Sean

There are a number of reasons to run BGP. (I'm assuming we are talking about running BGP with an ISP for internet access.)

To take your original point. Lets say you have one connection to your ISP and your ISP allocates you a public range of addressing for servers that you want people to access from the Internet.

Do you need BGP for this. No you don't. You can have a default-route pointing to the upstream ISP router for all internal access to the Internet and the ISP will be advertising the addresses they have allocated to you, (as part of a larger range), to the rest of the Internet. So BGP is largely redudant in this situation.

However where BGP would be useful is if you have an independent IP address space and you are responsible for advertising this addressing to the rest of the internet.

"I would imagine that if service from ISP A went down, no one would know how to get to my web server unless I were running BGP and exchanging routes to my public IP to the ISP (and the Internet). Thus, traffic to my web server would get rerouted through ISP B to reach my web server."

Well yes and no. If you are using addressing from ISP A to address your web server it is unlikely you can also get ISP B to advertise this addressing also. So if your connection to ISP A goes down so does access to your web server. Hence the reason for using independent IP addressing and advertising to both ISPs.

Jon

Jon,

I think I understand your reply. What may be missing from my understanding is related to public IP addressing.

You used the term "independent IP address". Is it possible to obtain public IP address from an entity other than the ISP? Would I be able to purchase a range from the IANA?

If so, I think that is where my gap in understanding is coming from. I had always assumed the ISP handed out public IPs in all situations.

But if I can purchase my own addresses, then i can see why BGP would be help. I would be responsible for advertising my addresses to the ISP (or the Internet if I wanted to be an "Internet router") so that people could find me on the Intenet.

Am I close?

"Am I correct?"

Not exactly. If you had a public IP, both ISPs could advertize it for you.

However, you're on the right track with dual homing. Using BGP with your ISPs, you can selectively advertize what you want to each ISP (within certain constraints) and you can chose what appears to be the best outbound path to reach other destinations.

When single homed, there's no better path choice possiblitity.

As an aside, Cisco's later IOSs support a new technology known as OER (v1) or PfR (v2). This technology can find best performing multi-home Internet path, and direct traffic to it, and can do it with or without Internet routes.