Catalyst 4500 Series Switch Cisco IOS Software Configuration Guide, 12.2(31)SG

Port Unicast and Multicast Flood Blocking

This chapter describes how to configure multicast and unicast flood blocking on the Catalyst 4500 series switch. This chapter contains these topics:

•Overview of Flood Blocking

•Configuring Port Blocking

Note For complete syntax and usage information for the switch commands used in this chapter, see the Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html

If the command is not found in the Cisco Catalyst 4500 Command Reference, you can locate it in the larger Cisco IOS library. Refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/ps6350/index.html

Overview of Flood Blocking

Occasionally, unknown unicast or multicast traffic is flooded to a switch port because a MAC address has timed out or has not been learned by the switch. (This condition is especially undesirable for a private VLAN isolated port.) To guarantee that no unicast and multicast traffic is flooded to the port, use the switchport block unicast and switchport block multicast commands to enable flood blocking on the switch.

Note The flood blocking feature is supported on all switched ports (including PVLAN ports) and is applied to all VLANs on which the port is forwarding.

Configuring Port Blocking

By default, a switch floods packets with unknown destination MAC addresses to all ports. If unknown unicast and multicast traffic is forwarded to a switch port, there might be security issues. To prevent forwarding such traffic, you can configure a port to block unknown unicast or multicast packets.

Note Blocking of unicast or multicast traffic is not automatically enabled on a switch port; you must explicitly configure it.

Blocking Flooded Traffic on an Interface

Note The interface can be a physical interface (for example, GigabitEthernet 1/1) or an EtherChannel group (such as port-channel 5). When you block multicast or unicast traffic for a port channel, it is blocked on all ports in the port channel group.

To disable the flooding of multicast and unicast packets to an interface, perform this task:

Switch# configure terminal

This example shows how to block unicast and multicast flooding on a GigabitEthernet interface1/1 and how to verify the configuration:

Switch# configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# interface gigabitethernet1/1

Switch(config-if)# switchport block multicast

Switch(config-if)# switchport block unicast

Switch(config-if)# end

Switch# show interface gigabitethernet1/1 switchport

Name: Gi1/3

Switchport: Enabled

<output truncated>

Port Protected: On

Unknown Unicast Traffic: Not Allowed

Unknown Multicast Traffic: Not Allowed

Broadcast Suppression Level: 100

Multicast Suppression Level: 100

Unicast Suppression Level: 100 

Resuming Normal Forwarding on a Port

To resume normal forwarding on a port, perform this task:

Switch# configure terminal