Detour Merging Procedure
This section describes the procedures used by a label-switched router (LSR) to determine which LSP to select when the LSR receives Path messages from different interfaces with identical SESSION and SENDER_TEMPLATE objects. When this occurs, the LSR needs to merge the path states.
The LSR employs the following procedure to determine when and how to merge path states:
- When all of the Path messages do not include a FAST_REROUTE or a DETOUR object, or when the LSR is the egress of the LSP, no merging is required. The messages are processed according to RSVP-TE.
- Otherwise, the LSR must record the path state in addition to the incoming interface. If the path messages do not share the same outgoing interface and next-hop LSR, the LSR considers them to be independent LSPs and does not merge them.
- For all the Path messages that share the same outgoing interface and next-hop LSR, the LSR uses the following procedure to select the final LSP:
- If only one LSP originates from this node, it is selected as the final LSP.
- If only one LSP contains a FAST_REROUTE object, it is selected as the final LSP.
- If there are several LSPs and some of them have a DETOUR object, eliminate those containing a DETOUR object from the final LSP selection process.
- If several final LSP candidates remain (that is, there are still both DETOUR and protected LSPs), select the LSPs with FAST_REROUTE objects.
- If none of the LSPs have FAST_REROUTE objects, select the ones without DETOUR objects. If all of the LSPs have DETOUR objects, select them all.
- Of the remaining LSP candidates, eliminate from consideration those that traverse nodes that other LSPs avoid.
- If several candidate LSPs still remain, select the one with the shortest ERO path length. If more than one LSP has the same path length, select one randomly.