Before getting into the details of Layer 2 multicasting protocols, let’s first take a look at how IP multicasting works in a more generic sense. Although IP-based communications have not been looked at in detail yet, you have learned that multicasting is a one-to-many method of transmission. In order to facilitate this, a special class of IP addresses are designated or reserved for multicasts – Class D, or those addresses fall into the numerical range between 224 and 239 in the first octet of an IP address. For example, the address 126.96.36.199 would be a valid multicast destination address. Later in this book I’ll cover multicast addresses in more detail.
The primary multicast protocol used on the Internet is the Internet Group Management Protocol (IGMP). IGMP is a Network layer protocol whose primarily responsibility is managing the groups of hosts that which to receive a multicast, just like the name suggests. When a user opens a multicast-enabled application on their system, their computer sends an IGMP message to the local router. This message essentially tells the router that it should forward the requested multicast traffic onto this network. In turn, this router contacts its upstream router, telling that router to forward the particular multicast traffic requested. Overall, this ensures that forwarding a multicast to the entire Internet is unnecessary. Only those routers that actually have network hosts that “need” the multicast will have the traffic forwarded to them.
In fact, there may be many hosts on this network that wish to receive the same multicast transmission. In this case, as they open their multicast applications, they will simply begin processing the same frames forwarded by the router – again, the multicast is only sent once, but multiple hosts are listening and accepting the traffic. In order to reduce unnecessary traffic on the network, the local router will periodically send out what is known as an IGMP host membership query. If no systems respond to these queries, the router will ultimately stop forwarding this multicast traffic to the network.
Recall again that by default, a switch will forward multicast frames to all ports. What this means is that even though there are only perhaps two or three hosts that wish to receive the multicast, it will still be forwarded to all systems, most of which will simply discard the traffic. This can place a heavy load on both the hosts and any switches, especially in cases where many different multicast streams need to be forwarded.
In order to deal with this traffic more effectively, Cisco Catalyst switches can use one of two different Layer 2 multicast features – the Cisco Group Management Protocol (CGMP) and IGMP Snooping.