Overview

Table 1 describes multicast service processes.

**Table 1** Multicast service process in a user-side multicast scenario
Item	Description	Remarks
Multicast program join	To join a multicast program, after going online, a user sends to an IGMP-capable BRAS an IGMP Report message of a multicast program. Upon the receipt of the message, the BRAS identifies the user and the multicast program that the user wants to join.	-
Multicast program leave	To leave a multicast program, a user sends to an IGMP-capable BRAS an IGMP Leave message. Upon the receipt of the message, the BRAS identifies the user and the multicast program that the user wants to leave.	-
Multicast program switchover	To switch to another multicast program, a user sends to an IGMP-capable BRAS an IGMP Leave message of the multicast program that the user wants to leave and an IGMP Report message of the multicast program that the user wants to join.	Users switch to another multicast program by performing Multicast program leave and Multicast program join.
Multicast program leave of all multicast groups by going offline	After going offline, a user terminates the IPoE or PPPoE connection, without sending IGMP Leave messages. To stop the unnecessary multicast traffic replication, IGMP removes all outbound interface information in the multicast entries of the user.	-

Related Concepts

Multicast program

A multicast program is an IPTV channel or program and is identified by a multicast source address and a multicast group.

Access mode

In user-side multicast, only the Point-to-Point Protocol over Ethernet (PPPoE) access and IP over Ethernet (IPoE) access modes are supported, and only session-based replication is supported.

PPPoE access mode: allows a remote access device to provide access services for hosts on Ethernet networks and to implement user access control and accounting. PPPoE is a link layer protocol that transmits PPP datagrams through PPP sessions established over point-to-point connections on Ethernet networks.
IPoE access mode: allows the BRAS to perform authentication and authorization on users and user services based on the physical or logical user information, such as the MAC address, VLAN ID, and Option 82, carried in IPoE packets. In IPv4 network access where a user terminal connects to an Ethernet interface of a BRAS through a Layer 2 device, the user IP packets are encapsulated into IPoE packets by the user Ethernet interface before they are transmitted to the BRAS through the Layer 2 device.

**Table 2** Differences between PPPoE access users and IPoE access users in user-side multicast
Item	Packet Encapsulation	Multicast Message Type (Unicast/Multicast)	Description
PPPoE access mode	Multicast traffic and IGMP messages exchanged between a user and a BRAS are encapsulated using PPPoE.	IGMP messages exchanged between a user and a BRAS are all unicast messages. Multicast traffic that a BRAS replicates to a user is sent in unicast PPPoE packets.	Multicast replication by interface + VLAN is not supported for the PPPoE access mode.
IPoE access mode	Multicast traffic and IGMP messages exchanged between a user and a BRAS are encapsulated using IPoE.	IGMP Query messages that a BRAS sends to a user are multicast messages encapsulated using IPoE. The destination MAC address is the multicast MAC address of the user. Multicast traffic that a BRAS replicates to a user is sent in multicast IPoE packets. The destination MAC address is the multicast MAC address of the user.	-

Multicast replication modes

Table 3 describes the multicast traffic replication modes on BAS interfaces of BRAS devices.

**Table 3** Multicast replication modes
Multicast Replication Mode	Multicast Replication Devices	Description	Usage Scenario	Advantage
Session-based multicast replication	BRAS. The BRAS is used as the multicast replication device because its downstream Layer 2 device is incapable of IGMP snooping.	The BRAS replicates multicast traffic to each session.	The downstream Layer 2 device of the BRAS is not capable of IGMP Snooping.	Users who fail in authentication cannot join multicast programs, which allows for improved management of them.
Multicast replication by interface + VLAN	BRAS' Downstream Layer 2 device. This device is capable of IGMP snooping. In other words, it is capable of multicast replication.	The BRAS replicates multicast traffic by interface + VLAN to users aggregated based on their VLANs. For users on the same VLAN who go online through the same interface and join the same multicast program, the BRAS replicates only one copy of the multicast traffic to the downstream Layer 2 device. Then the Layer 2 device replicates the multicast traffic to the users.	IGMP Report messages carry VLAN tags and multicast traffic forwarding across VLANs is not required.	The burden on the BRAS to replicate multicast traffic is alleviated and the bandwidth usage is reduced.
Multicast replication by VLAN	BRAS' downstream Layer 2 device. This device is capable of IGMP snooping. In other words, it is capable of multicast replication.	Users first join multicast VLANs and then BRAS replicates multicast traffic based on the multicast VLANs. The Layer 2 device replicates the received multicast traffic based on the VLANs that the users are on. For users who go online through the same interface and join the same multicast program, the BRAS replicates only one copy of the multicast traffic to the downstream Layer 2 device.	IGMP Report messages carry VLAN tags and multicast traffic forwarding across VLANs is required.	The burden on the BRAS to replicate multicast traffic is alleviated and the bandwidth usage is reduced.
Replication by interface	BRAS' Downstream Layer 2 devices. This device is capable of IGMP snooping. In other words, it is capable of multicast replication.	The BRAS replicates multicast traffic based on interfaces and the downstream Layer 2 device replicates the received multicast traffic based on sessions. It is a special case of multicast replication by VLAN, which is enabled by setting the VLAN value to 0.	By default, multicast replication by interface is enabled.	The burden on the BRAS to replicate multicast traffic is alleviated and the bandwidth usage is reduced.

If all of the preceding multicast replication modes are configured, the priority is as follows in descending order: replication by interface + VLAN, session-based replication, replication by multicast VLAN, and replication by interface.

In addition to multicast data packets replication, IGMP Query messages are sent based on the preceding multicast replication modes.