3Com MSR 50 Series Configuration Manual page 1291

get flooded in the public network, causing bandwidth waste and extra burden on
the PE devices.
To optimize multicast transmission, the MD solution establishes a dedicated
switch-MDT between the PE devices with private network multicast receivers and
multicast sources for any large-traffic private network multicast stream before it
enters the public network. Then, the multicast stream is switched from the
share-MDT to the switch-MDT, to deliver the multicast data to only those receivers
that need it.
The process of share-MDT to switch-MDT switching is as follows:
1 The source-side PE device periodically checks the forwarding rate of the VPN
multicast traffic. Share-MDT to switch-MDT switching takes place only when the
following conditions are both met:
The private network multicast data has passed ACL filtering, and
■
The traffic rate of the private network multicast stream has exceeded the
■
switching threshold and stayed higher than the threshold for a certain length
of time.
2 As in the previous example, PE 1 chooses an idle switch-group address from the
switch-group-pool and sends a switching notification message to all the other PE
devices down the share-MDT. This message contains the private network multicast
source address, the private network multicast group address and the switch-group
address.
3 Each PE device that receives this message checks whether it interfaces with a
private network that has receivers of that VPN multicast stream. If so, it joins the
switch-MDT rooted at PE 1; otherwise, it caches the message and will join the
switch-MDT when it has attached receivers.
4 After sending the switching notification, PE 1 waits a certain length of time and
then starts using the switch-group address to encapsulate the private network
multicast data, so that the multicast data is forwarded down the switch-MDT.
5 After the multicast traffic is switched from the share-MDT to the switch-MDT, PE 1
continues sending switch notifications periodically, so that subsequent PE devices
with attached receivers can join the switch-MDT. When a downstream PE device
has no longer active receivers attached to it, it leaves the switch-MDT.
n
For a given VPN instance, the share-MDT and the switch-MDT are both forwarding
tunnels in the same MD. A share-MDT is uniquely identified by a share-group
address, while a switch-MDT is uniquely identified by a switch-group address.
Each share-group is uniquely associated with a set of switch-group addresses,
namely a switch-group-pool.
Backward Switching from Switch-MDT to Share-MDT
After the private network multicast traffic is switched to the switch-MDT, the
multicast traffic conditions may change and no longer meet the aforesaid
switching criterion. In this case, PE 1, as in the example above, initiates a backward
MDT switching process. When any of the following conditions is met, the
multicast traffic is switched from the switch-MDT back to the share-MDT:
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