Default-MDT-based delivery
After the default-MDT is established, the multicast source forwards the VPN multicast data to the receivers in each site along the default-MDT. The VPN multicast packets are encapsulated into public network multicast packets on the local PE, and transmitted along the default-MDT. Then, they are decapsulated on the remote PE and transmitted in that VPN site.
VPN multicast data packets are forwarded across the public network differently in the following circumstances:
If PIM-DM or PIM-SSM is running in the VPN, the multicast source forwards multicast data packets to the receivers along the VPN SPT across the public network.
When PIM-SM is running in the VPN:
Before the RPT-to-SPT switchover, if the multicast source and the VPN RP are in different sites, the VPN multicast data packets travel to the VPN RP along the VPN SPT across the public network. If the VPN RP and the receivers are in different sites, the VPN multicast data packets travel to the receivers along the VPN RPT over the public network.
After the RPT-to-SPT switchover, if the multicast source and the receivers are in different sites, the VPN multicast data packets travel to the receivers along the VPN SPT across the public network.
When BIDIR-PIM is running in the VPN, if the multicast source and the VPN RP are in different sites, the multicast source sends multicast data to the VPN RP across the public network along the source-side RPT. If the VPN RP and the receivers are in different sites, the multicast data packets travel to the receivers across the public network along the receiver-side RPT.
For more information about RPT-to-SPT switchover, see "Configuring PIM."
The following example explains how multicast data packets are delivered based on the default-MDT when PIM-DM is running in both the public network and the VPN network.
As shown in Figure 73:
PIM-DM is running in both the public network and the VPN sites.
Receiver of the VPN multicast group G (225.1.1.1) in Site 2 is attached to CE 2.
Source in Site 1 sends multicast data to multicast group (G).
The default-group address used to forward public network multicast data is 239.1.1.1.
Figure 73: Multicast data packet delivery
A VPN multicast data packet is delivered across the public network as follows:
Source sends a VPN multicast data packet (192.1.1.1, 225.1.1.1) to CE 1.
CE 1 forwards the VPN multicast data packet along an SPT to PE 1, and the VPN instance on PE 1 examines the MVRF.
If the outgoing interface list of the forwarding entry contains an MTI, PE 1 processes the VPN multicast data packet as described in step 3. The VPN instance on PE 1 considers the VPN multicast data packet to have been sent out of the MTI, because step 3 is transparent to it.
PE 1 encapsulates the VPN multicast data packet into a public network multicast packet (11.1.2.1, 239.1.1.1) by using the GRE method. The source IP address of the packet is the MVPN source interface 11.1.1.1, and the destination address is the default-group address 239.1.1.1. PE 1 then forwards it to the public network.
The default-MDT forwards the multicast data packet (11.1.2.1, 239.1.1.1) to the public network instance on all the PEs. After receiving this packet, every PE decapsulates it to get the original VPN multicast data packet, and passes it to the corresponding VPN instance. If a PE has a downstream interface for an SPT, it forwards the VPN multicast packet down the SPT. Otherwise, it discards the packet.
The VPN instance on PE 2 looks up the MVRF and finally delivers the VPN multicast data to Receiver.
By now, the process of transmitting a VPN multicast data packet across the public network is completed.