Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 44)

20 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Traffic engineering database
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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 10,000
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.
When multiple LSPs are candidates for preemption, the device normally preempts the LSP with the
lowest priority. However, when preempting a higher priority LSP with a high bandwidth requirement
would allow lower priority LSPs with lower bandwidth requirements to avoid preemption, the
higher-priority LSP is preempted.
For example, consider an interface with 10,000 Kbps of reservable bandwidth, allocated to two
active LSPs: one with a setup priority of three, hold priority of two, and mean-rate of 5,000 Kbps;
and another with a setup priority of four, hold priority of three, and mean-rate of 2,500 Kbps. When
an LSP with a setup priority of one, hold priority of zero, and mean-rate of 7,500 Kbps is
established, the following take place.
1. Admission control: On the interface, there is 10,000 Kbps available to priority one. The
mean-rate for the new LSP is 7,500 Kbps, so the LSP passes admission control; bandwidth can
be allocated to it.
2. Bandwidth allocation: The hold priority for the new LSP is 0. On the interface, 10,000 Kbps is
available to priority zero. Of this amount, seven,500 Kbps is allocated to the new LSP.
3. LSP preemption: To reserve enough bandwidth for the new LSP, one of the active LSPs must be
preempted. The LSP with hold priority two uses 5,000 Kbps, and the LSP with hold priority
three uses 2,500 Kbps. Instead of preempting both LSPs, the device preempts the higher
priority LSP and its allocation of 5,000 Kbps. This clears enough bandwidth to allow both the
new LSP and the lower priority LSP to be active.