Cisco Systems 4500 Port Unicast and Multicast Flood Blocking

CHAPT ER
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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
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Port Unicast and Multicast Flood Blocking
This chapter describes how to configure multicast and unicast flood blocking on the Catalyst 4000
family switch. This chapter contains these topics:
• About Flood Blocking, page 49-1
• Configuring Port Blocking, page 49-1
Note For complete syntax and usage information for the switch commands used in this chapter, first look at
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 Catalyst 4500 Command Reference, it will be found in the larger
Cisco IOS library. Refer to the Cisco IOS Command Reference and related publications at this location:
http://www.cisco.com/en/US/products/ps6350/index.html
About 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.