Giving Some STATIC (Routes, that Is!)

The Original Static Route!

Last time we talked about the IPv4 routing table and how the information populates it in the first place, starting with directly connected routes.  The fact that interfaces with these routes have to be up and functioning to be in the table at all makes them inherently “believable” (the term used to describe routes).  Routes in the IPv4 routing table from this source are marked with a C (connected) and easily recognizable.

That brings us to the topic of static routes, which may evoke images of lightning (see above), or walking across a carpeted room and sending a shock to someone just for fun.  Certainly the word static is used in those contexts, but in this case it simply means fixed or stationary, and refers to the fact that these routes stay permanently in the routing table.  You might think that this sounds like a description of connected routes too, but in reality it means that the routes are manually configured by a network administrator.

You can think of static routes in terms of the large concrete barricades that separate the sides of traffic on the highway.  First, they stay in one place, they do not move, even when an object may strike them with some degree of force.  Static routes remain in the table once configured and do not simply vanish when things are taking place from a routing perspective.  Second, these barriers unforgivingly separate traffic, just as static routes continually direct traffic to their intended destination.  Finally, these concrete dividers are actually put there by highway workers, they do not simply magically appear, just as static routes have to be placed manually in the router’s configuration.

The configuration of a static is remarkably straightforward and uncomplicated, using the following syntax:

ip route <destination-prefix> <subnet-mask> <next-hop-address | exit-interface> [<distance>]

The destination prefix is a subnet and/or network using standard decimal notation (e.g., 192.168.2.0), followed by a subnet mask, just as you probably have seen and used numerous times in your networking job and/or studies.  The next part is critical to understand, as you have options you need to consider.  You can choose simply to specify the outgoing interface, or you can specify the IPv4 address of the device you want to send the traffic to (typically directly connected).  In either case, if the interface is down and/or the next hop is unreachable, the route will not appear in the table.  The optional distance command allows you to specify the administrative distance, which we will cover next time!

– Joe

Road Trip, the Sequel! (aka, Introduction to IP Routing II)

Road Trip! (aka, Introduction to IP Routing I)

As you may recall from the very first entries on this blog, networking is all about communication—getting information from one location to another.  IPV4 packets carry that data from a source address to a destination address and all, but how does the process know how to get there in the first place?  The simple version of the answer is just IP routing!  Routing is the process of figuring out the best path to travel between any two given points in the journey.

To explain this a little bit, let me describe how I remember family vacations when I was a kid.  My Dad would load up the station wagon (yes, the nerdiest possible vehicle in history, I know), load us all inside, and then head out on the road.  My father was not the typical “Red Foreman” dad (in case you don’t remember, he was the colorful character from That 70’s Show).  My Dad is an engineer, and approaches everything with a carefully calculated plan by which to succeed, without much tolerance for variance.  Out would come the map, and he would move through and figure out the safest, most efficient path of travel, all before we ever actually got out on the road.

In a nutshell, that’s almost the same exact process that network devices (routers) use to figure out how to send data from the source to the destination.  When there are multiple possible ways to get there, a process is used to determine the most efficient manner to get from end to end.  Each part of the journey involves various networks, addresses, interfaces and media, not unlike the various highways and access roads used on a long-distance road trip.  And no, none of the small packets in the back seat keep asking “Are we there yet” over and over!

Each segment of the “trip” of an IP packet is managed by a Layer 3 routing device, typically a router or Layer 3 switch.  These devices maintain a list of networks it can reach, and by which exit point from the device (interface) it uses to reach those.  Once the packet leaves that device, it becomes the responsibility of the next one in the chain, which uses a similar process (route lookup) to send traffic on to the next part of the journey.  Once it reaches the destination, the process is repeated for return traffic, just as a road trip usually involves heading back to the point of origin.

Next time, we will take a look at the various methods used for specifically accomplishing the forwarding process.

–          Joe