Sizing routers

Given all the different router models on the market today, and that most organizations still only use a T1 or two, or less, for their WAN connections, how do you decide which model of router to deploy?

Just a few years ago, when router processors were still pretty skimpy (usually under 100 MHz), some of the most popular enterprise models weren't actually capable of routing fast enough to fill up that 1.544 Mbps T1 circuit. Then, it made sense to spend a little extra to buy a faster router to really make the best of what was very expensive bandwidth, but those days are long gone. Any current model of router should be able to support multiple T1 circuits, and probably at least a 10 Mbps Ethernet link or two at "full line rate". Especially now that most traffic actually bypasses the CPU, with technologies like Cisco's CEF, and "Fast Switching" modes, throughput isn't really an issue until you start talking about T3s and one or more Fast Ethernet links, which brings us to our next consideration.

One of the primary reasons for choosing a router is its speeds and feeds. How many and what types of interfaces does it support? This is pretty simple to answer if all we're talking about is common WAN ports like serial, HSSI, ATM/DS3, and ISDN, but keep in mind that there may be other interfaces you'll want to connect in the future. These might include FXS or FXO ports or VoIP gateway modules for IP telephony.

When looking for a router to support special

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interfaces like this, be sure to consider the manufacturer's recommendations. Just because a low-end model accepts the interface card doesn't mean performance will be acceptable. Spending a little more up front can save you a lot of those "the network is slow" troubleshooting calls later.

Finally, even though the CPU isn't as critical for passing packets anymore, you should consider it and the maximum memory available because of special software you may want to run on the router. From firewalls and intrusion detection software to H.323 gatekeepers, to dial VPN concentrator features, running specialized applications on routers is becoming increasingly popular. So when you size your router, be sure to think about how much memory you'll need as well.

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.

This was first published in June 2004

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