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This chapter describes how to configure broadcast/multicast suppression on the Catalyst enterprise LAN switches.
This chapter consists of these sections:
These sections describe how broadcast/multicast suppression works:
Broadcast/multicast suppression prevents switched ports on a LAN from being disrupted by a broadcast storm. A LAN broadcast storm occurs when broadcast or multicast packets flood the LAN, creating excessive traffic and degrading network performance. Errors in the protocol-stack implementation or in the network configuration can cause a broadcast storm.
Broadcast/multicast suppression works by measuring incoming broadcast/multicast activity on a switch port. Broadcast/multicast activity can be measured in two ways:
Broadcast/multicast suppression is implemented either in hardware or in software. Hardware broadcast/multicast suppression uses the bandwidth-based method. Software broadcast/multicast suppression uses the packet-based method. Since packet sizes vary, bandwidth-based measurement is more accurate and effective than packet-based measurement.
When broadcast/multicast suppression is enabled, the switch monitors incoming packets on the port over a one-second time period. The switch uses the Individual/Group bit in the packet destination address to determine if the packet is a unicast or a broadcast/multicast. If the total number of broadcasts/multicasts exceeds a user-configured threshold, further activity is suppressed for the duration of a specified period.
Figure 17-1 shows the broadcast/multicast traffic patterns on a port over a given period. In this example, broadcast/multicast suppression occurs between the one-second intervals T1-T2 and T4-T5. During those one-second intervals, the amount of broadcast/multicast traffic exceeded the configured threshold.
Broadcast/multicast suppression is disabled by default. Bandwidth-based broadcast/multicast suppression takes precedence over packet-based broadcast/multicast suppression unless you disable bandwidth-based broadcast/multicast suppression (that is, you set the threshold value to 100 percent).
With bandwidth-based broadcast/multicast suppression, if the number of incoming broadcasts/multicasts on a port within a one-second time interval exceeds the configured threshold, the switch filters out all incoming broadcast/multicast packets on the port for the remainder of the one-second period.
Bandwidth-based broadcast/multicast suppression measures broadcast/multicast activity relative to total bandwidth. You can configure the percentage of total available bandwidth that can be used by broadcast/multicast traffic using the set port broadcast command. The higher the threshold, the more incoming broadcast/multicast packets are allowed to pass. A threshold value of 100 percent means that no limit is placed on broadcast traffic (suppression is disabled).
To determine whether your hardware supports bandwidth-based broadcast/multicast suppression, see the documentation for your hardware or use the show port capabilities command.
With packet-based broadcast/multicast suppression, if the number of incoming broadcasts/multicasts on a port within a one-second time interval exceeds the configured threshold, the switch filters out all incoming traffic (including unicast packets) on the port for the remainder of the one-second period.
Packet-based broadcast/multicast suppression measures broadcast/multicast activity relative to the total number of packets received on the port. You can configure the total number of packets that can be used by broadcast/multicast traffic using the set port broadcast command. The higher the threshold, the more incoming broadcast/multicast packets are allowed to pass. A threshold value of 0 packets means that no limit is placed on broadcast traffic (suppression is disabled on the port).
To determine whether your hardware supports packet-based broadcast/multicast suppression, see the documentation for your hardware or use the show port capabilities command.
Broadcast/multicast suppression requires supervisor software release 2.2 or later in addition to broadcast suppression-capable hardware.
Broadcast/multicast suppression capability is not available on all platforms and is hardware-dependent. Table 17-1 shows which switches have available hardware that supports broadcast/multicast suppression.
To determine whether a specific piece of hardware supports broadcast/multicast suppression, use the show port capabilities command.
| Suppression Method | Catalyst 5000 Family | Catalyst 4000 Family | Catalyst 2926G Series | Catalyst 2948G |
|---|---|---|---|---|
Bandwidth-based (hardware) suppression | Yes | No | Yes | No |
Packet-based (software) suppression | Yes | No | Yes | No |
These sections describe how to configure broadcast/multicast suppression:
To enable bandwidth-based broadcast/multicast suppression, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Specify the broadcast/multicast suppression threshold for one or more ports as a percentage of total bandwidth. | set port broadcast mod_num/port_num threshold% |
Step 2 Verify the broadcast/multicast suppression configuration. | show port broadcast [mod_num[/port_num]] |
This example shows how to enable bandwidth-based broadcast/multicast suppression and verify the configuration:
Console> (enable) set port broadcast 3/1 75% Port(s) 3/1-24 broadcast traffic limited to 75%. Console> (enable) show port broadcast 3 Port Broadcast-Limit Broadcast-Drop -------- --------------- -------------- 3/1-8 75 % - 3/9-16 75 % - 3/17-24 75 % - Console> (enable)
To enable packet-based broadcast/multicast suppression, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Specify the broadcast/multicast suppression threshold for one or more ports as a maximum number of broadcast or multicast packets allowed in a one-second period. | set port broadcast mod_num/port_num threshold |
Step 2 Verify the broadcast/multicast suppression configuration. | show port broadcast [mod_num[/port_num]] |
This example shows how to enable packet-based broadcast/multicast suppression and verify the configuration:
Console> (enable) set port broadcast 2/6 500 Port(s) 2/6 broadcast traffic limited to 500 packets/second. Console> (enable) set port broadcast 2/7 500 Port(s) 2/7 broadcast traffic limited to 500 packets/second. Console> (enable) set port broadcast 2/8 500 Port(s) 2/8 broadcast traffic limited to 500 packets/second. Console> (enable) show port broadcast Port Broadcast-Limit Broadcast-Drop -------- --------------- -------------- 1/1 - - 1/2 - - 2/1 - 0 2/2 - 0 2/3 - 0 2/4 - 0 2/5 - 0 2/6 500 p/s 0 2/7 500 p/s 0 2/8 500 p/s 0 2/9 - 0 2/10 - 0 2/11 - 0 2/12 - 0 Console> (enable)
To disable broadcast/multicast suppression on one or more ports, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Disable broadcast/multicast suppression on one or more ports. | clear port broadcast mod_num/port_num |
Step 2 Verify the broadcast/multicast suppression configuration. | show port broadcast |
This example shows how to disable broadcast/multicast suppression on one or more ports:
Console> (enable) clear port broadcast 3/1 Port 3/1-8 broadcast traffic unlimited. Console> (enable)
Posted: Fri Oct 1 13:16:26 PDT 1999
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