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Catalyst 2950 and Catalyst 2955 Switch Command Reference
0L-10102-01
Chapter 2 Catalyst 2950 and 2955 Cisco IOS Commands
switchport block
switchport block
Use the switchport block interface configuration command to prevent forwarding of unknown multicast
or unicast packets. Use the no form of this command to allow forwarding of unknown multicast or
unicast packets.
switchport block {multicast | unicast}
no switchport block {multicast | unicast}
This command is available only on these switches:
• Catalyst 2950 Long-Reach Ethernet (LRE) switches running Cisco IOS Release 12.1(14)EA1
or later
• Catalyst 2950G-12-EI, 2950G-24-EI, 2950G-24-EI-DC, 2950G-48-EI, and 2955 switches running
Cisco IOS Release 12.1(19)EA1 or later
Syntax Description
Defaults Unknown multicast and unicast traffic are 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 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, see 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.
multicast Specify that unknown multicast traffic should be blocked.
unicast Specify that unknown unicast traffic should be blocked.
Release Modification
12.1(14)EA1 This command was introduced.
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