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Default-MDT establishment in a PIM-SSM network
Figure 75 Default-MDT establishment in a PIM-SSM network
PE 1
BGP: 11.1.1.1/24
As shown in
A. The process of establishing a default-MDT is as follows:
1.
PE 1, PE 2, and PE 3 exchange MDT route information (including BGP interface address and
the default-group address) through BGP.
2.
PE 1 sends a subscribe message to PE 2 and PE 3. Each device on the public network creates
an (S, G) entry. An SPT is established in the MD with PE 1 as the root and PE 2 and PE 3 as the
leaves.
At the same time, PE 2 and PE 3 separately initiate a similar process, and establish an SPT with
itself as the root and the other PEs as the leaves.
3.
The three independent SPTs constitute the default-MDT in the PIM-SSM network.
In PIM-SSM, the term "subscribe message" refers to a join message.
Default-MDT characteristics
No matter which PIM mode is running on the public network, the default-MDT has the following
characteristics:
•
All PE devices that support the same VPN instance join the default-MDT.
•
All multicast packets that belong to this VPN are forwarded along the default-MDT to every PE
device on the public network, even if no active downstream receivers exist.
Default-MDT-based delivery
The default-MDT delivers multicast protocol packets and multicast data packets differently.
Multicast protocol packet delivery
To forward the multicast protocol packets of a VPN over the public network, the local PE device
encapsulates them into public network multicast data packets. These packets are transmitted along
the default-MDT and are then decapsulated on the remote PE device to go into the normal protocol
procedure. Finally, a distribution tree is established across the public network.
The following describes how multicast protocol packets are forwarded in different circumstances:
•
If the VPN network runs PIM-DM or PIM-SSM:
Hello packets are forwarded through MTI interfaces to establish PIM neighboring
relationships.
BGP: 11.1.3.1/24
PE 3
P
MD
Figure
75, PIM-SSM runs on the network, and all the PE devices support VPN instance
Default-Group: 232.1.1.1
Public instance BGP peers
SPT (11.1.1.1, 232.1.1.1)
SPT (11.1.2.1, 232.1.1.1)
SPT (11.1.3.1, 232.1.1.1)
PE 2
BGP: 11.1.2.1/24
211
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