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Share-MDT establishment in a PIM-SSM network
Figure 78 Share-MDT establishment in a PIM-SSM network
As shown in
, PIM-SSM is enabled in the network and all the PE devices support VPN
instance A. The process of establishing a share-MDT is as follows:
The public network on PE 1 sends the local BGP MDT routing information, including its BGP
interface address and the share-group address, to PE 2 and PE 3. PE 2 and PE 3 perform the same
operation to exchange their BGP MDT routing information with one another. After receiving the BGP
MDT information from PE1, PE 2 and PE 3 separately send a subscribe message for channel
subscription hop by hop toward the BGP interface of PE1. A (11.1.1.1, 232.1.1.1) entry is created on
devices on the path toward PE 1 in the public network. Thus an SPT is created in the network, with
PE 1 as its root, PE 2 and PE 3 as its leaves.
At the same time, PE 2 and PE 3 separately initiate a similar SPT establishment process. Finally,
three independent SPTs are established in the MD. In the PIM-SS M network, the three independent
SPTs constitute a share-MDT.
NOTE:
In PIM-SSM, subscribe messages are used equivalent to join messages.
Characteristics of a share-MDT
A share-MDT is characterized as follows, no matter what PIM mode is running in the public network:
•
All PE devices that support this VPN instance join the share-MDT.
•
All VPN multicast packets that belong to this VPN, including protocol packets and data packets,
are forwarded along the share-MDT to every PE device in the public network, even if they have
no active receivers downstream.
Share-MDT-based delivery
A share-MDT can be used for delivering multicast packets, including both multicast protocol packets
and multicast data packets. However, the transmission processes for these two types of multicast
packets are different.
Delivery of multicast protocol packets
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
Public instance BGP peers
MD
P
PE 3
PE 2
PE 1
BGP: 11.1.2.1/24
BGP: 11.1.1.1/24
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)
BGP: 11.1.3.1/24
Share-Group: 232.1.1.1