HP A6600 Configuration Manual page 236

PE 2 encapsulates the join message by means of GRE, with its BGP interface address as the multicast
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source address and the share-group address as the multicast group address, to convert it into a
normal, public network multicast data packet (11.1.2.1, 239.1.1.1). PE 2 then forwards it to the
public network.
• The multicast data packet (11.1.2.1, 239.1.1.1) is forwarded to the public network on all the PE
devices along the share-MDT. After receiving this packet, every PE device decapsulates it to turn it
back into a join message to be sent to the VPN RP. Then, each PE device checks the join message. If
any PE device finds that the VPN RP is in the site it interfaces with, it passes the join message to the
VPN instance on it. Otherwise, it discards the join message.
• When receiving the join message, the VPN instance on PE 1 considers that it received the message
from the MTI. PE 1 creates a local (*, 225.1.1.1) state entry, with the downstream interface being
the MTI and the upstream interface being the one that leads to CE 1. At the same time, it sends a
join message to CE 1, which is the VPN RP.
• After receiving the join message from the VPN instance on PE 1, CE 1 creates a local (*, 225.1.1.1)
state entry or updates the entry if it already exists. By now, the construction of an RPT across the
public network is completed.
For more information about GRE, see Layer 3—IP Services Configuration Guide.
Delivering multicast data packets
After the share-MDT is established, the multicast source forwards the VPN multicast data to the receivers
in each site along the distribution tree. The VPN multicast packets are encapsulated into public network
multicast packets on the local PE device, transmitted along the share-MDT, and then decapsulated on the
remote PE device and transmitted in that VPN site. VPN multicast data flows 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 to the
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receivers along the VPN SPT across the public network.
• On a VPN running PIM-SM:
Before SPT switchover, if the multicast source and the VPN RP are in different sites, the multicast
source forwards VPN multicast data to the VPN RP along the VPN SPT across the public network.
If the VPN RP and the receiver are in different sites, the VPN RP forwards VPN multicast traffic to
the receiver along the VPN RPT over the public network.
After SPT switchover, if the multicast source and the receiver are in different sites, the multicast
source forwards VPN multicast data to the receiver along the VPN SPT across the public
network.
For more information about RPT-to-SPT switchover, see
The following example explains how multicast data packets are delivered based on the share-MDT when
PIM-DM is running in both the public network and the VPN network.
As shown in
VPN multicast group G (225.1.1.1) in Site 2 is attached to CE 2, and Source in Site 1 sends multicast
data to multicast group G); the share-group address used to forward public network multicast data is
239.1.1.1.
Figure 70, PIM-DM is running in both the public network and the VPN sites, Receiver of the
"Configuring PIM."
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