The first LSP had been using 1,000 Kbps of this amount, but its hold priority is only three. Consequently, the first LSP is preempted, and its bandwidth allocation removed in order to make room for the new LSP.

Once this happens, the reservable bandwidth array for the interface looks like this:

Priority Unreserved Bandwidth
0 10,000
1 0
2 0
3 0
4 0
5 0
6 0
7 0

Active: LSP with setup 2, hold 1, mean-rate 1,000

Preempted: LSP with setup 6, hold 3, mean-rate 1,000

On this interface, the only LSP that could preempt the active LSP would be have a setup and hold priority of zero.