David DavisCables form layer 1 of the network — the physical layer. Properly connecting cables is fundamental to healthy network communication. Faulty connections could interrupt service or cause packets to be dropped. Without a clear understanding of cabling, you won’t be able to troubleshoot or design your LAN or WAN. Additionally, knowledge of cable types is important for certification tests like the Cisco CCNA/CCENT.

Cabling basics for network admins

  • Your cable works at layer 1 — Physical of the OSI Model. Also at that layer are the 1’s and 0’s that traverse the cable as an electrical or light signal (depending on what type of cabling you are using).
  • Unshielded twisted pair (UTP) copper cable is used for many functions in network cabling: Ethernet, Serial, ISDN, Console, and more.
  • While you could put different ends on a UTP cable, typically it will have a RJ45 end with 8 pins.
  • With a normal Ethernet cable, the wires run straight through, from one end to the other. Straight-through cables are used to connect a PC to a switch, as in Figure A.

Figure A

Graphic Courtesy of Cisco Systems
  • With a crossover cable (Figure B), the source and destination of the UTP wires are crossed. This allows you to use it to connect a PC to PC, switch to switch, or router to router.

Figure B

Graphic Courtesy of Cisco Systems

Now, how is cabling for Cisco routers and switches different?

Cisco console and AUX port cabling

There are a few differences between Cisco cabling and other network device cabling. Two things immediately come to mind:

  • Cisco routers, switches, and firewalls use a special “rolled” cable for console and auxiliary port access.
  • Cisco offers intelligent serial cabling.

One of the most confusing things to Cisco newcomers is the concept of the console cable. Other SMB and home-networking devices don’t usually have a console port. With those devices, they receive a DHCP IP address and then you can configure them over the network from there. With Cisco devices, there is no IP address on the device, and you must first use the console port and console cable to configure the router, switch, or firewall OOB (out of band).

The Cisco console cable is a special cable. It isn’t wired like an Ethernet cable. However, if you didn’t have a console cable, you could cut off the end of a straight-through Ethernet cable, change the pin out, and recrimp it to make it a console cable.

Below, you can see the pin out of a console cable. The console cable is a “rolled” cable, because if you look at the pins from one end to the other, it is as if the end was rolled over (the order is flipped), as in Figure C.

Figure C

Rolled over cable
Graphic Courtesy of Cisco Systems

Traditionally Cisco console cables were RJ45-RJ45 and then you would use a RJ45-DB9 adaptor to connect it to your PC’s serial port (COM port). Today, new Cisco devices come with console cables that have a DB9 adaptor integrated/molded to the cable on one end (Figure D). Keep in mind that the data moving across the console cable is serial data (not Ethernet).

Figure D

Graphic Courtesy of Cisco Systems

While what I said above concerning console cables is true for most Cisco devices, there are variations on the console cable. For more detailed information about Cisco console and AUX port cabling (including the pin-out for a console cable so that you can make your own), see this Cisco document Cisco Cabling Guide for Console and AUX Ports.

David Davis has worked in the IT industry for 15+ years and holds several certifications, including CCIE, CCNA, CCNP, MCSE, CISSP, VCP. He has authored hundreds of articles and numerous IT training videos. Today, David is the Director of Infrastructure at Train Signal.com. Train Signal, Inc. is the global leader in video training for IT Professionals and end users.

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