Troubleshooting

Efficient Video Distribution Networks with.Multicast: IGMP Querier and PIM-DM

Introduction

Dell PowerConnect

firmware revision 5.1 has added vast improvements in the areas of IGMP (Internet

Group Management Protocol) snooping and IP Multicast Failover. Starting with PowerConnect 5.1

firmware, Dell PowerConnect users have a new set of commands to configure IGMP snooping, MLD

snooping, MVR (Multicast VLAN Registration), and bridge multicast filtering. This white paper uses

several of the new commands to show how an administrator would set up common video distribution

networks.

PowerConnect 5.1 firmware is supported on the following Dell PowerConnect devices:

M6220 7024 7048P 8132

M6348 7024F 7048R 8132F

M8024 7024P 8024 8164

M8024k 7048 8024F 8164F

Multicast Overview

Multicast services are used to distribute streaming media, such as audio and video traffic, over the

network. These applications typically generate large amounts of traffic taking up network bandwidth.

Both multicast and broadcast allow a network device to send single packets to multiple destinations.

The difference is broadcast is designed to forward packets to all nodes on the VLAN, whereas multicast

is designed to only forward to nodes in a multicast group. Multicast is therefore a better solution when

trying to conserve network bandwidth. Also, since all nodes must process every frame they receive,

multicast saves processing cycles on nodes that do not need to receive the multicast frames.

The multicast model consists of groups, transmitters, and receivers. A typical multicast group contains

one transmitter (or sender) and one or more receivers. The receiver sends a “group join” message to

network switches and routers via the Internet Group Multicast Protocol (IGMP). The switches and

routers then forward multicast traffic only to the receivers that have joined the multicast group.

Dell PowerConnect switches by default will forward multicast frames to all interfaces in a VLAN. No

additional configuration is required to pass this traffic; however, the default behavior provides no

benefit over using broadcast. To limit unnecessary traffic on the network, an IGMP Querier (layer 2) or

PIM router (layer 3) must be seen on the network for multicast to stop broadcasting to all ports.

Starting with firmware 5.1.0.0, IGMP snooping is enabled on all switches; however, this does not keep

the multicast traffic from broadcasting until the switches can see a Querier or PIM router.

Note: By creating an IGMP Querier or PIM router on the network, switches are able to stop multicast

frames from being broadcasted and can forward those frames to transmitters and receivers in a

specified multicast group.

Switches can use IGMP snooping to automatically collect information about which interfaces are

participating in multicast groups. The switches then use this information to direct multicast traffic

away from devices that are not interested in the IP multicast traffic.

Switches that are not “multicast aware” can use static multicast groups to manually select the

interfaces that will pass multicast traffic.