255
Figure 79 Transmission of multicast protocol packets
The work process of multicast protocol packets is as follows:
1.
Receiver sends an IGMP membership report for multicast group G to CE 2. CE 2 creates a local
(*, 225.1.1.1) state entry and sends a join message to the VPN RP (CE 1).
2.
After receiving the join message from CE 2, the VPN instance on PE 2 creates a (*, 225.1.1.1)
state entry with the upstream interface being the MTI, and then PE 2 processes the join
message. Now, the VPN instance on PE 2 considers that the join message has been sent out
the MTI.
3.
PE 2 encapsulates the join message by means of Generic Routing Encapsulation (GRE), with
its BGP interface address as the multicast 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.
4.
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 de-encapsulates 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.
5.
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.
6.
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.
Delivery of 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
de-encapsulated 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:
1.
If PIM-DM or PIM-SSM is running in the VPN, the multicast source forwards multicast data to
the receivers along the VPN SPT across the public network.
Public instance BGP peers
MD
P
PE 1
PE 2
PE 3
BGP: 11.1.2.1/24
BGP: 11.1.3.1/24
Public instance join (11.1.2.1, 239.1.1.1)
BGP: 11.1.1.1/24
Share-Group: 239.1.1.1
RP
CE 1
CE 2
Source
Receiver
G: 225.1.1.1
S: 192.1.1.1/24
VPN instance join (*, 225.1.1.1)
Site 1
Site 2
