P2MP S2Ls can be preempted by or preempt other P2P or P2MP LSPs. SR OS supports P2MP S2L hard preemption as described in Hard preemption.
The P2P LSPs and P2MP S2L LSPs reserve the bandwidth they require throughout the network. The P2P and P2MP LSPs only compete for bandwidth if a link failure occurs and a single link is overloaded by additional P2P and S2L LSPs.
The user can configure the setup and hold priority on all LSP types to ensure the higher priority LSPs (P2P or S2L) can preempt lower priority LSPs.
Figure 1 illustrates a network example with three LSP types:
- gold P2P LSP, with setup 1 and hold 1
- silver P2MP LSP, with setup 3 and hold 3
- bronze P2P LSP, with setup 7 and hold 7
Figure 1. LSP priority example
If a link failure occurs between CR A1 and CR A3, as illustrated in Figure 2:
- The gold P2P LSPs take the shortest path and preempt all other P2P and P2MP LSPs that have lower hold values.
- The silver P2MP LSPs (S2L) take the second shortest path because there is insufficient bandwidth on the shortest path after the gold P2P LSPs reserve all the bandwidth. The bronze LSPs on this path are preempted.
- The bronze P2P LSPs take the longest path because there is insufficient bandwidth on the other two paths. They cannot preempt any other LSPs and can always be preempted by other LSPs that have higher priority values.
Figure 2. Link failure example
