About Flood Blocking - Cisco Catalyst 4500 Series Configuration Manual

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:
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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 Series Switch 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.
The flood blocking feature is supported on all switched ports (including PVLAN ports) and is applied
Note
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.
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About Flood Blocking, page 53-1
Configuring Port Blocking, page 53-1
C H A P T E R
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
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