HP FlexFabric 5950 series Configuration Manual page 223

2. After receiving the join message from CE 2, the VPN instance on PE 2 creates a state entry (*,
225.1.1.1) and specifies the MTI interface as the upstream interface. The VPN instance on PE
2 considers the join message to have been sent out of the MTI interface, because step 3 is
transparent to the VPN instance.
3. PE 2 encapsulates the join message into a public network multicast data packet (11.1.2.1,
239.1.1.1) by using the GRE method. In this multicast data packet, the source address is the
MD source interface IP address 11.1.2.1, and the destination address is the default-group
address 239.1.1.1. PE 2 then forwards this packet to the public network.
4. The default-MDT forwards the multicast data packet (11.1.2.1, 239.1.1.1) to the public network
instance on all the PE devices. After receiving this packet, every PE device decapsulates it to
get the original join message to be sent to the VPN RP. Then, each PE device examines the
VPN RP address in the join message. If the VPN RP is in the site to which a PE device is
connected, the PE passes the join message to the VPN instance on the PE. Otherwise, the PE
discards the join message.
5. When receiving the join message, the VPN instance on PE 1 considers the received message
to be from the MTI. PE 1 creates a local state entry (*, 225.1.1.1), with the downstream interface
being the MTI and the upstream interface being the one that leads to CE 1. At the same time,
the VPN instance sends a join message to CE 1, which is the VPN RP.
6. After receiving the join message from the VPN instance on PE 1, CE 1 creates a local state
entry (*, 225.1.1.1) or updates the entry if the entry already exists.
By now, the construction of an RPT across the public network is completed.
Multicast data packet 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 device, and transmitted along the default-MDT.
Then, they are decapsulated on the remote PE device 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
•
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 77:
213