It has always seemed a bit strange to me that the console ports on networking gear are 8P8C (popularly referred to as "RJ45" even though that's not strictly applicable here) connectors and require an adapter to connect to a DE9 serial port. What is the reason for this? Is it entirely to make better use of panel real estate?

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    The cynical would say it was originally to allow Cisco to sell extra cables and adapters. Commented Jan 4, 2013 at 2:01
  • @David: Well, probably, but I guess my real question is "what is the stated reason". :)
    – tgies
    Commented Jan 4, 2013 at 2:03
  • To sell extra cables and adapters, and save 17 cents over the cost of replacing it with a standard that's actually used anywhere in the last decade (USB for example). Commented Jan 4, 2013 at 2:03
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    Well, on terminal servers with 10 or more ports, it does save a huge amount of panel real estate. And if you're going to connect the network switch to a terminal server (which was the usual setup), an RJ45 console cable was what you'd use, not an adapter. Commented Jan 4, 2013 at 2:06
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    I would guess that RJ45 sockets are cheaper then DB9. Cisco, just have a huge volume for RJ45 sockets.
    – Zoredache
    Commented Jan 4, 2013 at 5:47

2 Answers 2


Out Of Band management is the only thing I can think of. A specific port that gets you into the device that doesn't rely on the communications-plane being managed. In the beginning, all comms techs HAD serial laying about, and it was easy: just plug the port into your serial terminal and configure. No mucking about with Ethernet, DHCP, BootP, or RARP, or figuring out what IP the switch auto-configures for itself.

And then inertia happened.


Density, cost and the smaller form factor are probably the primary reasons.

Look at a Cisco 2960 48-port front panel, for instance... I think the RJ45 is a better bet for serial than a DB-9 in that case.

Also having easy ability to pin-out for Cat5 runs to serial console/terminal servers.

  • Probably, but I'm really hoping someone can provide a more or less "definitive" answer backed by reliable sources.
    – tgies
    Commented Jan 4, 2013 at 2:08

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