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Using the 'show cam' and 'clear cam' commands

I just inherited a pair of Catalyst 4000s, with roughly 80 hosts connected to each one. The trouble is, I have virtually no idea which ports are connected to where.

I can use the 'show cam dynamic' command which gives me a list of MAC addresses connected to the ports, but for many of the ports it shows many addresses -- I guess that it caches them? I.e. I connect a host to port 1, then a few days later, connect a different host; and it updates the MAC address of the port but remembers what it used to be.

show cam dynamic

(output snipped)

1 00-0c-f1-b9-16-28 3/14 [ALL] 1 00-0c-f1-f3-80-68 3/14 [ALL] 1 00-0c-f1-f3-80-74 3/14 [ALL]

I notice that there's a 'clear cam dynamic' command. If I go ahead and run 'clear cam dynamic', what are the implications for the network? I'm not using STP or anything like that, just 'regular' IP services. We have a large number of AppleTalk machines, if it matters (I believe STP has problems with AppleTalk).

Will the switch quickly regenerate the CAM list for each port, or is it a time-consuming (and thus will bring my network down) process?

Thanks!

Chris

Reply to
Chris
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It will recreate the table very quickly

clear the cam table off hours if you are concerned

You will see several MAC addresses on a port if that port is connected to another switch or if there is a hub connected to the switch.

CAM entries timeout so if the devuice is removed it will not remain in the CAM table

Reply to
Merv

That's not a bad idea, I'll clear them after-hours. Don't know why I didn't think of that... :-)

There is a 4-port EtherChannel between the two switches, and obviously a whole bunch of ports are listed on those ports; but there were more regular ports with multiple devices than I expected. Then, of course, there's the challenge of going from MAC address to hostname...

Reply to
Chris

This should not be happening. The actual physical port on the switch should go into a down state when a host is disconnected and the CAM table should then flush all entries. When a new host is connected the CAM table for the port repopulates with the new MAC address. Either you have a hub or switch connected to the port or their is an anomoly with your OS. Try updating to the latest revision of your train (verify a valid version through your SE) and see if the problem goes away.

Reply to
Roman

If after clearing the CAM table, you find you have multiple address on a port where you know for sure that there is only one device, then that would need furhter investigation.

Reply to
Merv

Thanks Roman. I'll have to verify what ports have switches connected (there should be very few), then 'eyeball' those ports and see what is really connected. I do know that the version of CatOS I'm running is very old -- it's 6.1(4) on a Cat 4006 with 64MB of DRAM.

Reply to
Chris

Much good advice already.

Dynamic cam entries are created from the source address of incoming packets. By default the cam age time is 300 secs.

Unless a packet has been received on a particular port with the matching source address the entry is removed after 300 secs.

STP operations can reduce this to 15 secs.

So:- you don't get stale entries.

It is quite unusual but you should check that none of the entries are static.

I forget now how to display the aging time in catos but I am sure that a search of Cisco will turn it up.

Reply to
anybody43

I'll search Cisco, thanks. I went from the 4x100Mbit EtherChannels this morning to a 2x1000Mbit, so hopefully it should be a little easier to find the devices I'm looking for.

Chris

Reply to
Chris

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