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

 
 
Chris
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      05-01-2006
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

 
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Merv
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      05-01-2006
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

 
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Chris
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      05-02-2006
Merv wrote:
> 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


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...

 
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Roman
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      05-02-2006
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.

 
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Merv
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      05-02-2006

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.

 
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Chris
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      05-02-2006
Roman wrote:
> 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.


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.

 
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anybody43@hotmail.com
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      05-03-2006
> but for many of the ports it shows
> many addresses -- I guess that it caches them? I


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.

 
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Chris
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      05-03-2006
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> 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.


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

 
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