One final way of controlling the traffic outbound from your network is to accept only those routes from each provider that are directly attached to them and use a default route to reach the rest of the network in the Internet. In other words, Router A would accept only routes announced from ISP A that belong to it and its customers.
The trick, in this case, is to effectively filter the routes out that do not belong to your provider or their customers. There are two ways to achieve the same result: the easy way and the not-so-easy way.
• The Easy Way— Ask your providers only to advertise to you their routes and their customer's routes. Any provider will be glad to comply.
A variation involves asking your provider to set a community on their routes and their customer's routes. All you have to do is filter out all the routes that do not have the agreed upon community marking.
Your choice, along with the use of local preference, will guarantee the shortest path to the destinations received.
• The Not-So-Easy Way— Set up a filter to accept only routes with an AS_PATH length of 1 or 2. The value of 1 will identify your provider's routes, whereas the value of 2 will identify their customer's routes. This might work out well enough, but you will leave out any prefix on which the AS_PATH is prepended.
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