Cisco Systems RJ-45-to-AUX Cisco Group Management Protocol, Router Group Management Protocol, 148

This process looks simple. As a host joins the multicast group, its port number is added to the CAM table; as the host leaves, its port is removed from the CAM table. However, this entire process forgets about the switch’s processor. The switch must also receive the multicasts in order to receive the join and leave messages. If a steady stream of multicast traffic is received by the switch, the processor will be so busy checking the multicast traffic that it will have no time to process any other traffic. To overcome this problem, Cisco redesigned its switch to enable the ports to examine frames for Layer 3 information. When the port sees an IGMP frame, the frame will only be forwarded to the processor. This way, the processor will see all the IGMP traffic but will not receive any non−IGMP multicast traffic.

Let’s look at the entire process. When a host broadcasts an IGMP report, the report is processed by the switch and sent out to every port. The router will begin to send multicast traffic to the switch. The switch will add the source port to the CAM table along with the first port. As more hosts send IGMP reports, their ports are added to the forwarding table. When the router sends a general query to the switch, the switch will send the multicast to every port. All the multicast group members will send a reply to the router (all IGMP replies are sent to the processor). The processor will send one IGMP report to the router so that it will continue to receive the multicast traffic. When a host sends a leave group message, the switch sends a general query out the same port to ensure that no other host wishes to receive the multicast (if the port is connected to another switch/hub). It will remove the port from the CAM table if it does not get a reply.

IGMP snooping is very efficient and is vendor independent. Unfortunately, it is only available for the enterprise class Catalyst switches.

Cisco Group Management Protocol

Cisco Group Management Protocol (CGMP) is a lightweight Layer 2 protocol that will only work between a Cisco router and a Cisco switch. CGMP is supported by almost the full line of routers and switches. A router and a switch utilize CGMP to communicate the multicast member states in the CAM table entries. The router communicates with the host using IGMP, but these IGMP messages are ignored by the switch.

When a host sends a membership report to the multicast router, the router sends a CGMP message to the switch with the host’s MAC address and the multicast MAC address. The CGMP switch uses the host’s MAC address to look up the port. Depending on the CGMP message, the switch will either remove the port from or add it to the CAM table of the specific multicast MAC address.

This entire process moves the administration of the multicast from the switch to the router. The switch only has to follow the instructions and add/remove MAC addresses from the CAM table (which is what it is designed to do).

Tip When you enable the router’s interface with CGMP, ensure that the CGMP−enabled switch is already connected to that interface. When CGMP is enabled on the router, the router will send an assign router port message to the switch.

Router Group Management Protocol

Router Group Management Protocol (RGMP) is used by the switch to manage the multicast router. It is supported by Catalyst enterprise LAN switches. It lets a switch reduce multicast traffic by sending multicasts to the routers that are configured to receive them.

All the routers on the network must support RGMP and have PIM−SM running. IGMP snooping must also be enabled on the switch. The router sends a hello packet to the switch. The hello packet tells the switch when it will receive multicast traffic. This message is called an RGMP join. When the router wants to stop receiving multicast traffic, it sends an RGMP leave message to the switch. These messages greatly reduce the multicast traffic on the network.

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