Path Computation Fallback for PCC-initiated LSPs

For PCC-initiated SR-TE LSPs, the router supports fallback to a local path computation method when the configured PCEP sessions are down or the PCE is unreachable. Fallback to a local computation method can be used when all configured PCEs are signaling overload and the redelegation timer has expired so that the LSP cannot be redelegated.

This capability is supported by both active stateful and passive stateful LSPs. Active stateful LSPs are fully delegated to the PCE by being both PCE-computed (pce-computation) and PCE-controlled. Passive stateful LSPs are PCE-computed.

Note: For passive stateful LSPs, it is important that the retry-timer and retry-limit values exceed the redelegation-timer value, otherwise; the LSP may go operationally down before the fallback path computation has occurred.

A fallback path computation method is configured as follows:

configure>router> 
   mpls 
      lsp <xyz>
         fallback-path-computation-method [none]
         pce-computation
         pce-control

If the fallback path computation method is configured as none, MPLS uses the default method based on the configured path, which is hop-to-label path computation for SR-TE LSPs.

The fallback-path-computation-method command is only valid if pce-computation is enabled.

The fallback mechanism is only triggered if the PCC informs MPLS that PCEP is down. It is not triggered while the PCC is administratively down or if it is not yet configured.

Note: On the first local path computation following a fallback, MPLS is not aware of the list of SRLGs or administrative groups that are used by the original path computed by PCE. As a result, MPLS can only provide a list of hops or links to avoid on the first computation.

PCE reports are sent, where applicable, with the delegation bit cleared.