3Com S7906E - Switch, Configuration Manual

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Shart-MDT establishment in a PIM-SSM network
Figure 1-7 Shart-MDT establishment in a PIM-SSM network
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
As shown in Figure 1-7 , 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 instance 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. Upon receiving the BGP MDT
information from PE1, PE 2 and PE 3 respectively 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 respectively 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.
Characteristics of a share-MDT
As discussed above, a share-MDT is characterized as follows, no matter what PIM mode is running in
the public network:
z
All PE devices that support this VPN instance join the share-MDT.
z
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
When a multicast source and the receivers are located in different sites of a VPN, multicast protocol
packets must be transmitted across the public network. Protocol packets are encapsulated into normal
multicast data packets for the public network on the local PE device, transmitted along the share-MDT,