Path Computation Fallback for PCC-Initiated LSPs

For PCC-initiated RSVP-TE and SR-TE LSPs, the router supports fallback to a local path computation method in the case where the configured PCEP sessions are down or the PCE is unreachable, or when all configured PCEs are signaling overload and the redelegation timer expires while all configured LSPs are signaling overload so that the LSP cannot be redelegated. The fallback method can be configured to be the local CSPF or none. In the latter case, MPLS uses the hop-to-label translation (SR-TE LSPs) or the explicit IGP path (RSVP-TE LSPs).

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

Note: For the passive stateful case, 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>
         pce-control
         path-computation-method {pce | local-cspf} 
         fallback-path-computation-method {none | local-cspf}

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

The fallback-path-computation-method command is only valid for path-computation-method pce, irrespective of whether pce-control is configured. It is mutually exclusive with the path-computation-method local-cspf and no path-computation-method commands.

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