The operation of Draft-rosen multicast within a
Layer 3 VPN domain with provider tunnels operating in any-source (ASM)
multicast mode occurs in multiple stages, which are shown in Figure 34 and described on the following pages.
Figure 34: Multicast Over Layer 3 VPN Operation
Figure 34 shows the various stages that multicast
packets pass through in a Layer 3 VPN environment.
Stage 1: PIM hello messages
PIM is configured as part of a VPN routing instance and
the configuration is committed. For Juniper Networks M Series Multiservice
Edge Routers and Juniper Networks T Series Core Routers, a virtual
multicast tunnel interface (mt-fpc/pic/port.abcde) is created if a Tunnel Services Physical Interface Card (PIC)
is installed on the router. On Juniper Networks MX Series Ethernet
Services Routers, you can create a virtual multicast tunnel interface
by including the tunnel-services statement at the [edit
chassis fpcslot-numberpicnumber] hierarchy level. For more
information about configuring tunnel interfaces on MX Series routers,
see the JUNOS System Basics Configuration Guide. The virtual multicast tunnel interface is used to communicate between
the PIM instance within the VRF and the master PIM instance.
A PIM hello message is sent from the VRF across the mt interface. When this happens, a GRE header is prepended to
the PIM hello message with fields containing the VPN group address
and the loopback address of the PE router.
A PIM register header is prepended to the hello message
as the packet is looped through the pe (PIM encapsulation)
interface. This header contains the destination address of the SP-RP
and the loopback address of the PE router.
The packet is sent to the SP-RP.
The SP-RP de-encapsulates the top header off the packet
as it travels through the pd (PIM de-encapsulation) interface
and sends the remaining GRE encapsulated hello message to all of the
PE routers.
The master PIM instance on the PE router handles the GRE
encapsulated packet. Because the VPN group address is contained in
the packet, the master PIM instance de-encapsulates the packet and
sends the hello message over the mt interface to reach the
desired VPN group address within the VRF.
Stage 2: PIM join message
Router CE5 is interested in receiving from multicast source 224.1.1.1, so a PIM Join message is sent from Router CE5 to
Router PE3.
The PIM Join message is sent through the mt interface
and a GRE header is prepended to it. The GRE header contains the VPN
group ID and the loopback address of Router PE3.
The GRE encapsulated Join message is sent to other PE
routers.
Router PE2 receives the packet. Because the VPN C-RP is
behind Router PE2, Router PE2 sends the packet through the mt interface, which strips off the GRE header.
The PIM Join message is now sent to the VPN C-RP.
Stage 3: Multicast forwarding
The source behind Router CE1 is sending to group 224.1.1.1. The designated router (DR) behind the CE router encapsulates this
packet into a PIM register.
Because the packet already has the PIM register header,
it is forwarded to the VPN C-RP by unicast routing over the Layer
3 VPN.
The VPN C-RP de-encapsulates the data packet and sends
it out the downstream interfaces (which include the return path interface
leading to Router PE3). Router P also forwards the packet to
Router PE3.
The data packet is sent through the mt interface
on Router PE2. In the process, the GRE header is prepended to the
packet.
The packet is sent to the PE routers with GRE header intact.
The “interested” PE routers strip the GRE
header off the packet and forward it to the CE routers that requested
the PIM join. If there are no PIM-join messages for this group at
this site, the PE router drops the packet.
When PIM is configured within
a routing instance, two mt interfaces are created:
mt-[xxxxx](xxxxx) range is 32768 through 49151 for mt-encap
mt-[yyyyy](yyyyy) range is 49152 through 65535 for mt-decap
PIM is run only on the mt-encap interface. The mt-decap interface is used to populate downstream
interface information.
