Monitoring CGMP

Monitoring CGMP

The operation of CGMP is easily verified by using debug and show commands on the router and switch. The network we will use to demonstrate the operation of CGMP is shown in Figure 4-14. The router will begin the CGMP process by sending a Join to the switch.

Figure 4-14: Host IGMP messages pass through the switch to the router.

Router#debug ip cgmp

07:59:15: CGMP: Sending self Join on Ethernet0

07:59:15: GDA 0000.0000.0000, USA 0010.7b3a.6171

08:00:15: CGMP: Sending self Join on Ethernet0

08:00:15: GDA 0000.0000.0000, USA 0010.7b3a.6171

Initially, the host sends an IGMP Group Membership Report to the router. To view this, execute the command debug ip igmp on the router:

router#debug ip igmp

09:04:55: IGMP: Received v2 report from 172.16.1.1 (Ethernet0) for 224.65.10.154.

To verify that the router has created an entry for the group, use the show ip igmp group command.

router#show ip igmp group     IGMP Connected Group Membership

Group Address

Interface

Uptime

Expires

Last Reporter

224.65.10.154

Ethernet0

00:00:12

00:02:48

172.16.1.1

The router then sends a CGMP Join to the switch (refer to Figure 4-15), which can be monitored using both the IGMP and CGMP debug commands.

Figure 4-15: After receiving an IGMP Report from the host, the router informs the switch with a CGMP Join message.

Router#debug ip igmp

02:11:18: CGMP: Received IGMP Report on Ethernet0 from 172.16.1.1 for 224.65.10.154.

02:11:19: CGMP: Sending Join on Ethernet0

GDA 0100.5E41.0A9A, USA 0010.7b3a.6171

When switch B receives the CGMP Join message from the router, a static CAM entry is created for the host.

switch (enable) show cam dynamic

VLAN

Dest MAC/Route Des

Destination Ports or VCs

1

0010.7b3a.6171

3/1

B (enable) show cam static

VLAN

Dest MAC/Route Des

Destination Ports or VCs

1

01-00-5e-41-0a-9a

3/1

Once the static CAM entry is in the table, multicast traffic that is received by the switch for group 224.65.10.154 is sent only to port 3/1 . When the host decides to leave the group, the host sends an IGMP Leave message to the router (see Figure 4-16). Here we are assuming that the host is using IGMP version 2.

Figure 4-16: The Host IGMP Leave message triggers Membership Queries from the router.

When the router receives the Leave message from the host, the router sends multiple Membership Queries for the group to determine if there are any members remaining.

routeradebug ip igmp

09:04:54:IGMP: Received Leave from 172.16.1.1 (Ethernet0) for 224.65.10.154.

09:04:55:IGMP: Send v2 Query on Ethernet0 to 224.65.10.154.

09:04:56:IGMP: Send v2 Query on Ethernet0 to 224.65.10.154.

If there is no response to the query for the group, then the router deletes the state for the group on the interface and sends a CGMP Leave for the group to the switch.

routeradebug ip igmp

02:11:18: IGMP: Deleting 224.65.10.254 on Ethernet0

02:11:19: CGMP: Sending Leave on Ethernet0

GDA 0100.5E41.0A9A, USA 0000.0000.0000

What happens when the router receives an IGMP v1 Leave message? Hopefully, as you remember from Chapter 3, that there are no IGMP v1 Leave messages. If the host leaves the group, the traffic for group 224.65.10.254 continues to be forwarded to the host until the state for the group expires on the router. When the state for the group does so, a CGMP Leave message is sent to the switch, deleting the entry from the CAM table.

The process of leaving a group can be made more efficient if the switch can monitor IGMP Leave messages. This option is called Fast IGMPv2 Leave processing and is enabled on the switch with the command shown below.

switch (enable) set cgmp leave enable

With CGMP Leave enabled on the switch, the switch processes the IGMPv2 Leave messages and does not send them to the router. If the switch knows that other receivers for the group are on the same port or VLAN, then no action is required. If the switch knows that this is the last receiver to leave the group, then an IGMP Leave message is sent to the router. To disable this feature, use:

switch(enable) set cgmp leave disable