Introduction
Slow performance of LAN due to a high degree of multicast traffic or frequent disruption of multicast streams and how to filter multicast traffic
Core issue
The purpose of the Cisco Group Management Protocol and Internet Group Management Protocol (IGMP) snooping is to restrain multicast traffic in a switched network. By default, a LAN switch floods multicast traffic within the broadcast domain and this can consume a lot of bandwidth if many multicast servers are sending streams to the segment.
Multicast traffic becomes flooded because a switch usually learns MAC addresses by looking into the source address field of all the frames it receives. But because a multicast MAC address is never used as a source address for a packet and because such addresses do not appear in the MAC address table, the switch has no method for learning them.
Resolution
To resolve this issue, choose from one of these options:
- The first option is to configure static MAC addresses for each group and each client. This solution works well, but it is neither scalable nor dynamic. You can do it on a Catalyst 4000, 4500, 5000, 5500, 6000 or 6500 switch by issuing one of these commands:
- set cam static multicast_mac mod/port
- set cam permanent multicast_mac mod/port
These two commands have the same effect except that the static entries disappear at reboot and permanent entries do not.
- The second option is to use Cisco Group Management Protocol, which is a Cisco proprietary protocol that runs between the multicast router and the switch. It enables the Cisco multicast router to understand IGMP messages sent by hosts and informs the switch about the information contained in the IGMP packet.
- The last and most efficient option is to use IGMP snooping. With IGMP snooping, the switch intercepts IGMP messages from the host itself and updates its MAC table accordingly. Advanced hardware is required to support IGMP snooping.
Details
set cam
To add entries into the CAM table, set the aging time for the CAM table, and configure traffic filtering from and to a specific host, use the set cam command.
set cam {dynamic | static | permanent} {unicast_mac | route_descr} mod/port [vlan]
set cam {static | permanent} {multicast_mac} mod/ports.. [vlan]
set cam {static | permanent} filter {unicast_mac} vlan
set cam agingtime vlan agingtime
Syntax Description
dynamic | Specifies entries are subject to aging. |
static | Specifies entries are not subject to aging. |
permanent | Specifies permanent entries are stored in NVRAM until they are removed by the clear cam or clear configcommand. |
unicast_mac | MAC address of the destination host used for a unicast. |
route_descr | Route descriptor of the "next hop" relative to this switch; valid values are from 0 to 0xffff. |
mod/port | Number of the module and the port on the module. |
vlan | (Optional) Number of the VLAN; valid values are from 1 to 4094. |
multicast_mac | MAC address of the destination host used for a multicast. |
mod/ports.. | Number of the module and the ports on the module. |
filter | Specifies a traffic filter entry. |
agingtime | Sets the period of time after which an entry is removed from the table. |
agingtime | Number of seconds (0 to 1,000,000) dynamic entries remain in the table before being deleted. |
Defaults
The default configuration has a local MAC address, spanning tree address (01-80-c2-00-00-00), and CDP multicast address for destination port 1/3 (the supervisor engine). The default aging time for all configured VLANs is 300 seconds.
The vlan variable is required when you configure the traffic filter entry.
Setting the aging time to 0 disables aging.
Command Types
Switch command.
Command Modes
Privileged.
Usage Guidelines
If the given MAC address is a multicast address (the least significant bit of the most significant byte is set to 1) or broadcast address (ff-ff-ff-ff-ff-ff) and you specify multiple ports, the ports must all be in the same VLAN. If the given address is a unicast address and you specify multiple ports, the ports must be in different VLANs.
The MSM does not support the set cam command.
If you enter a route descriptor with no VLAN parameter specified, the default is the VLAN already associated with the port. If you enter a route descriptor, you may only use a single port number (of the associated port).
The MAC address and VLAN for a host can be stored in the NVRAM and are maintained even after a reset.
The vlan value is optional unless you are setting CAM entries to dynamic, static, or permanent for a trunk port, or if you are using the agingtime keyword.
If a port or ports are trunk ports, you must specify the VLAN.
Static (nonpermanent) entries remain in the table until you reset the active supervisor engine.
You can specify 256 permanent CAM entries.
Enter the route_descr variable as two hexadecimal bytes in the following format: 004F. Do not use a "-" to separate the bytes.
Note 
Static CAM entries that are configured on the active supervisor engine are lost after fast switchover. You must reconfigure CAM entries after fast switchover.
Examples
This example shows how to set the CAM table aging time to 300 seconds:
Console> (enable) set cam agingtime 1 300
Vlan 1 CAM aging time set to 300 seconds.
Console> (enable)
This example shows how to add a unicast entry to the table for module 2, port 9:
Console> (enable) set cam static 00-00-0c-a0-03-fa 2/9
Static unicast entry added to CAM table.
Console> (enable)
This example shows how to add a permanent multicast entry to the table for module 1, port 1, and module 2, ports 1, 3, and 8 through 12:
Console> (enable) set cam permanent 01-40-0b-a0-03-fa 1/1,2/1,2/3,2/8-12
Permanent multicast entry added to CAM table.
Console> (enable)
This example shows how to add a traffic filter entry to the table:
Console> (enable) set cam static filter 00-02-03-04-05-06 1
Filter entry added to CAM table.
Console> (enable)
Related Commands
Reference
Multicast in a Campus Network: CGMP and IGMP Snooping
Rebuilding the Multicast Entries with CGMP and Spanning Tree Topology Changes
For information on what multicast commands are available, for which series and which software, refer to Multicast Catalyst Switches Support Matrix
