IGMP and Layer 2 Switching
Layer 2 switches operate at the MAC layer
- IP multicast traffic maps to a L2 (MAC) multicast address, which is treated at L2 as a broadcast (IEEE behavior)
- L2 switches therefore actually broadcast multicast traffic to all their ports -- this can be very detrimental to performance!
- Yes you can define static entries on the switches to control multicast at L2, but the administrative overhead can be enormous, and static entries do not react to dynamic behaviors
The solution is to enhance the intelligence of the layer 2 switches:
- IGMP snooping: sneak peek into the layer 3 content of a multicast packet to make forwarding decisions
- may have performance penalties (compared to CGMP)
- CGMP (Cisco Group Multicast Protocol)