MPLS / RSVP-TE help - Priority setup / hold values

[mozmarr124]

Hi, I am really trying to get my head around how LSP's are pre-empted. I understand the values of 0 and 7 (highest priority and lowest priority so that LSPs will always be pre-empted or stay as they are) however I don't understand any of the other values in between 2-6. How do they relate to the importance of an LSP or LSP's? Is it if you have a number of LSPs contending so they can be rated for importance? Also, do the priority values relate to any specific bandwidth level? If anyone can explain in simple plain English (idiots guide!) that would be appreciated.

[Peter Paluch · Accepted solution]

Hi Stephen,

The setup and hold priorities on MPLS-TE tunnels are indeed somewhat confusing.

Keep in mind that when a new MPLS-TE tunnel is just being set up, the headend router performs the path calculation to determine the optimal path for this tunnel based on its bandwidth requirements and the available bandwidth on links in the network. If such path exists, then the priorities are ignored because there was still enough bandwidth in the network to support this particular tunnel, and so it was not necessary to preempt any other existing tunnel.

If, however, such a path does not exist because the available bandwidth is lower than what the tunnel requires, now it comes to the priorities. The router will compare the setup priority of the tunnel just being established to the hold priorities of the tunnel that already exist, and if the setup priority of the new tunnel is higher than the hold priority of an existing tunnel, the new tunnel is allowed to preempt the existing tunnel.

So, in terms of importance of tunnels, these two priorities describe two independent qualities of a tunnel:

• Setup priority: How important is it to bring this tunnel up even at the expense of tearing down another existing tunnel?
• Hold priority: Once established, how important is to keep this tunnel operational at the expense of not allowing another tunnel to come up?

This article sums up these points nicely:

http://wiki.kemot-net.com/mpls-te-path-setup-priority-hold

The priority levels themselves do not directly relate to any bandwidth levels - it is up to you as a network administrator to decide why a particular tunnel is specifically important so that it comes up at the expense of another tunnels, and possibly stays alive.

Would this help a little? Please feel welcome to ask further!

Best regards,
Peter

[Goodwinthegr8]

Hi there,

I got a query here. Is it necessary that the Hold priority needs to be Setup priority? 

Thanks,

Subhajit 

[amilad]

Nice explanation thanks

[hossam helal]

Thank you