Detour Merging Process

This section describes the process used by a router to determine which LSP to select when the router receives Path messages from different interfaces with identical Session and Sender Template objects. When this occurs, the router needs to merge the path states.

The router employs the following process to determine when and how to merge path states:

• When all the Path messages do not include a Fast Reroute or a Detour object, or when the router is the egress of the LSP, no merging is required. The messages are processed according to RSVP traffic engineering (TE).
• Otherwise, the router 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 router, the router 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 router, the router uses the following process to select the final LSP:
  • If only one LSP originates from this node, select it as the final LSP.
  • If only one LSP contains a Fast Reroute object, select it 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 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.
• Once the final LSP has been identified, the router must transmit only the Path messages that correspond to this LSP. All other LSPs are considered merged at this node.

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