Basic concepts in MDT-based MVPN
This section introduces the following basic concepts in MDT-based MVPN:
MVPN—An MVPN logically defines the transmission boundary of the multicast traffic of a VPN over the public network. It also physically identifies all the PEs that support that VPN instance on the public network. Different VPN instances correspond to different MVPNs.
Multicast distribution tree (MDT)—An MDT is a multicast distribution tree constructed by all PEs in the same VPN. MDT includes default-MDT and data-MDT.
Multicast tunnel (MT)—An MT is a tunnel that interconnects all PEs in an MVPN. The local PE encapsulates a VPN multicast packet into a public network multicast packet and forwards it through the MT over the public network. The remote PE decapsulates the public network multicast packet to get the original VPN multicast packet.
Multicast tunnel interface (MTI)—An MTI is the entrance or exit of an MT, equivalent to an entrance or exit of an MVPN. MTIs are automatically created when the MVPN for the VPN instance is created. PEs use the MTI to access the MT. The local PE sends VPN data out of the MTI. The remote PEs receive the private data from their MTIs. An MTI runs the same PIM mode as the VPN instance to which the MTI belongs. PIM is enabled on MTIs when a minimum of one interface in the VPN instance is enabled with PIM. When PIM is disabled on all interfaces in the VPN instance, PIM is also disabled on MTIs.
Default-group—A default-group is a unique multicast address assigned to each MVPN on the public network. It is the unique identifier of an MVPN on the public network and helps build the default-MDT for an MVPN on the public network. A PE encapsulates a VPN multicast packet (a multicast protocol packet or a multicast data packet) into a public network multicast packet. The default-group address is used as the public network multicast group.
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—An MVPN is assigned a unique data-group for MDT switchover. The ingress PE selects a least used address from the data-group range to encapsulate the VPN multicast packets when the multicast traffic of the VPN arrives at the PE. Other PEs are notified to use the address to forward the multicast traffic for that VPN. This initiates the 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, PEs with downstream receivers join a data-group to build a data-MDT. The ingress PE forwards the encapsulated VPN multicast traffic along the data-MDT over the public network.