preempt (SEP segment view)

In a SEP segment, some ports are blocked to prevent loops.

When a link in the SEP segment fails, the blocked interface in Up state changes to the Forwarding state after receiving a fault notification packet. If the preemption mode is not configured, when all link faults are rectified or the last two interfaces enabled with SEP complete neighbor negotiations, interfaces send blocking status packets to each other. The interface with the highest priority is then blocked, and the other interfaces enter the Forwarding state.

The blocked interface is determined by the administrative priority and user-defined priority of each interface. The priority value of an interface contains 16 bits. The higher 8 bits are defined by the system, and the lower 8 bits are set by the user. The value of the lower 8 bits ranges from 1 to 128, and the default value is 64. The interface with the highest priority is blocked.

If the preemption mode is configured on the device where the primary edge interface is located, the specified interface transitions to the Blocking state and sends blocking status packets to other ports.

SEP supports the following preemption modes:

• Delayed preemption

  After all interface faults are rectified, the faulty interface does not send any fault notification packet. If the primary edge interface does not receive any fault notification packet within 3 seconds, it starts the delay timer. When the delay timer expires, the devices in the SEP segment initiate blocked interface preemption.

  

  You must set the preemption delay when delayed preemption is used because there is no default delay time.

  After delay preemption is configured, you can run the undo preempt delay command in the SEP segment view to delete the configuration of delay preemption.

• Manual preemption

  In this mode, if the link status databases of the primary and secondary edge interfaces are complete, the primary edge interface sends preemption packets to block the specified interface. Then the specified interface sends blocking status packets to request the original blocked interface to transition to the Forwarding state.

  

  • Manual preemption is a one-off operation and will not take effect again after the preemption is complete on the SEP segment. To change the blocked interface, run the preempt command and specify the delay parameter to configure delayed preemption.

  • Manual preemption causes a short link disconnection in the SEP segment.

Blocked interface preemption is triggered when the following conditions are met:

• The primary edge interface in the SEP segment is selected.

  To configure interface roles, run the sep segment segment-id [ edge [ no-neighbour ] { primary | secondary } ] command in the interface view.

• The method of selecting the interface to block is configured on the device where the primary edge interface is located.

  To configure the method of selecting the interface to block, run the block port { sysname sysname interface interface-type interface-number | hop hop-id | optimal | middle } command in the SEP segment view.

• The topology of the SEP segment must be complete.