H3C Technologies H3C S12500 Series Switches - Manual (Page 266)

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6. PE 1 encapsulates the multicast data by means of 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 (11.1.1.1, 239.1.1.1). PE 1 then forwards it
to the public network.
7. The multicast data packet (11.1.1.1, 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 VPN multicast data packet, and passes it to the corresponding VPN instance. If any
PE has a downstream interface for an SPT, it forwards the VPN multicast packet down the SPT.
Otherwise, it discards the packet.
8. The VPN instance on PE 2 searches the MVRF and finally delivers the VPN multicast data to
Receiver. By now, the process of transmitting a VPN multicast packet across the public network is
completed.
MDT switchover
Switching from share-MDT to switch-MDT
When a multicast data packet of a VPN is transmitted through the share-MDT on the public network, the
packet is forwarded to all PE devices that support that VPN instance, no matter whether any active
receivers exist in the attached sites. When the rate of the customer multicast traffic of that VPN is high,
multicast data might get flooded on 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 VPN multicast receivers and multicast sources for any large-traffic VPN 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 switchover is as follows:
1. The source-side PE (PE 1 in this example) device periodically checks the forwarding rate of the VPN
multicast traffic. Share-MDT to switch-MDT switchover takes place only when the following criteria
are both met:
{
The VPN multicast data has passed the filtering by an ACL rule for share-MDT to switch-MDT
switchover, and
{
The traffic rate of the VPN multicast stream has exceeded the switchover threshold and stayed
higher than the threshold for a certain length of time.
2. PE 1 chooses an idle switch-group address from the switch-group-pool and sends an MDT
switchover message to all the other PE devices down the share-MDT. This message contains the
VPN multicast source address, the VPN multicast group address and the switch-group address.
3. Each PE device that receives this message checks whether it interfaces with a VPN 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 MDT switchover message, PE 1 waits a certain length of time and then starts
using the switch-group address to encapsulate the VPN 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
MDT switchover messages 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.