MAC Address Table Instability

Another problem caused by a looped environment is MAC address tables (CAM tables in CatOS) being constantly updated. Take the network in Figure 8-3, for example. With 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.

MAC address table inconsistencies

Figure 8-3. MAC address table inconsistencies

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 | 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 | include 0030.1904.da60
   1    0030.1904.da60    DYNAMIC     Fa0/10
3550-IOS# sho mac-address-table | include 0030.1904.da60
   1    0030.1904.da60    DYNAMIC     Fa0/10
3550-IOS# sho mac-address-table | include 0030.1904.da60
   1    0030.1904.da60    DYNAMIC     Fa0/2
3550-IOS# sho mac-address-table | include 0030.1904.da60
   1    0030.1904.da60    DYNAMIC     Fa0/3
3550-IOS# sho mac-address-table | include 0030.1904.da60
   1    0030.1904.da60    DYNAMIC     Fa0/2
3550-IOS# sho mac-address-table | include 0030.1904.da60

This switch is directly connected to the device in question, ...

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