To provide Multicast VPN (MVPN) Service, Service Providers (SPs) need
to instantiate multicast tunnels (known as "P-tunnels") that enable
the Provider Edge (PE) routers of a given VPN to transmit multicast
packets to each other. Some SPs organize their networks in a
hierarchical manner, with the PE routers in "edge areas", and the
edge areas connected to each other via a "core area". A P-tunnel that
connects PE routers in different edge areas can be thought of as
having three segments: a segment through one edge area, a segment
through the core area, and a segment through the second edge area.
It is desirable to preserve the independence of the core area by
allowing it to use a different tunneling technology than that used in
the edge areas. However, it is not desirable for the core area
Border Routers (BRs) to participate in the MVPN-specific signaling,
or even to have any knowledge of which MVPNs are in the edge areas
that attach to it. This document specifies a simple method for
segmenting MVPN P-tunnels at the BRs, subject to these constraints.