Flooding Controls

With the Flooding Controls page, you can:

• Reduce the flood of unknown unicast packets.
• Configure broadcast storm control.
• Disable flooded unicast and multicast traffic on a port.

Use the Network Port Table on this page to assign a network port on a switch; unknown unicast packets are forwarded only to the port that you specify in this table.

Use the Flooding Controls Table to set a threshold for the number of broadcast packets that can be received from a port before forwarding is blocked. Using this table, you can also set a threshold for reenabling the normal forwarding of broadcast packets, and disable the flooding of unknown unicast and multicast packets. Flooding controls affect all VLANs on multi-VLAN ports and trunk ports (Enterprise Edition Software only).

Reducing the Flood of Unknown Unicast Packets

By configuring a network port, you can reduce the flooding of unknown unicast packets. When you define a network port for the VLAN, unknown unicast packets in the corresponding VLAN are forwarded only to that network port. Typically, a network port is connected to an upstream neighbor such as the Catalyst 5500 or 8500 switch. Configuring a network port does not affect the forwarding of known unicast, multicast, and broadcast packets.

When a network port is configured, the switch deletes all dynamic addresses on the port and disables address learning to conserve space for addresses on other ports.

With the Network Port Table, you can:

• Enable a network port.
• Disable a network port.

Enabling a Network Port

The following rules apply when you enable a network port:

• A network port can be a static-access port, a multi-VLAN port, a port group (all ports in the group act as the network port), or a trunk port (Enterprise Edition Software only). Both the multi-VLAN port and the trunk port become the network port for all the VLANs associated with that port.
• If you make a network port a multi-VLAN port or trunk port (Enterprise Edition Software only) without configuring the new VLANs with a network port, the existing network port becomes the network port for all the new VLANs on the port.
• If you try to add a network port to a VLAN that already has one, an error message displays telling you a network port is already configured.

The following restrictions apply when you enable a network port:

• A network port cannot be a secure port, a monitor port, an ATM port, or a dynamic-access port (Enterprise Edition Software only). You can assign a dynamic-access port to a VLAN in which another port is the network port.
• Each VLAN can have up to one network port.
• A network port cannot be in a destination-based port group.

To enable a network port in the Network Port Table:

1. From the Interface drop-down list, select an interface.
2. Click <<Enable<<.
   The selected interface and VLAN ID appear in the Network Port list.

Disabling a Network Port

To disable a network port:

1. In the Network Port list, select a network port.
2. Click>>Disable>>.

Configuring Broadcast Storm Control

A broadcast storm occurs when a switch port receives a large number of broadcast packets. Forwarding these packets can cause the network to slow down or time out. The broadcast rate and forwarding rate are maintained on a per-port basis, not on a per-VLAN basis.

With the Flooding Controls Table, you set two thresholds that define the beginning and the end of a broadcast storm. The rising threshold is the number of broadcast packets per second that a switch port can receive before forwarding is blocked. The falling threshold reenables the normal forwarding of broadcast packets.

By default, broadcast storm control is disabled.

To enable broadcast storm control management for each port in the Flooding Controls Table:

1. In the Filter State: Requested/Actual column, select the Enable checkbox.
2. In the Trap State: Requested/Actual column, select the Enable checkbox to generate an SNMP trap when a threshold is crossed.
   Make sure you configure a trap manager on the SNMP Configuration page to receive this trap.
3. In the Threshold: Rising field, enter a number from 0 to 4294967295 (broadcast packets per second).
   The default is 500 packets per second. Make sure you set the rising threshold larger than the falling threshold.
   Note: Setting the threshold to more than 148000 packets per second might not be useful because that rate is approximately the maximum rate on a 100-Mbps link (in one direction). The higher the threshold, the less effective storm control will be.
4. In the Threshold: Falling field, enter a number from 0 to 4294967295 (broadcast packets per second) to determine when to deactivate broadcast storm control on the port.
   The default is 250 packets per second.
5. Click Apply.

Note: The Current column displays the number of broadcast packets per second arriving on the port. The Trap Sent column displays the number of traps that have been generated for the port.

Disabling Flooded Unicast and Multicast Traffic on a Port

By default, the switch floods packets having unknown destination MAC addresses to all ports in the VLAN. Flooded traffic does not cross VLAN boundaries, except for multi-VLAN ports, which flood traffic to all VLANs to which they are connected.

Some configurations do not require flooding. For example, a port that has only manually assigned addresses has no unknown destinations, and flooding serves no purpose. Therefore, you can disable the flooding of unicast and multicast packets on a per-port basis.

Note: If a port is a network port, you cannot change the settings for unicast and multicast traffic.

To disable flooded traffic of unicast and multicast packets:

1. In the Receive Unknown MACs column, deselect the Unicast and Multicast checkboxes for the port.
2. Click Apply.