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.