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, first look at the Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:

http://www.cisco.com/en/US/products//hw/switches/ps4324/index.html

If the command is not found in the Catalyst 4500 Series Switch Command Reference, it will be found in the larger Cisco IOS library. Refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/ps6350/index.html

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.

Note Starting with Cisco IOS Release 12.2(52)SG, only IPV4 and IPv6 unknown multicast traffic flooding is blocked; Layer 2 unknown multicast flooding is not. This behavior stems from a fix for the following problem: when you configure blocking of unknown multicast flooding on a port, broadcast traffic to the port is also blocked.

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.