9.2 IGMP
9.2.5
IGMP in a switched environment
In the multilayer campus model, IP multicast traffic must traverse a Layer 2 switch, especially at the access layer. Because IP multicast traffic maps to a corresponding Layer 2 multicast address, multicast traffic is delivered to all ports of a Layer 2 switch.

For example, a video client wants to watch a 1.5-Mbps IP multicast-based video feed sent from a corporate video server. The video client sends an IGMP join message to the video server. The next-hop router for the client logs the IGMP join message. IP multicast traffic is transmitted downstream to the video client. The switch detects the incoming traffic and examines the destination MAC address to determine where the traffic should be forwarded. Because the destination MAC address is a multicast address and there are no entries in the switching table for where the traffic should go, the 1.5-Mbit video feed is simply sent to all ports.

Switches must have an architecture that allows multicast traffic to be forwarded to a large number of attached group members without unduly loading the switch fabric. This function allows the switch to provide support for the growing number of new multicast applications without impacting other traffic. Layer 2 switches also need some degree of multicast awareness to avoid flooding multicasts to all switch ports.

Multicast control in Layer 2 switches can be accomplished in several ways:

  • Virtual LANs (VLANs) can be defined to correspond to the boundaries of the multicast group. This is a simple approach; however, it does not support dynamic changes to group membership and adds to the administrative burden of unicast VLANs.
  • Layer 2 switches can snoop IGMP queries and reports to learn the port mappings of multicast group members. This allows the switch to dynamically track group membership. However, snooping every multicast data and control packet consumes a lot of switch processing capacity and, therefore, can degrade forwarding performance and increase latency.

The traditional role of the router as a control point in the network can be maintained by defining a multicast router-to-switch protocol. CGMP allows the router to configure the multicast-forwarding table in the switch to correspond with the current group membership.