Supported Platforms
Multicast over Layer 3 VPNs
You can configure multicast routing over a network running a Layer 3 VPN that complies with RFC 4364. This section describes this type of network application and includes these topics:
Multicast over Layer 3 VPNs Overview
In the unicast environment for Layer 3 VPNs, all VPN state information is contained within the PE routers. However, with multicast for Layer 3 VPNs, Protocol Independent Multicast (PIM) adjacencies are established in one of the following ways:
- You can set PIM adjacencies between the CE router and the PE router through a VRF instance at the [edit routing-instances instance-name protocols pim] hierarchy level. You must include the group-address statement for the provider tunnel, specifying a multicast group. The rendezvous point (RP) listed within the VRF-instance is the VPN customer RP (C-RP).
- You can also set the master PIM instance and the PE’s IGP neighbors by configuring statements at the [edit protocols pim] hierarchy level. You must add the multicast group specified in the VRF instance to the master PIM instance. The set of master PIM adjacencies throughout the service provider network makes up the forwarding path that becomes an RP tree rooted at the service provider RP (SP-RP). Therefore, P routers within the provider core must maintain multicast state information for the VPNs.
For this to work properly, you need two types of RP routers for each VPN:
- A C-RP—An RP router located somewhere within the VPN (can be either a service provider router or a customer router).
- An SP-RP—An RP router located within the service
provider network.
Note: A PE router can act as the SP-RP and the C-RP. Moving these multicast configuration tasks to service provider routers helps to simplify the multicast Layer 3 VPN configuration process for customers. However, configuration of both SP-RP and VPN C-RP on the same PE router is not supported.
To configure multicast over a Layer 3 VPN, you must install a Tunnel Services Physical Interface Card (PIC) on the following devices:
- P routers acting as RPs
- PE routers configured to run multicast routing
- CE routers acting as designated routers or as VPN-RPs
For more information about running multicast over Layer 3 VPNs, see the following documents:
- Internet draft draft-rosen-vpn-mcast-02.txt, Multicast in MPLS/BGP VPNs
- Multicast Protocols Configuration Guide
The sections that follow describe the operation of a multicast VPN. Figure 1 illustrates the network topology used.
Figure 1: Multicast Topology Overview
Sending PIM Hello Messages to the PE Routers
The first step in initializing multicast over a Layer 3 VPN is the distribution of a PIM Hello message from a PE router (called PE3 in this section) to all the other PE routers on which PIM is configured.
You configure PIM on the Layer 3 VPN routing instance on the PE3 router. If a Tunnel Services PIC is installed in the routing platform, a multicast interface is created. This interface is used to communicate between the PIM instance within the VRF routing instance and the master PIM instance.
The following occurs when a PIM Hello message is sent to the PE routers:
- A PIM Hello message is sent from the VRF routing instance over the multicast interface. A generic routing encapsulation (GRE) header is prepended to the PIM Hello message. The header message includes the VPN group address and the loopback address of the PE3 router.
- A PIM register header is prepended to the Hello message as the packet is looped through the PIM encapsulation interface. This header contains the destination address of the SP-RP and the loopback address of the PE3 router.
- The packet is sent to the SP-RP.
- The SP-RP removes the top header from the packet and sends the remaining GRE-encapsulated Hello message to all the PE routers.
- The master PIM instance on each PE router handles the GRE encapsulated packet. Because the VPN group address is contained in the packet, the master instance removes the GRE header from the packet and sends the Hello message, which contains the proper VPN group address within the VRF routing instance, over the multicast interface.
Sending PIM Join Messages to the PE Routers
To receive a multicast broadcast from a multicast network, a CE router must send a PIM Join message to the C-RP. The process described in this section refers to Figure 1.
The CE5 router needs to receive a multicast broadcast from multicast source 224.1.1.1. To receive the broadcast, it sends a PIM Join message to the C-RP (the PE3 router):
- The PIM Join message is sent through the multicast interface, and a GRE header is prepended to the message. The GRE header contains the VPN group ID and the loopback address of the PE3 router.
- The PIM Join message is then sent through the PIM encapsulation interface and a register header is prepended to the packet. The register header contains the IP address of the SP-RP and the loopback address of the PE3 router.
- The PIM Join message is sent to the SP-RP by means of unicast routing.
- On the SP-RP, the register header is stripped off (the GRE header remains) and the packet is sent to all the PE routers.
- The PE2 router receives the packet, and because the link to the C-RP is through the PE2 router, it sends the packet through the multicast interface to remove the GRE header.
- Finally, the PIM Join message is sent to the C-RP.
Receiving the Multicast Transmission
The steps that follow outline how a multicast transmission is propagated across the network:
- The multicast source connected to the CE1 router sends the packet to group 224.1.1.1 (the VPN group address). The packet is encapsulated into a PIM register.
- Because this packet already includes the PIM header, it is forwarded by means of unicast routing to the C-RP over the Layer 3 VPN.
- The C-RP removes the packet and sends it out the downstream interfaces (which include the interface back to the CE3 router). The CE3 router also forwards this to the PE3 router.
- The packet is sent through the multicast interface on the PE2 router; in the process, the GRE header is prepended to the packet.
- Next, the packet is sent through the PIM encapsulation interface, where the register header is prepended to the data packet.
- The packet is then forwarded to the SP-RP, which removes the register header, leaves the GRE header intact, and sends the packet to the PE routers.
- PE routers remove the GRE header and forward the packet
to the CE routers that requested the multicast broadcast by sending
the PIM Join message.
Note: PE routers that have not received requests for multicast broadcasts from their connected CE routers still receive packets for the broadcast. These PE routers drop the packets as they are received.
