Discussion

Solution
Configure the more important LSP so that it is more likely to be set up and remain up:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R5 priority 0 0
Then configure intermediate priority LSPs, those less likely to be set up and remain up:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R6 priority 1 3
aviva@R1# set label-switched-path R1-to-R4 priority 4 5
Finally, set up the least important LSP, the one least like to be set up and most likely
to be torn down if there is insufficient bandwidth:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R5-low priority 7 7
Discussion
When you offer a higher level of service for some customers, you want RSVP to be
able to establish those customers’ LSP at all times, even when there might not be
enough bandwidth available on some links because other LSPs are already established.
To provide this level of service, you can prioritize each customer’s LSP so that
some are more important than others. Then, when RSVP calculates the path and sets
up the LSP, RSVP will always be able to establish the more important LSPs, even at
the cost of tearing down a less important LSP. The relative LSP priorities are determined
in advance, when RSVP is establishing the LSP, not when traffic is being forwarded.
If a link has insufficient bandwidth, RSVP establishes the more important
(higher-priority) LSPs first and tears down lower-priority LSPs if necessary.
This recipe prioritizes the four LSPs that originate on router R1. To configure an
LSP’s priority, you include two values in the priority statement, the setup priority
and the hold priority. You express these two priorities with a number from 0 through
7, where 0 is best and 7 is worst. A setup priority of 0 means that this LSP can preempt
any other LSP whose hold priority is worse than 0. Similarly, a hold priority of
0 means that once the LSP is set up, it cannot be preempted. A hold priority of 1
means that it can be preempted only by an LSP with a setup priority of 0. The default
setup priority is 7, which means that one LSP cannot preempt another. The default
hold priority is 0, which means that another LSP cannot preempt this one.
The four LSPs configured in this recipe have different priorities. The LSP R1-to-R5 is
the one being used for the most important customer. It will always be set up (setup
priority of 0) and can never be preempted (hold priority of 0) when RSVP is setting
up another LSP. The other three LSPs, in order from highest to lowest priority, are
R1-to-R6 (setup priority of 1, hold priority of 3), R1-to-R4 (setup priority of 4, hold
priority of 5), and R1-to-R5-low (setup priority of 7, hold priority of 7). The setup
and hold priority values for the different LSP are all evaluated relative to each other.
538 | Chapter 14: MPLS
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