We have a number of Catalyst 6500 switches running catos 8.5(2). Most of the time a sh cam <macaddress> will return the blade and port number, but in a significant number of cases it returns 0 entries found. This occurs even if the device (pc/printer) is connected and working well. Can anybody explain why this might happen please?
It could be a couple of things
1) The mac-address has been aged out of the cache and you won't see it again until something connects to it ( or you could ping it )
2) The mac-address table can hold a finite number of entries. If the table is full you will not get any more entries.
Both of these apply to devices that are layer 2 adjacent to the 6500. If the device is on the other side of an external router then you won't see it's mac-address just the mac-address of the external router interface that connects to the 6500.
The devices can be either recently ataached (as in just plugged in) or for some time as in printers that may have been attached for weeks, but get regular use. It is possible that the table could be full as the switches are often heavily used, but as we do not use hubs, there would tyupically only be one mac address per port. I would have expected that there would be sufficient space to cover all the available ports? Or perhaps not?
The CAM table should be able to hold everything you have as it does not sound that you have thousands of devices on your LAN. If there has not been activity for a bit from that MAC it will get flushed from the CAM so try pinging just before you look for entries in the CAM table.
Check out especially the heading "Loss of Dynamic MAC Addresses with Distributed Switching"