Upgrading and Repairing Networks (5th Edition)

Most routers are similar to computers in that they run an "operating system." You can think of the router as a small computer that has only one basic function: Sort out where incoming packets need to go. To preserve your investment, most manufacturers store the router's operating system in nonvolatile memory that can be reprogrammed when updates or fixes are released. When the router is first powered up, it runs a small bootstrap program that is used to locate and load the operating system. Most routers also allow you to load the router OS using a trivial FTP (tftp) server. This type of FTP server doesn't require a password and shouldn't be used in an insecure environment. However, within a LAN, using a tftp server to download router OS data can make it easier to keep track of different versions of the OS and the routers that are configured to use them.

Cisco's OS for its main router line is called IOS, and it has been updated many, many times over the years and adapted to the newest equipment. Cisco's website can tell you which versions are available for the different routers it manufactures. If you deal heavily in Cisco equipment, a good website for you to bookmark is the Cisco documentation website. Although Cisco routers do ship with a CD that contains most of this documentation, you'll find up-to-date copies at these websites:

www.cisco.com/en/US/support/index.html

www.cisco.com/univercd/home/home.htm

Configuring a router usually begins after you take it out of the box and plug in everything. For each interface (that is, network connection), you need to configure the appropriate information. For example, for a 10BASE-T or Fast Ethernet port connecting to a LAN that uses TCP/IP, you must specify an IP address and subnet mask for the port.

There are also tasks you need to perform to configure the routing protocols that the router uses. The information you need for this depends, of course, on the routing protocols you plan to use.

Other configuration issues you need to address include access and security. If the router is being used as part of a firewall, or if there is a need within your network, you might have to set up access control lists to permit or deny network traffic from passing through any of the network interfaces.

One important thing to keep in mind is that the router's OS and the configuration information you enter are separate entities. Both the OS and the configuration file can be stored in nonvolatile memory (NVRAM), and both usually can be downloaded from a tftp server. However, the configuration file that is created when you initially set up a router should be saved in more than one place for backup purposes. Indeed, keeping a printed copy of the information contained in the configuration file can be a helpful troubleshooting tool.