CHAPTER
64-1
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
64
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:
• About Flood Blocking, page 64-1
• Configuring Port Blocking, page 64-1
Note For complete syntax and usage information for the switch commands used in this chapter, see the
Cisco IOS Command Reference Guides for the Catalyst 4500 Series Switch.
If a command is not in the Cisco Catalyst 4500 Series Switch Command Reference , you can locate it in
the Cisco IOS Master Command List, All Releases.
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.
Note Blocking of unicast or multicast traffic is not automatically enabled on a switch port; you must explicitly
configure it.
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