What Is Static Routing And What Do We Use It For?

by Jim Goughenour
What Is Static Routing And What Do We Use It For?

So rather than bore you all with the standard, dictionary definition of static routing, I thought it might be a more interesting read if I put the answer to the question in a context based on my experience.  So here we go.

For those of you that know anything about Ma Bell, the Baby Bells, or what used to be the former telecommunications giant AT&T that then went through divestiture and was broken into a bunch of smaller phone companies, then you probably know the name BellSouth – as this was the Baby Bell in the Carolinas from which this experience is drawn. What you may not know is that telephone systems, much like the Internet backbone, has hierarchical layers to it, that go from the Central Office, or CO for short, to larger switching centers to regional switching centers and so forth. So said another way, there is a hierarchical relationship between all the phone company locations in your neck of the woods. In any event, to put it simply, when you make a call to a location that is within your local CO, or switching center, the call is routed and delivered without ever having to leave that central office. By contrast, when you call someone in a different switching center or CO, the call is routed over to that central office and then delivered on the receiving side. Depending on where you live, and the hierarchical relationships there, when you make a call to a different city, the call might join other calls going first to a regional switching center and then on to the local switching center, or it might go from one Regional Center to another Regional Center and then on the local CO where the call is intended.  If you are calling someone overseas, then it goes across a transatlantic telephone cable and then eventually gets delivered.  As a sidebar, if you want to do some interesting reading by learning more about the TAT-14.

So now that the groundwork is laid, lets talk about one of the features of the Public Switched Telephone Network (PSTN). If you are like me, at some point in your life, you choose to call 911 – you know, like when your cat would not emerge from the drainage pipe under your driveway and everyone is freaking out because it is getting ready to downpour rain and you are afraid – well, you get the picture.  So when you call 911, there are a number of things that happen to get that call to the right place. Now if you are in Charlotte, North Carolina, what happens to that call is first, it is routed to what is called a primary public service access point (no, not the WiFi kind), which is manned 24 hours, 7 days a week, 365 days a year (huge thanks to all those folks that sit behind a monitor and serve at the pleasure of the public for all our needs). When the emergency operator answers the call, one of the things they might ask is do you need police, fire, or medic (another huge thanks to all our front-line workers for working so hard and continuing to show up during the global pandemic). Suppose that you say you need fire, one of the questions they will ask is where you need fire. I promise there is a point to all of this.  Well, for me, I live in a small community outside of the city limits of Charlotte, and our community has a very highly respected Volunteer Fire Department.  So the operator will take your call, and they will ask you to hold the line while they transfer you to Mint Hill, where there is what is called a secondary public service access point (PSAP for short).  Mint Hill PSAP answers, and you tell them or confirm with them where you live, and fire engines are on the way.

So you ask yourself, what in the world does all this have to do with static routing in the IP world? Well, the answer is really simple – your voice on the line once it hits the PSTN is packetized. And what do we do with packets in a network environment? We route them to where they need to go. So before I get too deep in the weeds on all the moving parts of the PSTN and the integration that it has with routers and switches, let me just stop right here and tell you about some of the benefits and drawbacks of static routing.

Simply said, static routing, when all the physical lanes and interfaces are up and working, takes very little logical thought on behalf of the routing device to act on the packets. Static routing is very simple to implement, and effectively becomes a rule in a routing device that says “if you are going to this destination, go this way!” Sorry, had to take a moment to watch the music video with Run DMC and Aerosmith in that amazing collaboration.

Static routing also ranks very high in the routing device when it comes to believability. In other words, the router will trust that you (the administrator) know what you are talking about when you enter the commands to program it. Now lets talk about some of the downsides of static routing. Static routing has no way of communicating to the peer router that it exists.  Static routing has no ability to do anything other than what you told it to do (and I know – you are saying what about floating static routes? That’s not what I’m talking about). What I am talking about is the fact that unlike dynamic routing protocols that have a whole multicast relationship with each other and signaling ability to advise of changed pathways due to a changing environment, static routing has none of that.

In Conclusion

So to tie all this together, let me circle back around to my cat in the drain and my call to 911. The Primary PSAP answered my call, and when I told them I needed the fire department at my house. They routed my call to the Secondary PSAP in Mint Hill, and that PSAP is who dispatched the fire truck to my house and brave firefighters filled the drain pipe with a small but steady stream of water and out ran my cat. That forwarding from the primary PSAP to the secondary PSAP was facilitated by static routing. And that brings me to my closing point – static routing is used when the network is not expected to change, and I hope for all our sakes, the 911 system remains the steadfast, static, engineered, monitored workhorse that serves all of us.

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