Like much in life, there are levels of routing and switching that can be accomplished with minimal effort to establish simple connectivity, and then there are more advanced levels, achieved through careful tuning and consideration that result in a work of art. If designing optimized networks is something you aspire to, then one of the most important things you can do is put some thought into how you want traffic to flow through the network. To do that, consider some of these questions, which you can use as part of a checklist when you design a network:
Do layer 2 and layer 3 match? Be careful to configure the network so that Spanning-Tree and your routing protocol compliment each other instead of conflict.
Is routing synchronous? That is, if you have multiple paths, does traffic flow in and out the same path? or does it go out one and come back in on the other? Sometimes it doesn't matter, other times it's critical, but mostly, it's part of load-balancing which in turn is an important part of tuning your network.
Is traffic taking the most optimized path inside your hardware? Many manufacturers deploy proprietary features that can greatly improve performance by making forwarding decisions in the line cards instead of the CPU on a router or switch. This may or may not be default behavior, and even if you enable it, configuring certain other features can disable it without warning you.
If you have multiple exit points in your network, is traffic taking the best one? This can be particularly tricky, especially if your network is global.
Are you using the best topology that fits your budget? Traffic between spokes on a hub, particularly common in Frame-Relay networks, is usually suboptimal.
Can your network benefit from prioritizing traffic or doing traffic engineering? Now that support for Policy-Based-Routing has matured considerably in most hardware platforms, splitting traffic by application (e.g. VoIP) for specific circuits is now a possibility. It can save a lot of money for some organizations.
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.
