You have several choices, depending upon how you connect to your ISP and how much you trust your ISP. For example, can you depend upon the link level protocol to detect that a link to your ISP is down? A true point-to-point T1 or frame relay with end-to-end keepalives means you don't have to run BGP just to detect link outages. However, if your ISP is not a first tier ISP, you might want to run BGP anyway (using a private ASN, no registration or fees required) and accept only routes to a couple of prefixes which indicate that your ISP has upstream connectivity. You can then use those subnets to trigger your default route. That way, if a POP loses connectivity with the ISP but not with you, you won't keep sending packets into a black hole.
If you cannot depend on the link level protocol to detect link outages, you will need to run BGP in order to detect link down. You can use the same approach as above or just accept a default route from your ISP.
Don't even think of taking full routes with only a 26xx router, but you don't need to in order to get the 99.9% of the availability benefits of running BGP.
You should also run an IGP such as OSPF or EIGRP between your 26xx's so that IBGP will know how to get between the two routers and each router can learn whether or not the other has a path to the Internet. This is optional if there is only one path between the two routers, but you might as well get a head start on future expansion.
These scenarios and more are discussed in a brief white paper on my web site (http://www.networkingunlimited.com/white008.html). But for all the gory details you'll need to read Chapter 8 of my book (the configuration examples in my book are posted on my web site).
Keep in mind that for maximum availability you will need to work with your ISP. Also keep in mind that 100% availability is impossible to achieve, all you can do is add more nines after the decimal point. And each nine is an order of magnitude more expensive.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
