Subject: [mpls] Answer to: "Does egress know where the penultimate hop in P2MP Egress Local Protection?"

From: Huaimo Chen <[email-masked]>
Date: Mon, 26 Sep 2011 20:44:16 +0000

Hi Yimin,

    The following is the answer with some more details to your question "In egress protection case: do you assume the egress router knows where the penultimate hop?" in IETF 81. 
    Ingress router should know the path for LSP, and the penultimate hop of the egress router. When ingress router has the explicit path for the LSP, it knows the penultimate hop of the egress router. When there are loose hops in the path for the LSP, the loose hops will be expanded or computed to the explicit hops during the signaling of the LSP. Thus the penultimate hop of the egress router is known eventually. 

A path message for the LSP contains an EGRESS_BACKUP_SUB_LSP object comprising information about the egress router and the corresponding backup egress router. The explicit path from the penultimate hop of the egress router to the backup egress router may be included in the path message. The penultimate hop of the egress router creates a backup LSP to the backup egress router when it knows that it is the penultimate hop of the egress router and needs to set up the backup LSP for protecting the egress router through analyzing the EGRESS_BACKUP_SUB_LSP object.

Best Regards,
Huaimo