Welcome to the introduction to routers. Here we will analyse routers in quite some depth about what they do and how they work. We must point out that knowledge on the OSI model is recommended, plus understanding of how data is sent across the network medium. If you find the information a bit too confusing or don’t quite understand it, I suggest you go back to the others articles and do some reading on the OSI model and protocols.

What are they and what do they do?

Routers are very common today in every network area; this is mainly because every network these days connect to some other network, whether it’s the Internet or some other remote site. Routers get their name from what they do, which is to route data from one network to another.

For example, if you have a company that has an office in New York and another one in Chicago, then to connect the two sites you would use a leased line/WAN connection, for which you need to connect a router at each end. Any traffic which needs to travel from one site to another will be routed via the routers, while all the other unnecessary traffic is filtered (blocked), thus saving you valuable bandwidth and money.

So, you can say “A router is a device which makes communication possible between two or more different networks present in different geographical locations.”

You guys might be thinking, “To make communication possible between two or more different networks present in the same geographical location, what are the steps to be taken?”

Answer: Configuring a Software Router or Implementing the CIDR concept

There are two types of routers:

  1. Hardware routers
  2. Software routers.

So what’s the difference?

When people talk about routers, they usually don’t use the terms “hardware” or “software” router but we are using them for the purpose of distinguishing between the two.

Hardware routers are dedicated hardware that runs special software created by their vendors to give them the routing capabilities, plus a whole lot more functions. Hardware routers are most common amongst companies as they are faster and more reliable. In the early days, hardware routers were expensive; however, their prices today are extremely low for cheaper-brand models.

The picture below shows a Cisco 1700, 2600 series modular router that offers a lot more than simple routing capabilities

Software routers perform similar tasks as the above hardware routers (routing data), but they don’t come in small flashy boxes. A software router could be a Windows, Linux, or Novell NetWare server. All network servers have built-in routing capabilities.

Most people use them for Internet gateways and firewalls but there is one big difference between the hardware and software routers. You cannot (in most cases) simply replace the hardware router with a software router. Why? Simply because the hardware router has the necessary hardware built in to allow it to connect to the special WAN link (frame relay, ISDN, ATM etc), where your software router (e.g., a NT server) would have a few network cards, one of which connects to the LAN and the other goes to the WAN via the hardware router.

Many companies are manufacturing routers:

  • * Cisco
  • * Juniper
  • * Dlink
  • * Alcatel
  • * Nortel
  • * Multicom
  • * Cyclades
  • * Linksys
  • * 3Com

Fig. 2 (source: http://seekingalpha.com/article/109717-sector-overview-networking)

In the above figure, it is clear that Cisco has a monopoly (above 51% share) in the routers market.

Cisco divided the router into three layers

  • * Access layer router
  • * Distribution layer router
  • * Core layer router

Access layer routers are used by small organizations and are also known as desktop or company layer routers. Examples are router Series 800, 1000, 1600, 1700, and 2500.

Distribution layer routers are used by ISPs and are also known as ISP layer routers. Examples are Router Series 2600, 3200, 3600, and 3700.

Core layer routers are used by global ISPs and are also known as backbone routers. Examples are router series 6400, 7200, 7300, 7400, 7500, 7600, 10000, and 12000.

Fig. 3 explains the router’s booting sequence; please take a close look at that.

Now, we are going to take a closer look at the router’s hardware.

Serial port:

  • * It is known as a remote administrative port.
  • * It is generally used for WAN link establishment.
  • * A serial cable is used for this port with 60 pin/26 pin (smart serial).

Console port:

  • * It is known as a local administrative port
  • * It is generally used for initial configuration, password recovery and local administration of the router. It is an RJ45 port.
  • * It is the most delicate port on the router, so you should make less use of the console port.

Auxiliary port:

  • * It is known as a remote administrative port.
  • * It is used for remote administration.
  • * It’s an RJ-45 port.
  • * A console or a rollover cable is to be used.

Internal Components:

  • * ROM—A bootstrap program is located here. It is same as the BIOS of the PC. The current version of the bootstrap program is 15.3.
  • * Flash—Internetwork Operating System (IOS) developed by Cisco is stored here. IOS is command line interface.
  • * NVRAM—Non-volatile RAM, similar to hard disk. It is also known as permanent storage or startup configuration. Generally, the size of NVRAM is 32 KB.
  • * RAM—It is also known as temporary storage or running configuration. Minimum size of RAM is 2MB. The size of RAM is greater than NVRAM in the router.
  • * Processor: speed and capability depends on models.

In the table below, router modes at many levels are described:

In the example shown in Fig.6, the workstations see the router as their “gateway.” This means that any machine on this LAN that wants to send a packet (data) to the Internet or anywhere outside its local area network (LAN) will send the packet via the gateway. The router (gateway) will know where it needs to send it from there on so it can arrive at its destination.

This explains the reason you need to add an internet protocol (IP) number for a gateway, when you have a LAN at home or in the office, in your TCP/IP network properties on your windows workstation.

The above figure shows only one example of how routers connect so the LAN gets Internet access. Let’s have a look how two offices, one in New York and another one in Chicago, would use routers to connect them.

The routers in the above picture connect using a particular WAN protocol, e.g. frame relay.

In reality, there would be a cable (provided by your service provider) which connects to the WAN interface of the router and from there the signal goes straight to your service provider’s network and eventually ends up at the other router’s WAN interface.

Depending on the type of router you get, it will support one of the most commonly used WAN protocols: ISDN, frame relay, ATM, HDLC, and PPP. We will discuss these protocols in other articles or you can find some resources by other writer too.

It’s important to note down and remember a few of the main features of a router:

  • * Routers are Layer 3 devices.
  • * Routers will not propagate broadcasts, unless they are programmed.
  • * Most serious routers have their own operating system.

Routers use special protocols between them to exchange information about each other (not data). This concludes our brief introduction to routers and its characteristics.

If you want to know more about Cisco router’s protocols and technology behind IGP, EGP routing specifically, as they are the most popular and preferred router devices in the world, then please visit other articles on the resource page or keep visiting the Intense School resource page to get more on that. I hope this article is informative for you; please use the below comment box to get in touch with me. Readers’ feedback and suggestion is always a valuable asset for any writers, so I am waiting for yours.


  • * Cisco Routers for the Desperate, 2nd Edition by Michael W. Lucas
  • * Cisco CCNA Simplified – By Paul Browning
  • * Cisco Field Manual: Router Configuration – By David Hucaby and Steve McQuerry
  • * Guide to Cisco Certified Network Associate certification by Todd Lamlee
  • * Guide to Cisco Certified Network Associate by Richard Deal.