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Priorities and Preemption
Priorities and preemption are used 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 CR-LSP, a device can tear down an established CR-LSP and use the bandwidth assigned to that CR-LSP to establish a desired CR-LSP. This is called preemption. The following preemption modes are supported:
- Hard preemption: A CR-LSP with a higher priority can -directly delete preempted resources assigned to a CR-LSP with a lower priority. Some traffic is dropped on the CR-LSP with a lower priority during the hard preemption process. The CR-LSP with a lower priority is immediately deleted after its resources are preempted.
- Soft preemption: A CR-LSP with a higher priority can directly preempt resources assigned to a CR-LSP with a lower priority, but the CR-LSP with a lower priority is not deleted. During the soft preemption process, the bandwidth assigned to the CR-LSP with a lower priority gradually decreases to 0 kbit/s. Some traffic is forwarded while some may be dropped on the CR-LSP with a lower priority. The CR-LSP with a lower priority is deleted after the soft preemption timer expires.
CR-LSPs use setup and holding priorities to determine whether to preempt resources. The setup priority must be lower than or equal to the holding priority for a tunnel.
The priority and preemption attributes are used in conjunction to determine resource preemption among tunnels. If multiple CR-LSPs are to be established, CR-LSPs with high priorities can be established by preempting resources. If resources (such as bandwidth) are insufficient, a CR-LSP with a higher setup priority can preempt resources of an established CR-LSP with a lower holding priority.
The following tunnels are established on the network shown in Figure 4-12.
- Tunnel 1: established over the path LSRA → LSRF → LSRD. Its bandwidth is 155 Mbit/s, and its setup and holding priority values are 0.
- Tunnel 2: established over the path LSRB → LSRF → LSRC. Its bandwidth is 155 Mbit/s, and its setup and holding priority values are 7.
- If hard preemption is used, since Tunnel 1 has a higher priority than Tunnel 2, LSRF sends an RSVP message to tear down Tunnel 2. As a result, some traffic on Tunnel 2 is dropped if Tunnel 2 is transmitting traffic.
- If soft preemption is used, LSRF sends LSRB a Resv message. After LSRB receives this message, LSRB reestablishes Tunnel 2 over another path LSRB→LSRE→LSRD→LSRC. LSRB switches traffic to the new path before tearing down Tunnel 2 over the original path.
