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 come to believe that 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.
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 DYNAMICFa0/10
3550-IOS#sho mac-address-table | include 0030.1904.da60
1 0030.1904.da60 DYNAMICFa0/10
3550-IOS#sho mac-address-table | include 0030.1904.da60
1 0030.1904.da60 DYNAMICFa0/2
3550-IOS#sho mac-address-table | include 0030.1904.da60
1 0030.1904.da60 DYNAMICFa0/3
3550-IOS#sho mac-address-table | include 0030.1904.da60
1 0030.1904.da60 DYNAMICFa0/2
3550-IOS#sho mac-address-table | include 0030.1904.da60
This switch is directly connected to ...
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