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This chapter describes how to configure broadcast suppression on the Catalyst 6000 family switches.
This chapter consists of these sections:
Broadcast suppression prevents switchports on a LAN from being disrupted by a broadcast storm on one of the interfaces. 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 17-1 shows the broadcast traffic patterns on an interface 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 family switches is implemented in hardware. The suppression circuitry monitors packets passing from an interface 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 switchport 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 family switches:
To enable broadcast suppression, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config-if)# switchport broadcast threshold | Specify the broadcast suppression threshold for an interface as a percentage of total bandwidth. | ||
| Router# show broadcast interface interface-num | In privileged EXEC mode, verify the broadcast suppression configuration. |
This example shows how to enable bandwidth-based broadcast suppression and verify the configuration:
Router(config-if)# switchport broadcast 75 Router# show broadcast interface fastEthernet 5/9 Interface Limit Suppressed --------- ----- ---------- FastEthernet5/9 75 0 Router#
To disable broadcast suppression on an interface, perform this task:
| Command | Purpose |
|---|---|
Router(config-if)# no switchport broadcast threshold | Disable broadcast suppression on an interface. |
This example shows how to disable broadcast suppression on an interface and verify the new configuration:
Router(config-if)#no switchport broadcast 75Router#show broadcast interface fastEthernet 5/9Interface Limit Suppressed--------- ----- ----------FastEthernet5/9 100 0Router#
The show broadcast command displays all interfaces that do not have a 100 percet default setting. If you specify the interface, the show broadcast command displays the interface configuration regardless of whether broadcast suppression has been configured. In the first example on page 18-4, the third column shows the number of packets that have been suppressed on that interface since the switch was reloaded or that module was inserted.
To display broadcast suppression configurations, perform this task:
| Command | Purpose |
|---|---|
Router# show broadcast interface interface_num | Display broadcast suppression on an interface. |
This example shows how to display broadcast suppression configuration information for a specific interface:
Router#show broadcast interface fa 5/9Interface Limit Suppressed--------- ----- ----------FastEthernet5/9 60 0Router#
This example shows how to display broadcast suppression configuration information for all interfaces that do not have the default setting:
Router# show broadcast Interface Limit Suppressed --------- ----- ---------- FastEthernet5/8 80 0 FastEthernet5/9 60 0 Router#
Posted: Mon Jan 3 14:04:23 PST 2000
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