Mdt Switchover - HP 6125XLG Ip Multicast Configuration Manual

2. CE 1 forwards the VPN multicast data packet along an SPT to PE 1, and the VPN instance on PE
1 examines the MVRF.
If the outgoing interface list of the forwarding entry contains an MTI, PE 1 processes the VPN
multicast data packet as described in step 3. The VPN instance on PE 1 considers the VPN
multicast data packet to have been sent out of the MTI, because step 3 is transparent to it.
3. PE 1 encapsulates the VPN multicast data packet into a public network multicast packet (11.1.1.1,
239.1.1.1) by using the GRE method. The source IP address of the packet is the MD source
interface 11.1.1.1, and the destination address is the default-group address 239.1.1.1. PE 1 then
forwards it to the public network.
4. The default-MDT forwards the multicast data packet (11.1.1.1, 239.1.1.1) to the public network
instance on all the PE devices. After receiving this packet, every PE device decapsulates it to get the
original VPN multicast data packet, and passes it to the corresponding VPN instance. If a PE
device has a downstream interface for an SPT, it forwards the VPN multicast packet down the SPT.
Otherwise, it discards the packet.
5. The VPN instance on PE 2 looks up the MVRF and finally delivers the VPN multicast data to
Receiver.
By now, the process of transmitting a VPN multicast data packet across the public network is
completed.

MDT switchover

Switching from default-MDT to data-MDT
When a multicast packet of a VPN is transmitted through the default-MDT on the public network, the
packet is forwarded to all PE devices that support that VPN instance. This occurs whether or not any
active receivers exist in the attached sites. When the rate of the multicast traffic of that VPN is high,
multicast data might get flooded on the public network. This increases the bandwidth use and burden on
the PE devices.
To optimize multicast transmission of large VPN multicast traffic that enters the public network, the MD
solution introduces a dedicated data-MDT. The data-MDT is built between the PE devices that connect
VPN multicast receivers and multicast sources. When specific network criteria are met, a switchover from
the default-MDT to the data-MDT occurs to forward VPN multicast traffic to receivers.
The process of default-MDT to data-MDT switchover is as follows:
1. The source-side PE device (PE 1, for example) periodically examines the forwarding flow of the
VPN multicast traffic. The default-MDT switches to the data-MDT only when the following criteria
are both met:
The VPN multicast data has passed the ACL rule filtering for default-MDT to data-MDT
switchover.
PE 1 receives the multicast traffic of the VPN for a certain length of time.
2. PE 1 selects a least-referenced address from the data-group address range and sends an MDT
switchover message to all the other PE devices down the default-MDT. This message contains the
VPN multicast source address, the VPN multicast group address, and the data-group address.
3. Each PE device that receives this message examines whether it interfaces with a VPN that has
receivers of that VPN multicast stream.
If so, it joins the data-MDT rooted at PE 1. Otherwise, it caches the message and will join the
data-MDT when it has attached receivers.
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