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Catalyst 3750-X and 3560-X Switch Command Reference
OL-29704-01
Chapter 2 Catalyst 3750-X and 3560-X Switch Cisco IOS Commands
switchport block
switchport block
Use the switchport block interface configuration command on the switch stack or on a standalone switch
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.
switchport block {multicast | unicast}
no switchport block {multicast | unicast}
Syntax Description
Defaults Unknown multicast and unicast traffic is not blocked.
Command Modes Interface configuration
Command History
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 nonprotected ports. If unknown multicast or unicast traffic is not
blocked on a protected port, there could be security issues.
With multicast traffic, the port blocking feature blocks only pure Layer 2 packets. Multicast packets that
contain IPv4 or IPv6 information in the header are not blocked.
Blocking unknown multicast or unicast traffic is not automatically enabled on protected ports; you must
explicitly configure it.
For more information about blocking packets, see the software configuration guide for this release.
Examples This example shows how to block unknown unicast traffic on an interface:
Switch(config-if)# switchport block unicast
You can verify your setting by entering the show interfaces interface-id switchport privileged EXEC
command.
multicast Specify that unknown multicast traffic should be blocked.
Note Only pure Layer 2 multicast traffic is blocked. Multicast packets that
contain IPv4 or IPv6 information in the header are not blocked.
unicast Specify that unknown unicast traffic should be blocked.
Release Modification
12.2(53)SE2 This command was introduced.
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