Multicast Filtering

Setting up multicast filtering improves the performance of networks that carry multicast traffic.

This chapter explains multicasts, multicast filtering, and how multicast filtering can be implemented on your Switch. It covers the following topics:

• What is a Multicast?

• What is Multicast Filtering?

• Multicast Filtering and Your Switch

A multicast is a packet that is sent to a subset of endstations in a LAN, or VLAN, that belong to a multicast group. If the network is set up correctly, a multicast can only be sent to an endstation if it has joined the relevant group.

A typical use of multicasts is video-conferencing, where high volumes of traffic need to be sent to several endstations simultaneously, but where broadcasting that traffic to all endstations would seriously reduce network performance.

Multicasts are similar to broadcasts - by default, they are sent to all endstations on a LAN or VLAN. Multicast filtering is the system by which endstations only receive multicast traffic if they register to join specific multicast groups. With multicast filtering, network devices only forward multicast traffic to the ports that are connected to registered endstations.

Figure 44 The effect of multicast filtering

Your Switch provides automatic filtering support for two multicast systems:

• IEEE 802.1p, which uses the GARP Multicast Registration Protocol (GMRP)

• IGMP (Internet Group Management Protocol)

In addition, you can manually configure the filtering and forwarding of multicasts using Transcend^® Network Management software.

IEEE 802.1p Multicast Filtering

The IEEE 802.1p standard defines a system that allows network devices to use a GARP Multicast Registration Protocol (GMRP) to register endstations with multicast groups. GMRP is protocol-independent, which means that it can be used on all LANs and VLANs that contain network devices and endstations which support IEEE 802.1p.

IEEE 802.1p multicast filtering works as follows:

1 .   If an 802.1p endstation wants to receive traffic for a multicast group, it sends out a join packet with a known multicast address to declare that it would like to join that group.

2 .   When the join packet arrives at a port on a Switch with 802.1p multicast learning enabled, the Switch specifies that the port is to forward traffic for the multicast group and then sends a similar packet to all other ports.

3 .   When traffic for the multicast group appears on the network, the Switch units only forward the traffic to ports that received a join packet.

Enabling 802.1p Multicast Learning

For information about enabling 802.1p multicast learning for an individual port on your Switch, see "Configuring a Port" . For information about enabling 802.1p multicast learning for a whole Switch or stack, see "Configuring the Advanced Stack Settings" .

For information about configuring IEEE 802.1p functionality on an endstation, refer to the user documentation supplied with your endstation or the endstation's Network Interface Card (NIC).

IGMP Multicast Filtering

IGMP is the system that all IP-supporting network devices use to register endstations with multicast groups. It can be used on all LANs and VLANs that contain an IP router and other network devices which support IP.

IGMP multicast filtering works as follows:

1 .   The IP router (or querier) periodically sends query packets to all the endstations in the LANs or VLANs that are connected to it.

If your network has more than one IP router, the one with the lowest IP address becomes the querier. If your network loses its IP router connections, the Switch unit with the lowest IP address becomes the querier - if this occurs, multicast filtering can only occur on the Default VLAN (VLAN 1).

2 .   When an IP endstation receives a query packet, it sends a report packet back that identifies the multicast group that the endstation would like to join.

3 .   When the report packet arrives at a port on a Switch with IGMP multicast learning enabled, the Switch specifies that the port is to forward traffic for the multicast group and then forwards the packet to the router.

4 .   When the router receives the report packet, it registers that the LAN or VLAN requires traffic for the multicast groups.

5 .   When the router forwards traffic for the multicast group to the LAN or VLAN, the Switch units only forward the traffic to ports that received a report packet.

Enabling IGMP Multicast Learning

For information about enabling IGMP multicast learning, see "Configuring the Advanced Stack Settings" .

For information about configuring IGMP functionality on an endstation, refer to the user documentation supplied with your endstation or the endstation's Network Interface Card (NIC).

Manual Filtering

You can manually configure your Switch to filter and forward multicasts using Transcend Enterprise Manager. This system can be used when endstations in your network do not support IEEE 802.1p or IGMP.