: 0030.1904.da60 F0/1 Figure 8-3. MAC address table inconsistencies all of the switches interconnected and spanning tree disabled, Switch A will assume the MAC address for the PC directly connected to it is sourced from a different switch. This happens very quickly during a broadcast storm and, in the rare instances when you see this behavior without a broadcast storm, chances are things are about to get very bad very quickly. With this network in place and spanning tree disabled, I searched for the MAC address of the PC on Switch A using the show mac-address-table I include 0030.1904.da60 command. I repeated the command as fast as I could and got the following results: 3550-IOS#sho mac-address-table I include 0030. 1904. da60 0030.1904. da60 DYNAMIC Fa0/10 3550-IOS# sho mac-address-table include 0030.1904.da60 0030.1904.da60 DYNAMIC Fa0/10 3550-IOS# sho mac-address-table include 0030.1904.da60 0030.1904.da60 DYNAMIC Fa0/2 3550-IOS# sho mac-address-table include 0030.1904.da60 0030.1904.da60 DYNAMIC Fa0/3 3550-IOS# sho mac-address-table I include 0030.1904.da60 0030.1904.da60 DYNAMIC Fa0/2 3550-IOS# sho mac-address-table I include 0030.1904.da60 This switch is directly connected to the device in question, yet at different times it seems to believe that the best path to the device is via Switch B or Switch C. Remember that a switch examines each packet that arrives on a port and assigns the packet's source MAC address to that port in its MAC address/CAM table. Because devices can and do move, the switch will assume that the last port on which the MAC address was observed is where the address now resides. Because the broadcast packets originating from the PC are constantly cycling through the looped network, wherever the packet comes into the switch is where the switch will believe that MAC address belongs. MAC Address Table Instability Preventing Loops with Spanning Tree The obvious way to prevent loops is to follow the same advice a doctor might give you when you complain, "It hurts when I do this"-don't do it! O