239.1.1.1 Is Accepted by Ports 3/1 and 7/3

Switch3 (enable) show cam static 2

* = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry. X = P

ort Security Entry

VLAN  Dest MAC/Route Des  Destination Ports or VCs / [Protocol Type]

----  ------------------  ----------------------------------------------------

2     01-00-5e-01-01-01*  3/1,7/3

Figure 9-10 shows Host2 connected to Switch3, a Catalyst 5509 device, which also has an RSM blade. Switch3 is configured for CGMP.

Step 1. Host3 sends a IGMP membership report for 239.1.1.1.

Step 2. The switch forwards a message to the RSM.

Step 3. The RSM receives the unsolicited IGMP membership report. It updates its IGMP group table.

Step 4. RSM then updates the mroute table by creating (*, G) and (S, G) for 239.1.1.1. It also puts VLAN 2 in forwarding state to receive 239.1.1.1 traffic.

Step 5. The router translates the IGMP membership report to a Layer 2 CGMP message and forwards it to the switch using CGMP well-known multicast MAC address, 0100.0CDD.DDDD with SNAP value of 0x2001:

- The GDA field will have the translated MAC address of 239.1.1.1= 0010.5e01.0101.

- The User Source Address (USA) will be the MAC address of the client (Host3) that sent the IGMP membership report.

Step 6. The switch examines the CGMP packet. If it does not have a CAM entry for the multicast MAC address, it will create a CAM entry for GDA and associate the router port to it. The switch looks at the USA field and again examines the CAM table. The switch will have a CAM entry for the USA field because Host2 initiated the request for the multicast stream. As a result, the switch also adds the Host2 port to the GDA entry.

Step 7. Any subsequent multicast traffic destined to 239.1.1.1 will be forwarded to the RSM and the host port. In this example, the host port is 7/3.

The leave process works the same. The router receives the IGMP leave message from the host and translates the information to CGMP and forwards it to the switch. The switch then removes the client port from the CAM entry for that GDA.

With more recent implementations, the Catalyst 4000 and 5000 family can actually handle the IGMP leave process locally rather than forwarding it to the router. To enable this feature, use the following command: