4000 Router Series IP Multicast Configuration Guide

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Default-MDT-based delivery

The default-MDT delivers multicast protocol packets and multicast data packets differently.

Multicast protocol packet delivery

To forward the multicast protocol packets of a VPN over the public network, the local PE device

encapsulates them into public network multicast data packets. These packets are transmitted along the

default-MDT, and then decapsulated on the remote PE device to go into the normal protocol procedure.

Finally a distribution tree is established across the public network.

The following describes how multicast protocol packets are forwarded in different circumstances:

• If the VPN network runs PIM-DM or PIM-SSM:

{ Hello packets are forwarded through MTI interfaces to establish PIM neighboring relationships.

{ A flood-prune process (in PIM-DM) or a join process (in PIM-SSM) is initiated across the public

network to establish an SPT across the public network.

• If the VPN network runs PIM-SM:

{ Hello packets are forwarded through MTI interfaces to establish PIM neighboring relationships.

{ If the receivers and the VPN RP are in different sites, a join process is initiated across the public

network to establish an RPT.

{ If the multicast source and the VPN RP are in different sites, a registration process is initiated

across the public network to establish an SPT.

NOTE:

PIM mode must be the same for all interfaces that belong to the same VPN, including those interfaces tha

are bound with the VPN instance and the MTI interfaces on PE devices.

As shown in Figure 62, PIM-SM is running in both the public network and the VPN network, Receiver for

the VPN multicast group G (225.1.1.1) in Site 2 is attached to CE 2, and CE 1 of Site 1 acts as the RP for

group G (225.1.1.1). The default-group address used to forward public network data is 239.1.1.1.