HP FlexNetwork 7500 Series Configuration Manual page 192

Table 16 Basic MD VPN concepts
Concept
Multicast domain (MD)
Multicast distribution tree (MDT)
Multicast tunnel (MT)
Multicast tunnel interface (MTI)
Default-group
Default-MDT
Data-group
Data-MDT
Introduction to MD VPN
The main points in MD VPN implementation are as follows:
•
The public network of the service provider supports multicast:
The PE devices must support the public network and multiple VPN instances.
Each instance runs PIM independently.
VPN multicast traffic between the PE devices and the CE devices is transmitted on a
per-VPN-instance basis. However, the public network multicast traffic between the PE devices
and the P devices is transmitted through the public network.
•
An MD logically defines the transmission boundary of the multicast traffic of a specific VPN over
the public network. It also physically identifies all the PE devices that support that VPN instance
on the public network. Different VPN instances correspond to different MDs.
As shown in
MD that provides services for a particular VPN instance. All the VPN multicast traffic in that
VPN is transmitted within that MD.
•
Inside an MD, all the private traffic is transmitted through the MT. The process of multicast traffic
transmission through an MT is as follows:
Description
An MD is a set of PE devices that are in the same VPN instance.
Each MD uniquely corresponds to a VPN instance.
An MDT is a multicast distribution tree constructed by all PE
devices in the same VPN. MDT types include default-MDT and
data-MDT.
An MT is a tunnel that interconnects all PEs in an MD for delivering
VPN traffic within the MD.
An MTI is the entrance or exit of an MT, equivalent to an entrance or
exit of an MD. PE devices use the MTI to access the MT. An MTI
handles only multicast packets, not unicast packets. The MTI
interfaces are automatically created when the MD for the VPN
instance is created.
On the public network, each MD is assigned a unique multicast
address, called a default-group. A default-group is the unique
identifier of an MD on the public network. It helps build the
default-MDT for an MD on the public network.
A default-MDT uses a default-group address as its group address.
In a VPN, the default-MDT is uniquely identified by the
default-group. A default-MDT is automatically created after the
default-group is specified and will always exist on the public
network, regardless of the presence of any multicast services on
the public network or the VPN.
When the multicast traffic of a VPN reaches or exceeds a threshold,
the ingress PE device assigns it an independent multicast address
called data-group. It also notifies other PE devices that they must
use this address to forward the multicast traffic for that VPN. This
initiates the switchover to the data-MDT.
A data-MDT is an MDT that uses a data-group as it group address.
At MDT switchover, PE devices with downstream receivers join a
data-group, building a data-MDT. The ingress PE forwards the
encapsulated VPN multicast traffic along the data-MDT over the
public network.
Figure 61, the ellipse area in the center of each VPN instance plane represents an
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