Assuming you already have a functional network, there are many reasons why you may want to consider purchasing a new router. Obviously, scalability is one reason. You may have run out of Ethernet ports on your router. Your router may be near its shelf life (perhaps it has been serviced too frequently and is off of warranty and/or support), and you know it is time for a newer model. You may have a new requirement to support a different protocol than you have been using - such as RIP2, BGP, IGMP, HSRP, IGRP or OSPF - and perhaps your existing router does not support this protocol. You might want to implement VLANs into your network. Perhaps you need to work with spanning tree. You may also have a new requirement to provide better failover and redundancy that your existing router just does not provide. Perhaps you want an integrated VPN solution to be part of your router, without having to rely on a firewall. (There are pros and cons of doing it this way, but clearly if there are budget constraints, an integrated packet filtering router that also provides VPN services for remote client connectivity may really help you.)
Wireless is another area to consider. Many routers, particularly home-based solutions, offer integrated routers based on wireless solutions, though I would personally recommend a separate access point for your business when doing wireless - one which is optimized for security. Though it clearly works well for home use (and I use one myself at home), I would certainly not purchase a Linksys router/switch/wireless access point for my business.
Support: Some things to consider
When upgrading to a new router, you must make sure that you understand the type of support that you will be getting from your vendor. If you try to go with a low-priced router, it may not have the technical staff to help you deal with the problems that will develop. One must also consider the platforms that WAN engineers typically use, as it will be easier to support your environment with industry-standard products that are fully utilized by the majority of companies, small and large.
For both the enterprise and SMB, it is important to choose a manufacturer that cares enough about their products to educate engineers who are responsible for deploying their solutions (and not just box pushers). For example, Cisco network engineers typically will try to get a CCNA or (if they are really amazing) CCIE certification from Cisco, which showcases their skill sets to employers. Nortel offers an NCA at their highest level, which represents a highly advanced level of technical design and analytical expertise for complex Nortel Networks solutions, also widely acknowledged throughout the industry and regarded a symbol of excellence. Nortel has the greatest telephony experience, having grown up in that world, so I would seriously consider using their infrastructure if you are looking for strong VOIP integration. 3Com also offers certification in their technologies. Their router cert is the 3Com Certified WAN Specialist, which demonstrates skills in designing and implementing 3Com WAN solutions, working with 3Com routers and protocols including OSPF and BGP4.
Product lines: What to expect
Let's look at what a few of the major router vendors offer from their product lines.
For SMBs, Cisco has the 1800 Series, which provides WLAN capabilities along with advanced with advanced security services and management features such as hardware encryption acceleration, IPSec VPN (AES, 3DES, DES), firewall protection, inline intrusion prevention, Network Admission Control, and URL filtering support to allow their smaller customers to implement resilient, scaleable solutions. Cisco helps its clients to optimize networks via a range of products and services, which they offer through their dealer channel. Through their Cisco Registered Partner program, they ensure that their partners are certified annually and have the knowledge and information at their disposal to fully support their customers. 3Com and Nortel also offer similar services. In addition to their SMB routers, Cisco also offers enterprise-wide models, including the 7600 series, their high-end model which offers integrated, high-density Ethernet switching, carrier-class IP/MPLS routing, and 10-Gbps interfaces.
3Com also has a strong product line, and I am particularly impressed with their 6000 series model, as it has every feature you can possibly think of, including fault tolerance and advanced traffic management and control features. I have deployed 3Com-only switch and routing infrastructures with great success in the past.
On the Nortel front, they have teamed with Microsoft to support Microsoft's Network Access Protection (NAP), an extensible standards-based technology that allows users to more securely access their corporate networks and reduce the complexity of network access for IT administrators. They are also working together with other industry-leading security companies to develop industry standards, network designs and products intended to secure critical information by protecting the communications infrastructure as well as user computing devices like desktop and laptop computers. This partnership is important, because despite the fact that many WAN folks hate Microsoft, those people will still need to deal with Microsoft on the PC client side.
Regarding Nortel's product line, I'm impressed with the Nortel Multiprotocol Router 5430, which is being marketed to remote offices that have outgrown smaller branch office routers. It can support concurrent, compute-intensive applications such as IP Quality of Service, IP multicast, compression and VPNs. It also has support for multiple WAN technologies -- including ATM T1/E1 and T3/E3, frame relay, PPP and ISDN.
Cisco has its own propriety product called the Cisco Systems' Network Admission Control (NAC), which will also carefully analyze any PC that wants to attach to your network, to check for the presence and status of antivirus and personal firewall software and report on the configuration of the machine. I feel Nortel has the advantage here, because most desktop clients are Microsoft, and it is to their advantage that unlike Cisco, they have chosen to partner with them, and not develop their own proprietary standards.
In conclusion, I will reiterate that when looking to purchase new routers for the enterprise, one must look for VPN capability, mulit-protocal support, integration with other networks, security enhancements (I.E (3-DES encryption) and direct vendor support. Though I would not hesitate to price out alternate solutions, I prefer industry-standard products for the enterprise.