Cisco Group Management Protocol

With the Cisco Group Management Protocol (CGMP) window, you can:

CGMP reduces the unnecessary flooding of IP multicast packets by limiting the transmission of these packets to groups of CGMP clients that request them. Clients (end stations) automatically join and leave groups that receive IP multicast traffic, and the switch dynamically changes its forwarding behavior according to join and leave requests.

CGMP works with the Internet Group Management Protocol (IGMP) and requires a connection to a router running CGMP and IGMP. When the router receives an IGMP request from a client, it forwards this information to the switch in a CGMP packet. The switch uses this information to alter its forwarding behavior.

Disabling CGMP

CGMP is enabled by default. To disable CGMP:

  1. Select the Protocol tab from the CGMP window.
  2. From the CGMP Protocol drop-down list, select the Disable.
  3. Click Apply to put your change in effect.
  4. Click OK to close the CGMP window.

Enabling the CGMP Fast Leave Feature

Normally, a CGMP-enabled router does not remove a CGMP group until all members are removed from the group, but you can accelerate group removal from the switch and the router by using the CGMP Fast Leave feature. When a member leaves a group, Fast Leave searches for other members of the group (devices receiving IP multicast packets from a particular port on the switch). If there are no other members on the port, the switch removes the port from the group. If there are no other ports in the group, the switch notifies the routers connected to the VLAN to delete the entire group.

Fast Leave is disabled by default. To enable the CGMP Fast Leave feature:

  1. Select the Protocol tab from the CGMP window.
  2. From the Enable CGMP Fast Leave drop-down list, select Enable.
  3. Click Apply to put your changes on this tab in effect.
  4. Click OK to close the CGMP window.

Note: The CGMP Fast Leave feature works only if CGMP is enabled. The client must be running IGMP Version 2 for the CGMP Fast Leave feature to function properly.

Modifying the Router Hold-Time

The router hold-time is the number of seconds the switch waits before removing a router entry. If the removed router is the last router entry on a VLAN, then all groups on the VLAN are removed.

To modify the router hold-time setting:

  1. Select the Protocol tab from the CGMP window.
  2. In the Router Hold-Time field, enter a number from 10 through 6000.
    The default is 300 seconds.
  3. Click Apply to put your changes on this tab in effect.
  4. Click OK to close the CGMP window.

Removing Management Groups

The Management Groups tab lists each VLAN ID on which the switch is forwarding IP multicast packets, the IGMP MAC address of the router or group in the VLAN, and the switch port forwarding the multicast traffic to the group. If the CGMP Fast Leave feature is disabled, you can speed the removal of a group (and its end stations) by removing the port entry from this tab.

To remove a management group:

  1. Select the Management Groups tab on the CGMP window.
  2. In the table on this tab, select one or more management groups.
  3. Click Remove to remove the management groups you selected.
    To remove all management groups, click Remove All. The Management Groups table is updated with your changes.
  4. Click OK to close the CGMP window.

Removing Router Ports

Another way to speed the removal of multicast groups is through the Router Ports table. The Router Ports table lists each VLAN ID on which the switch is forwarding IP multicast packets, the IGMP MAC address of the router port, the router expiration time, and the router port number.

When the router expiration time shown in the entry reaches 0 seconds, and if the router is the last member of the VLAN, the multicast group table is cleared.

To remove a CGMP-enabled router and all members associated with its multicast group:

  1. Select the Router Ports tab on the CGMP window.
  2. In the table on this tab, select one or more ports to remove.
  3. Click Remove to remove the ports that you selected.
    To remove all router ports, click Remove All. The Router Ports table is updated with your changes.
  4. Click OK to close the CGMP window.