Answer
If you configure a virtual tunnel (vt) interface on the egress PE router and that router is also a transit router for the point-to-multipoint LSP, the penultimate hop router sends just one copy of each packet over the link to the egress PE router. A vt interface can perform two lookups on an incoming packet, one for the multicast MPLS lookup and one for the IP lookup. This applies to Layer 3 VPNs and VPLS point-to-multipoint LSPs.
This is the behavior of vt-interface in different services:
- NG-MVPN: The MPLS lookup is performed first, with MPLS packet copying. One of the copies has a null label and is sent to vt-ifl. The null label in this case is a label pop operation. When the packet is received from vt-ifl, IP lookup and copying is performed in VRF.
- VPLS: Initial behavior is the same as in NG-MVPN. The MPLS lookup is performed first, with MPLS packet copying. One of the copies has a null label and is sent to vt-ifl. The null label in this case is a label pop operation. However, in VPLS, when a packet is received from vt-ifl, Ethernet MAC lookup and copying is performed in the VPLS edge (VE) or the virtual switch interface (VSI).
- CCC: Initial behavior is the same as in NG-MVPN and VPLS. The MPLS lookup is performed first, with MPLS packet copying. One of the copies has a null label, however, this label is sent to _egress CCC interface. A second lookup is not needed.
For more information, click Layer 2 VPNs and VPLS Feature Guide.