Traffic Engineering with MPLS by Osborne Eric & Simha Ajay

Author:Osborne, Eric & Simha, Ajay [Osborne, Eric]
Language: eng
Format: mobi
Publisher: Cisco Press
Published: 2002-07-16T16:00:00+00:00

Tunnel Preemption

The last thing you have to do is make sure that the subpool tunnel can preempt nonsubpool tunnels. This is done with the command tunnel mpls traffic-eng priority, which was covered in Chapter 3, "Information Distribution." Why do you have to allow DS-TE tunnels to preempt non-DS-TE tunnels? Because of the way subpool bandwidth is advertised. The subpool is, as the name implies, a subset of the global bandwidth pool on an interface, not a separate bandwidth pool. If you don't allow the DS-TE LSP to preempt a global LSP, the global LSP can reserve bandwidth that would then no longer be available for the subpool.

If the subpool bandwidth were advertised as a separate bandwidth pool, rather than as a subset of existing bandwidth, you could easily end up with bandwidth fragmentation, as the following example shows.

If you have a link configured with ip rsvp bandwidth 150000 sub-pool 45000, this means that you are advertising that you have 150 Mbps (150,000 Kbps) of global reservable bandwidth on the link, and 45 Mbps of that 150 Mbps is subpool bandwidth. At this point, the link is advertising the output shown in Example 6-7.

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