When unicast services use IGP shortcuts, IGP shortcut next-hops are installed in the RTM. Therefore, for multicast P2MP MLDP, the leaf node resolves the root using these IGP shortcuts. Currently MLDP cannot be resolved using IGP shortcuts. To avoid this, MLDP does a lookup in the multicast RTM. IGP shortcuts are not installed in MRTM. The command configure router ldp resolve-root-using forces MLDP do next-hop lookups in the RTM or MRTM.
By default, the configure router ldp resolve-root-using command is set to ucast-rtm and MLDP uses the unicast RTM for resolution of the FEC in all cases. When MLDP uses the unicast RTM to resolve the FEC, it does not resolve the FEC if its next hop is resolved using an IGP shortcut.
To force MLDP resolution to use the multicast RTM, use the configure router ldp resolve-root-using mcast-rtm command. When this command is enabled:
For FEC resolution using IGP, static or local, the ROOT in this FEC is resolved using the multicast RTM.
A FEC being resolved using BGP is recursive, so the FEC next-hop (ASBR/ABR) is resolved using the multicast RTM first and, if this fails, it is resolved using the unicast RTM. This next-hop needs to be recursively resolved again using IGP/Static-Route or Local, this second resolution (recursive resolution) uses the multicast RTM only; see Figure 1.
When configure router ldp resolve-root-using ucast-rtm is set, MLDP uses the unicast RTM to resolve the FEC and does not resolve the FEC if its next hop is resolved using an IGP shortcut.
For inter-AS or intra-AS, IGP shortcuts are limited to each AS or area connecting LEAF to ASBR, ASBR to ASBR, or ASBR to ROOT.
In Figure 1, the FEC between LEAF and ASBR-3 is resolved using an IGP shortcut. When the configure ldp resolve-root-using is set to mcast-rtm, the inner Root 100.0.0.14 is resolved using the multicast RTM first. If the multicast RTM lookup fails, then a second lookup for 100.0.0.14 is done in the unicast RTM. Resolution of 100.0.0.14 results in a next-hop of 100.0.0.21 which is ASBR-3, therefore ASBR-3 100.0.0.21 is resolved only using multicast RTM when mcast-rtm is enabled.