Usage Scenario
The setup and holding priorities and the preemption function are configured to allow TE tunnels to be established preferentially to transmit important services, preventing random resource competition during tunnel establishment.
If there is no path meeting the bandwidth requirement of a desired tunnel, a device can tear down an established tunnel and use bandwidth resources assigned to that tunnel to establish a desired tunnel. This is called preemption. The following preemption modes are supported:
- Hard preemption: A tunnel with a higher setup priority can preempt resources assigned to a tunnel with a lower holding priority. Consequently, some traffic is dropped on the tunnel with a lower holding priority during the hard preemption process.
- Soft preemption: After a tunnel with a higher setup priority preempts the bandwidth of a tunnel with a lower holding priority, the soft preemption function retains the tunnel with a lower holding priority for a specified period of time. If the ingress finds a better path for this tunnel after the time elapses, the ingress uses the make-before-break (MBB) mechanism to reestablish the tunnel over the new path. If the ingress fails to find a better path after the time elapses, the tunnel goes Down.
To enable soft preemption, run the mpls te soft-preemption command.
Prerequisites
MPLS TE has been configured as a tunnel protocol using the tunnel-protocol mpls te command.
P2MP-TE has been configured using the
mpls te p2mp-mode command.
Configuration Impact
The priority and preemption attributes are used in conjunction to determine resource preemption among tunnels. If multiple tunnels are to be established, tunnels with higher setup priorities can be established by preempting resources. If resources, such as bandwidth, are insufficient, a tunnel with a higher setup priority can preempt resources of an established tunnel with a lower holding priority.