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This chapter describes how to configure broadcast suppression on the Catalyst 6000 and 6500 series switches.
This chapter consists of the following sections:
Broadcast suppression prevents switched ports on a LAN from being disrupted by a broadcast storm on one of the ports. 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 suppression uses filtering that measures broadcast activity on a LAN over a one-second time period and compares the measurement with a predefined threshold. If the threshold is reached, further broadcast activity is suppressed for the duration of a specified time period. Broadcast suppression is disabled by default.
Figure 13-1 shows the broadcast traffic patterns on a port over a given period of time. In this example, broadcast suppression occurs between time intervals T1 and T2 and between T4 and T5. During those time periods, the amount of broadcast traffic exceeded the configured threshold.
The broadcast suppression threshold numbers and the time interval combination make the broadcast suppression algorithm work with different levels of granularity. A higher threshold allows more broadcast packets to pass through.
Broadcast suppression on the Catalyst 6000 and 6500 series switches is implemented in hardware. The suppression circuitry monitors packets passing from a port to the switching bus. Using the Individual/Group bit in the packet destination address, the broadcast suppression circuitry determines if the packet is unicast or broadcast. It keeps track of the current count of broadcasts within the one-second time interval, and when a threshold is reached, filters out subsequent broadcast packets.
Because hardware broadcast suppression uses a bandwidth-based method to measure broadcast activity, the most significant implementation factor is setting the percentage of total available bandwidth that can be used by broadcast traffic. A threshold value of 100 percent means that no limit is placed on broadcast traffic. Using the set port broadcast command, you can set up the broadcast suppression threshold value.
Because packets do not arrive at uniform intervals, the one-second time interval during which broadcast activity is measured can affect the behavior of broadcast suppression.
These sections describe how to configure broadcast suppression on the Catalyst 6000 and 6500 series switches:
To enable broadcast suppression, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Specify the broadcast 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 suppression configuration. | show port broadcast [mod_num[/port_num]] |
This example shows how to enable bandwidth-based broadcast 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 disable broadcast suppression on one or more ports, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Disable broadcast suppression on one or more ports. | clear port broadcast mod_num/port_num |
This example shows how to disable broadcast suppression on one or more ports:
Console> (enable) clear port broadcast 3/1 Port 3/1-8 broadcast traffic unlimited. Console> (enable)
Posted: Thu Apr 8 14:26:10 PDT 1999
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