Multicast Flooding

Multicast Flooding

Because no specific host is associated with the multicast MAC address in the content-addressable memory (CAM) table, multicast traffic is flooded throughout the VLAN. (See Figure 9-1.) This type of setup creates unnecessary traffic on the VLAN and wastes precious network resources. Each host machine has to process the packet as it arrives on the interface card, thus, wasting CPU cycles. If the volume of the multicast stream is high enough, it could potentially cause other relevant and control traffic to be dropped. This problem worsens if the VLAN is extended to various other campus switches.

One easy method to address this problem is to manually map the ports that require the multicast traffic to the multicast MAC address. For example, Host2 on port 10/1 is interested in receiving the multicast stream 239.1.1.1 as shown in Figure 9-2.