When soft preemption is enabled for P2MP S2L LSPs, the S2L preemption is governed by the timer value configured using the config>router>rsvp preemption-timer classic CLI command or configure router rsvp preemption-timer MD-CLI command.
When the S2L is preempted at an LSR node, the preempting node sends to the head-end node an Resv refresh message with the "preemption pending" flag set or a PathErr message with error code=34 (Reroute) and a value=1 (Reroute request soft preemption). The preemption timer (configured as described above) starts. When the timer expires, the node performs MBB on the affected adaptive CSPF LSP. Both IGP metric and TE metric based CSPF LSPs are included. If an alternative path that excludes the flagged interface is not found, the LSP is placed on a retry list in a similar way to the global revertive procedure at a head-end node.
When the preemption timer expires, the preempting node tears down the S2L and sends a path error to the head-end node, and the head-end node places the S2L on the retry list.