ARP Statistics

John Blakley · ‎09-22-2008

Is there a way to see ARP statistics under a 3750 switch? We're having an issue where it doesn't seem like the ARP table is refreshing correctly. I have a port that shows to be connected, but there's no mac address listed.

Thanks!

John

HTH, John *** Please rate all useful posts ***

Richard Burts · ‎09-22-2008

John

Are we really talking about the ARP table and are you sure that there should be an entry in the ARP table? There will be a listing in the ARP table only if the switch has communicated to the device on layer 3 (if the switch has pinged the device, etc, or has routing enabled and has routed packets to the device). I wonder if what you should be looking at is the mac-address-table to verify that a device is connected.

HTH

Rick

HTH

Rick

John Blakley · ‎09-22-2008

Makes sense for ARP, but can you explain below to me?

sh int g3/0/5 it shows up and connected.

sh mac add table int g3/0/5 shows no mac address

I'm having to move servers over to another VLAN, but I need to figure this out before doing it. I don't want to screw anything up. :-)

Thanks!

--John

HTH, John *** Please rate all useful posts ***

Richard Burts · ‎09-22-2008

John

There are several things that might produce symptoms such as you describe. If the port is configured as a span port destination (or monitor port destination depending on which model of switch) then the switch sends traffic to the monitor port but does not receive traffic from the monitor port. The result is that there is a connected device but no entry in the mac-address-table. Of if for some reason the device that is connected has not sent any traffic during the period of the mac address ageing time, then the switch will have aged out the mac address in the table and not relearned it. Some devices like printers do this a lot. Or there might be a situation where there is assymetric traffic which could result in this situation (explanations of unicast flooding sometimes refer to this issue).

So there are some situations where there is a connected device but no entry in the mac-address-table.

HTH

Rick

HTH

Rick