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Aaron’s blog on Networking, and Enterprise Technology

I’m not crazy if I use the term “router-on-a-stick”

If many of you have watched Jeff Dunham’s shows, you’ll no doubt be familiar with José, the Jalepeño on a stick.  (Jeff Dunham is a hilarious ventriliquist).  So, don’t think I’m nuts or crazy if I use the term “router-on-a-stick”.  It’s not a reference to José or being silly.

Why do I bring this up?  Well, I was talking to a technician, trying to get him to understand VLAN’s, and that you actually have to have a router (Layer3 device) in order for the two VLAN’s to communicate.  He asked, “I have 20 VLANS!  I can’t have that many interfaces on my small 1800 router!”.

Hence, the coined term, Router on a stick.  Basically, you use a single cabled interface (the stick if you straighten out the cable — sorry for the visual), and turn it into multiple logical interfaces.  You can then assign virtual IP addresses on each virtual interface, and route between them, still only using the 1 physical interface.

I was then asked, most appropriately, “How do you get multiple VLAN’s over a single cable?”.  Ahhh…. now you are talking about trunking.

VLAN’s are nothing more than the exact piece of data (the frame in this case, since we are talking Layer2), with 1 ity-bity-tiny difference…. a VLAN tag.  A frame that is tagged in a certain VLAN cannot cross another VLAN boundry.  So, you create a trunk interface, to pass all the frames in certain VLANs, or all of them.

Trunking can be done in two ways…. ISL (blehhh… don’t use it) and 802.1q.  If you build a trunk between the switch interface and the router, you can then pass all the trunk data to the router, and have the newly created logical interfaces receive the packet and send it on for routing.

No doubt, I have left gaps in my explanation.  I did this because I only wanted to give you an overview, and the definitions.  I didn’t get into detail, because many others have done this (and probably a better job than me).  Cisco also has a quick document written up.  http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a00800949fd.shtml

Good Luck, and don’t hesitate to ask questions, or correct me areas where I may be mistaken (I do that from time to time 🙂 )


2 responses to “I’m not crazy if I use the term “router-on-a-stick”

  1. Roland Dobbins August 9, 2008 at 9:00 am

    It’s a good way to describe this concept; however, be advised that the term ‘router-on-a-stick’ has been in use in operational circles for at least a decade, if not longer.

  2. Aaron Paxson August 9, 2008 at 9:49 am

    Hi Roland! Can you explain what you mean by “operational circles”?

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