Description: The bits of tBgpInstanceIgpShortcut identify the Layer-2 tunneling mechanisms used for IGP shortcuts. If a bit is set, the corresponding tunneling mechanism will be used to resolve paths to BGP next-hops.

The tunneling mechanisms represented by each bit are: te(0) - RSVP Traffic Engineering ldp(1) - LDP /32 Route Forwarding Equivalence Class (FEC) ip(2) - IP (IP-in-IP or GRE) mpls(3)- MPLS

Evaluation of tunnel preference is based on the following order, in descending priority: 1. Traffic Engineered Shortcut (RSVP-TE) 2. LDP /32 Route FEC Shortcut 3. IP Shortcut (IP-in-IP or GRE) 4. Actual IGP Next-Hop.

The 'mpls' option instructs BGP to first attempt to resolve the BGP next-hop to an RSVP LSP. If no RSVP LSP exists or the existing ones are down, BGP automatically searches for the LDP LSP with a FEC prefix corresponding to the same /32 prefix in the tunnel table and resolves the BGP next-hop to it.