BGP Implementation Choices
The final decision is determining what routes to accept from each ISP. This can range from merely accepting a default route (used to detect whether the link is up or down) to accepting all routes (called "running defaultless"). The former is usually insufficient, because it doesn't protect you from an ISP that has an internal failure cutting them off from the rest of the Internet. The latter requires using expensive "carrier-class" routers with lots of memory installed. Fortunately, there are some in-between choices.
Rather than using a simple default route, you can use a conditional default route to protect against ISP failure behind the ISP's router that serves your site. A conditional default route is a default route that's defined by a router only if a specific address is already in that router's routing table. Each ISP is used for a default route only if it advertises one or more routes that indicate that it's receiving advertisements from the rest of the Internet. That way, you'll always use a default route that promises to be useful.
Another option is to have the ISP send you just its local routes. That way, you can optimize your outbound routing to avoid sending packets that could be locally delivered to the wrong ISP, adding to delivery delays. Care must be taken when using this option, however, because some ISPs have so many local routes that there's no cost benefit in the size of the routers required to handle them, compared to running defaultless.
Options can also be combined. In some cases, taking local routes and a conditional default route can provide almost all the availability benefits of running defaultless, while still allowing the use of low-cost routers. More details and example configurations for all the approaches discussed can be found in Chapter 8 of High Availability Networking with Cisco (Addison-Wesley, 2001, ISBN 0-201-70455-2). As is always the case in networking, a good understanding of the requirements and the available capabilities is essential to maximizing cost effectiveness.