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Overview

In a unicast environment for Layer 3 VPNs, all VPN states are contained within the provider edge (PE) routers. With multicast over Layer 3 VPNs, two PIM adjacencies are established: one between the customer edge (CE) and PE routers through a VRF instance, the second between the master PIM instance (configured at the [edit protocols pim] hierarchy level) and the IGP neighbors of this PE. The set of these master PIM adjacencies throughout the service provider's network makes up the forwarding path, which will eventually be a rendezvous point (RP) tree. The tree is rooted at the RP contained within the service provider's network. Because of this, core provider transit routers within the service provider's network must maintain multicast state information for the VPNs.

For multicast in Layer 3 VPNs to work correctly, there must be two types of rendezvous points for each VPN. The VPN-RP is an RP that resides within the VPN, and the service provider rendezvous point (SP-RP) resides within the service provider network.


A PE router can act as an SP-RP but cannot be the VPN-RP of a VPN. The VPN-RP must be located at a CE router or elsewhere within the VPN.

The operation of multicast within a Layer 3 VPN domain occurs in multiple stages, which are described on the following pages.


Figure 16: Multicast over Layer 3 VPNs Operation

Figure 16 shows the various stages that multicast packets pass through in a Layer 3 VPN environment.

  1. PIM is configured as part of a VPN routing instance and the configuration is committed. A virtual multicast tunnel interface (mt-x/y/z.abcde) is created if a Tunnel Services PIC exists on the router. This interface is used to communicate between the PIM instance within the VRF and the master PIM instance.
  2. A PIM HELLO is sent from the VRF across the mt interface. When this happens, a GRE header is prepended to the PIM HELLO 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 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 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 instance de-encapsulates the packet and sends the HELLO over the mt interface to reach the desired VPN group address within the VRF.
  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 PIM Join message is then sent through the pe interface and a register header is prepended to the packet. The data contained within the register header is the IP address of the SP-RP and the loopback address of router PE3.
  4. The PIM Join message is sent to the SP-RP using unicast routing.
  5. Upon arrival at the SP-RP, the register header is stripped off and the packet is sent (with the GRE header intact) to all the PE routers.
  6. Router PE2 receives the packet. Because the VPN-RP is behind router PE2, router PE2 sends the packet through the mt interface, which strips off the GRE header.
  7. The PIM Join message is now sent to the VPN-RP.
  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-RP by unicast routing over the Layer 3 VPN.
  3. The VPN-RP de-encapsulates the data packet and sends it out the downstream interfaces (which include the return path interface leading to router CE3). Router CE3 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 next sent through the pe interface where the register header is prepended to the data packet.
  6. The packet is forwarded to the SP-RP, which removes the register header from the packet.
  7. The packet is sent to the PE routers with GRE header intact.
  8. 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 as follows:

PIM is run only on the mt-encap interface. The mt-decap interface is used to populate downstream interface information.

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