Soft-Preemption Over Frr Backup Tunnels - Cisco CRS Configuration Manual

Soft-preemption over FRR Backup Tunnels

This sample illustration explains the TE path cost limit application:
Figure 20: MPLS TE path cost limit application
Here, the path cost limit for the LSP is set at 50. To move the LSP away from the link between F and G, the
link cost is increased to 50.
The total path cost is the aggregate of individual costs assigned to the links through which the LSP traverses.
The effect of specifying a limit to the path cost (admin-weight) are:
• For new LSPs, if the path cost limit is crossed, the LSP is considered invalid and does not get signaled
• For existing LSPs, if the path cost limit is crossed, the LSP is considered as 'failed'. If the current LSP
• To recover from a cost limit failure, re-optimization will be triggered using any available path option.
Soft-preemption over FRR Backup Tunnels
The soft-preemption over FRR backup tunnels feature enables to move LSP traffic over the backup tunnels
when the LSP is soft- preempted. MPLS TE tunnel soft-preemption allows removal of extra TE traffic in a
graceful manner, by giving the preempted LSP a grace period to move away from the link. Though this
mechanism saves the traffic of the preempted LSP from being dropped, this might cause traffic drops due to
congestion as more bandwidth is reserved on the link than what is available. When the soft-preemption over
FRR backup tunnel is enabled, the traffic of the preempted LSP is moved onto the FRR backup, if it is available
and ready. This way, the capacity of the backup tunnel is used to remove the potential congestion that might
be caused by soft-preemption.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
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across its calculated path. However, if an alternate path that is below the cost limit is available, then that
path is signaled.
fails (for both FRR and non-FRR LSPs), the standby LSP will be activated if it exists. If there is no
standby LSP, the tunnel will be re-optimized. If there is no standby LSP and no path is found for a
re-optimized tunnel then the tunnel is put in 'reroute pending' state and re-optimization is attempted
periodically.
Implementing MPLS Traffic Engineering