In the world of voice, not everything is wine and roses.  As much as we might want to transition everything over to digital IP phones and soft clients, the fact remains that there are some analog devices that still need connectivity on a new phone system.  The more common offender of this is the lowly fax machine.  Yes, even in this day and age we still need to rely on the tried-and-true facsimile machine to send photostatic copies of documents across the PSTN to a waiting party.  Never mind email or Dropbox or even carrier pigeon.  Fax machines seem to be the most important device connected to a phone system.  Normally, I leave the fax connections and their POTS lines intact without touching anything.  However, there are times when I don’t have that luxury.

In the case of the Cisco VoIP systems, that means relying on the Analog Terminal Adapter, or ATA.  The ATA allows you to connect an analog device to the unit, whether it be a fax machine or a cordless analog phone or even a fire alarm or postage machine.  It has many uses.  The configuration of the ATA is fairly straightforward under any CUCM system.  However, if you have a multitude of analog devices that you need to connect, you might opt to use the second analog port on the ATA.  The ATA 186 of the past and its current replacement, the ATA 187, both have 2 analog ports on the back.  There’s only one Ethernet port, though.  This is where the interesting part comes in to play.  If there are two analog ports but only one Ethernet port, how to I configure the MAC address for the second port?  All phone devices in CUCM must be identified by MAC address.  On an ATA, the primary MAC address printed on the bottom or the side of the box is the address for the first port.

If you want to use the second port, you’re going to have to do a little bit of disassembly.  Cisco uses a standard method to create a new MAC address:

1.  Take the MAC address for port 1.  For example, 00:00:DE:AD:BE:EF.

2.  Drop the first two digits from the MAC address.  In the example, 00:DE:AD:BE:EF.

3.  Append “01” to the end of the 10-digit address.  Example, 00:DE:AD:BE:EF:01.

Once you’ve completed those steps, take the MAC address you’ve just created and plug it into CUCM as a new ATA device.  Once you’ve completed the necessary steps to create the new device, it will register with the DN you’ve assigned to it.  Then you can start calling or faxing it to your heart’s content.

There’s no mention of the secondary MAC address anywhere on the web interface.  You’d figure it wouldn’t be hard to write some HTML function to read the MAC address and do the above operation.  The Cisco documentation buries this information deep inside the setup document.  I’ve even search Cisco’s very own support forums and found all manner of advice that doesn’t work correctly.  I decided that it was time to jot this information down in a handy place for the next time I need to remember how to configure the ATA’s second port.  I hope you find it useful as well.