Mac address table question

So I have the following set up

                       Trunk                     Trunk
   layer 3 switch-A -------------Layer 3 switch-B ------------Layer 2 switch-C  ------ Printer

I’m on Switch B and I’m trying to track down a printer with the mac address. I do a show mac address-table | inc xxxxxx and if the printer is hanging off of the layer 2 switch-C I get the expected information it points me to the trunk link between the two so I can trace it down. However if I do the same search from switch-B and the printer is hanging off switch-A the other layer 3 switch I don’t get any result from the show mac address-table | inc xxxxxx command. Now I know the device is up and working as the IP is pingable. The layer 3 are 4500’s and running IOS-XE. So why does it act this way? why can’t a mac address be traced down between the 2 layer 3 switches?

Hello Derek

It all depends on how your layer 3 switches have been configured and specifically how Switch A has been configured. If Switch A has been configured like a layer 2 switch, that is, no routing is taking place, then you should see the printer’s MAC address associated with the port on Switch B that connects to Switch A. However, if Switch A has routing enabled AND the VLAN on which the printer sits does not extend beyond the switch itself, then the printer’s MAC address will not show up in the MAC address table of Switch B. This is because a routing process sits between the printer (and its VLAN) and the networks on Switches B and C.

Remember that MAC addresses are only significant within a subnet/VLAN. Two subnets that are configured with routing for communication between them cannot and will not learn anything about the MAC addresses of devices found on the other subnet. So, the first thing that comes to mind is that you probably have the printer in a VLAN that does not extend beyond Switch A. Any communication between the printer and devices on switches B and C must occur through routing. That’s why you get answers to your pings.

If this is not the case, the MAC address should show up in the MAC address table of switch B. Here are several other troubleshooting steps that you should take to resolve the issue.

  1. First of all, the access port on Switch A where the printer is connected must be on a VLAN that is included in the trunk between Switches A and B.
  2. The trunk between switches A and B must be configured correctly, that is, it must have the appropriate VLANs allowed on both ends of the link
  3. A communication between the printer and another entity must have occurred recently so that the MAC address table of Switch B will be populated with the MAC address of the printer.

I hope this has been helpful!