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Dual PIM Draft-Rosen Multicast VPN Operation

 

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 1 and described on the following pages.

Figure 1: Multicast Over Layer 3 VPN Operation
Multicast Over Layer 3 VPN
Operation

Figure 1 shows the various stages that multicast packets pass through in a Layer 3 VPN environment.

  • Stage 1: PIM hello messages

  1. 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 fpc slot-number pic number] 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.

  2. 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.

  3. 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.

  4. The packet is sent to the SP-RP.

  5. 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.

  6. 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

  1. 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.

  2. 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.

  3. The GRE encapsulated Join message is sent to other PE routers.

  4. 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.

  5. The PIM Join message is now sent to the VPN C-RP.

  • Stage 3: Multicast forwarding

  1. 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.

  2. 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.

  3. 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.

  4. The data packet is sent through the mt interface on Router PE2. In the process, the GRE header is prepended to the packet.

  5. The packet is sent to the PE routers with GRE header intact.

  6. 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.