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:

•Overview of Flood Blocking

•Configuring Port Blocking

Note For complete syntax and usage information for the switch commands used in this chapter, refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at this location:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/index.htm.

Overview of 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.

Blocking Flooded Traffic on an Interface

Note The interface can be a physical interface (for example, GigabitEthernet 1/1) or an EtherChannel group (such as port-channel 5). When you block multicast or unicast traffic for a port channel, it is blocked on all ports in the port channel group.

To disable the flooding of multicast and unicast packets to an interface, perform this task:

 
Command
Purpose

Step 1  

Switch# configure terminal

Enters global configuration mode.

Step 2 

Switch(config)# interface interface-id

Enters interface configuration mode and enter the type and number of the switchport interface (for example, GigabitEthernet 1/1).

Step 3 

Switch(config-if)# switchport block multicast

Blocks unknown multicast forwarding to the port.

Step 4 

Switch(config-if)# switchport block unicast

Blocks unknown unicast forwarding to the port.

Step 5 

Switch(config)# end

Returns to privileged EXEC mode.

Step 6 

Switch#
show interface interface-id switchport

Verifies your entry.

Step 7 

Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

This example shows how to block unicast and multicast flooding on a GigabitEthernet interface1/1 and how to verify the configuration: 

Switch# configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# interface gigabitethernet1/1

Switch(config-if)# switchport block multicast

Switch(config-if)# switchport block unicast

Switch(config-if)# end

Switch# show interface gigabitethernet1/1 switchport

Name: Gi1/3

Switchport: Enabled


<output truncated>


Port Protected: On

Unknown Unicast Traffic: Not Allowed

Unknown Multicast Traffic: Not Allowed


Broadcast Suppression Level: 100

Multicast Suppression Level: 100

Unicast Suppression Level: 100

Resuming Normal Forwarding on a Port

To resume normal forwarding on a port, perform this task:

 
Command
Purpose

Step 1  

Switch# configure terminal

Enters global configuration mode.

Step 2 

Switch(config)# interface interface-id

Enters interface configuration mode and enter the type and number of the switchport interface (GigabitEthernet1/1).

Step 3 

Switch(config-if)# no switchport block multicast

Enables unknown multicast flooding to the port.

Step 4 

Switch(config-if)# no switchport block unicast

Enables unknown unicast flooding to the port.

Step 5 

Switch(config)# end

Returns to privileged EXEC mode.

Step 6 

Switch# show interface interface-id switchport

Verifies your entry.

Step 7 

Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.