Cisco Model D9479 Gigabit QAM Modulator Technische Referenzen

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4022934 Rev A 

 

LAN switches use forwarding tables, such as Layer 2 and Content Addressable 
Memory (CAM) tables, to direct traffic to specific ports based on the virtual LAN 
(VLAN) number and the destination MAC address of the frame. If the table does not 
contain an entry for the destination MAC address of the frame in the incoming 
VLAN, the switch sends the unicast frame to all forwarding ports within the 
respective VLAN. This condition is called "flooding." In this case, the packet is 
flooded out of all forwarding ports in its VLAN except for the port where it is 
received. 

The default aging time for an Address Resolution Protocol (ARP) table is 4 hours, 
but the CAM only holds the entries for 5 minutes (300 seconds). The switch sends 
out a frame to all forwarding ports in the VLAN whenever the destination MAC 
address is aged out from the CAM table. 

 

In order to prevent unicast flooding, you should set your CAM aging timer to be 
greater than or equal to the ARP timeout value.  

The default value for the L3 switch ARP timeout is 4 hours, and the default time for 
the L2 switch timeout is 300 seconds (5 minutes). To prevent unicast flooding, the L2 
switch CAM aging timeout should be set to a value greater than the ARP timeout of 
the L3 switch. 

You can use one of the following commands in order to increase the CAM aging 
timer for the affected VLAN to match the ARP aging time:  

 

If you are using CatOS, use the set cam agingtime command to change the CAM 
aging timer. 

 

If you are using Cisco IOS software, use the mac−address−table aging−time 
command to change the CAM aging timer. 

Note: If you are working in a Catalyst environment that runs a Hot Standby Router 
Protocol (HSRP), be sure that the CAM and ARP timers are synchronized. 

For more information, refer to Catalyst 6500/6000 Switches ARP or CAM Table Issues 
Troubleshooting, document ID 71079.