Unlike some devices, routers require quite a bit of configuration before they will be useful. In this tip, we'll look at the bare minimum tasks to get a Cisco router working. Note that there are a lot of differences between the old school "routers" like the venerable 2500, 2600, 3600 series, and the newer "layer 3 switches" models like the 3550 or 2948G-L3. However, the tasks, as far as configuring the routing function, are the same.
First, you need to configure the interfaces. You'll need to configure at least two, or else you probably didn't need to buy a router. In most simple networks, one interface will be a LAN interface (Ethernet) and the other a WAN interface (T1/E1, ISDN, etc.)
Each interface needs an IP address and subnet mask (assuming you're doing IP), and the "no shutdown" command. WAN interfaces will also probably require you to set the encapsulation (e.g. Frame Relay, or HDLC or PPP) or other interface-specific information, like SPIDs for ISDN or DLCI for Frame-Relay or PVC for ATM. You should also configure the "description" for each interface, although this is optional.
At this point, the router should be capable of routing traffic from one interface to another, but you will probably want to route beyond that, which requires either "static routes" or the use of a routing protocol, such as RIP or OSPF or BGP. In a small network, it may be sufficient to have a single "default route" pointing toward your WAN interface. You should use a routing protocol if you have more than a handful of routers, or if you have multiple paths between locations and either want to load-balance or have redundancy.
Although you now have the bare minimum, you should also configure login and enable passwords and turn off any features you don't need (e.g. the Web server interface and SNMP). In my next R&S tip, I'll cover several standard security commands that should be applied to most routers. In the meantime, this article from Cisco's Packet magazine is a good bit of reading to help you understand the fundamentals.
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.