Preemption on MPLS

Unanswered Question
May 11th, 2007

I’m currently doing a study about Traffic Engineering over MPLS

MPLS traffic engineering sets up LSP along links with available resources, thus ensuring that bandwidth is always available for a particular flow.The path computation is performed by the headend router, which is the source of the TE tunnel. The Headend routeur uses CSPF calculation which result with an ordered set of IP address that map to next-hop IP addresses of routers forming the LSP.

This path computation uses links related information resource availability including bandwidth per priority (0-7). These informations are flooded by IGP such as OSPF-TE and IS-IS-TE. Then, RSVP with TE extensions reserves appropriate resources on each LSR in the path defined by the headend router and assigns labels mapping to the TE tunnel LSP. Extensions added to RSVP (RSVP-TE) and link state protocols OSPF (OSPF-TE) and IS-IS (IS-IS-TE) enable the setting up of pre-emption. The available bandwidths on links are configured on priority basis. When setting up TE LSP path, if the requested bandwidth is in use by a lower priority session, the lower priority session can be pre-empted (RSVP send PATHERR and RESVERR messages to close the current connexion)

Here is, globally, what I understood about theoretical aspect.

What I would like to know, is, how pre-emption is applied in practice ?

How, starting from a query coming from a distant station, can it be established a new LSP which will preempt, if necessary, the LSP in place?

How is it possible to manage, dynamically, the creation of new LSP?

What I could see on the cisco routers configuration, specify command are used to create LSP (It is what I could call, static way, the domain administrator has in charge to write these command to configure his MPLS domain )

interface Tunnel0

ip unnumbered Loopback0

tunnel destination

tunnel mode mpls traffic-eng

tunnel mpls traffic-eng autoroute announce

tunnel mpls traffic-eng priority 1 1

tunnel mpls traffic-eng path-option 1 dynamic

tunnel MPLS traffic-eng bandwidth 100

How the establishment of a new LSP, enable the dynamic reconfiguration of all the LSR present in the LSP?

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bjornarsb Sun, 05/13/2007 - 03:37


You might be aware of this but check the following command:

tunnel mpls traffic-eng priority setup-priority [hold-priority]

Configures setup and reservation priority for a tunnel.

The setup-priority argument is the priority used when signaling an LSP for this tunnel to determine which existing tunnels can be preempted.

Valid values are from 0 to 7. A lower number indicates a higher priority. An LSP with a setup priority of 0 can preempt any LSP with a non-0 priority.

The hold-priority argument is the priority associated with an LSP for this tunnel to determine if it should be preempted by other LSPs that are being signaled.

Valid values are from 0 to 7, where a lower number indicates a higher priority.

Also have a look at this post:



Djule2804 Wed, 05/16/2007 - 00:55

Thank you very much for your response Bjornarsb.

Your response helped me.


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