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Table 16 Basic MD-VPN concepts
Conce
t Descri
tion
Multicast domain (MD)
An MD is a set of VPN instances running on PE devices that can send
multicast traffic to each other. Each MD uniquely corresponds to the
same set of VPN instances.
Multicast distribution tree (MDT)
An MDT is a multicast distribution tree constructed by all PE devices in
the same VPN. MDT types include default-MDT and data-MDT.
Multicast tunnel (MT)
An MT is a tunnel that interconnects all PEs in an MD for delivering
VPN traffic within the MD.
Multicast tunnel interface (MTI)
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.
Default-group
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.
Default-MDT
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.
Data-group
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
a switchover to the data-MDT.
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.
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.
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