Switchport Block - Cisco Catalyst 3550 Command Reference Manual

Chapter 2 Cisco IOS Commands

switchport block

Use the switchport block interface configuration command to prevent unknown multicast or unicast
packets from being forwarded. Use the no form of this command to allow forwarding unknown multicast
or unicast packets.
Syntax Description
multicast
unicast
Defaults Unknown multicast and unicast traffic are not blocked.
Command Modes
Interface configuration
Command History Release
12.1(4)EA1
Usage Guidelines
By default, all traffic with unknown MAC addresses is sent to all ports. You can block unknown multicast
or unicast traffic on protected or non-protected ports. If unknown multicast or unicast traffic is not
blocked on a protected port, there could be security issues.
Blocking unknown multicast or unicast traffic is not automatically enabled on protected ports; you must
explicitly configure it.
Note For more information about blocking packets, refer to the software configuration guide for this release.
Examples
This example shows how to block unknown multicast traffic on an interface:
Switch(config-if)# switchport block multicast
You can verify your setting by entering the show interfaces interface-id switchport privileged EXEC
command.
Related Commands Command
show interfaces
78-11195-09
switchport block {multicast | unicast}
no switchport block {multicast | unicast}
Specify that unknown multicast traffic should be blocked.
Specify that unknown unicast traffic should be blocked.
Modification
This command was first introduced.
switchport
Description
Displays the administrative and operational status of a switching
(nonrouting) port, including port blocking and port protection settings.
Catalyst 3550 Multilayer Switch Command Reference
switchport block
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