6

Routing Policy

Routing policy is considered by many to be one of the most complex concepts to grasp when designing an IP network. In fact, many engineers find it scary. Implementing policy is not inherently complex or difficult. However, policies are often used to enumerate exceptionally complex ideas and, therefore, it can be exceptionally tricky to write policy that consistently and accurately implements those complex requirements. The way in which you write your policy can dramatically impact the scalability of your network. Wellwritten policy is easy to understand, easy to update accurately and easy to troubleshoot. Poorly written policy is often none of those things. In general, the simpler the policy, the better. However, it is not always possible to create functional policy that can be implemented simply or elegantly.

This chapter will look at what policy is used for, how it can be implemented and some of the features of the main vendors’ syntaxes, which can be utilized to improve the overall scalability of your network.

6.1 WHAT IS POLICY FOR?

The whole point of policy is to be able to influence the way in which your network devices advertise and accept prefixes and thus the way in which they forward traffic. However, this is just the technical element of what policy is for. In the broader sense, policy allows you to implement your business aims.

It is clearly important that you operate your network in such a way to meet the aims of your company’s business plan. Otherwise, you are effectively working against the rest of your company! Routing policy provides you with the means to decide which paths are

Designing and Developing Scalable IP Networks G. Davies

2004 Guy Davies ISBN: 0-470-86739-6