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.