Get IT Done: Easier routing management with BGP

Understand how Border Gateway Protocol (BGP) helps networks achieve redundancy

Border Gateway Protocol (BGP) is a routing protocol that enables you to fortify your network for redundancy. And despite what a lot of people would have you believe, a moderate-size organization can make a cost-effective case for implementing BGP. If your company is heavily dependent upon Internet connectivity, read on to learn about the benefits of this network upgrade.

What is BGP?
BGP is a routing protocol—a set of rules a router uses when sending information across a network. Other types of routing protocols include static routes, RIP, and OSPF. Most network administrators are at least familiar with static routes and RIP.

BGP is designed to handle data flow in a complex network. It does this by evaluating segments of the network as one “object.” Before we get confused, the network segments I’m talking about are not necessarily subnets. They may be a collection of unrelated subnets grouped by geographic region. Figure A shows an example of how this network might look.

Figure A
A complex network can consist of unrelated subnets.

This sample network consists of three groups, each with three subnets. Each subnet is identified by a letter from A to I. What does this configuration mean to BGP? Essentially, it means that we can look at each group of subnets as just that—a group of subnets—or we can look at each group as an individual entity. For example, when we aggregate the subnets A, B, and C as one entity, we have summarized that network. And in order to reach those subnets, we must use a gateway that sits on the border of that summarized network.

What’s the benefit?
The ability to group these networks and evaluate them as one entity makes it easier to manage the routing between the network groups. However, when we look at network groups as single entities, we can’t use IP to address them. The subnets are not necessarily related and don’t follow a particular pattern. More importantly, they may be from different IP providers, which can happen when two companies merge or when you buy your first T1 from UUNet and your second T1 from Cable & Wireless. (We’ll get to why you would do that later.) Simpler routing protocols are unable to route to the different IP networks if the subnets are aggregated, but as you’ll see in a moment, BGP offers an IP alternative that makes such routing possible.

Autonomous system numbers to the rescue
If not IP, then what? Good question. An autonomous system number (ASN) is the answer. An ASN tells routers using BGP what subnets are in the autonomous network. In other words, if a router wants to get to your subnet A, it looks up that subnet in a BGP routing table, finds the associated ASN, and routes accordingly.

This process sounds a lot like a DNS lookup. In fact, it’s not much different and the benefits are similar. Using a DNS name, you can move your Web site from provider to provider, changing your IP address every time, and the effect is transparent. You simply update the DNS record so that the world can find you. Using an ASN, you can add IP subnets from different providers into your ASN so that you can switch IP providers without destroying your routing. As with DSN, the effect is essentially transparent.

Flexibility and redundancy
At this point, we’ve covered the basics of BGP: You get an ASN and assign your IP subnets to it; routing then occurs based on the ASN rather then the IP. Why is that so great? Because an ASN is autonomous. A regular IP subnet is tied to a particular provider. For example, say you buy a T1 from Sprint and they assign you an IP subnet for your company. If you decide to leave Sprint and move to UUNet, you must give that entire subnet back to Sprint and get a new one from UUNet. Those IP subnets are synchronized to the providers’ networks. You can’t move them—unless you’re using BGP.

BGP, of course, applies to all networks, not just Internet networks. And like IP addresses, you can’t just make up an ASN to use on the Internet. The American Registry for Internet Numbers (ARIN) manages the process of assigning ASNs to organizations. When you register your subnets with ARIN and get an ASN, your registered subnets become autonomous from your provider. They are no longer tied to Sprint or UUNet. Instead, they are tied to you and your ASN.

Because your IP subnets are not locked into routing through a specific provider, you can now route your previous Sprint IP subnet across a UUNet T1, a Cable & Wireless T1, or anyone else that supports BGP. One of the most common uses for BGP is to route a subnet across multiple T1s from different providers. That way, your Internet connectivity from one provider can disappear for days, and the other provider will keep running your traffic. The odds that both connections will be down are much lower than those of one or the other going down on its own.

BGP can accomplish this because when your router runs the BGP protocol, it advertises to its neighboring routers that it’s alive and well. It also makes sure that the neighboring routers (and connections) are alive and well. Should one of the connections go down, one or both routers will know it, and traffic will flow through an alternate route using the ASN.

Setup and costs
To set up BGP, you need an ASN. And to get an ASN, you need Internet connections from two or more providers. You must also apply for an ASN from ARIN. ARIN charges a $500 one-time fee for the ASN assignment, and it takes a few weeks to process the registration. In addition to the ASN, you must have the BGP protocol on your router and enough memory to hold the routing tables. Most administrators generally specify at least 64 MB of memory for the routing tables, with a preference for at least 128 MB.

A Cisco 3600 series router is an ideal modular router for running BGP and runs around $15,000 with the necessary modules. A pair of T1s from different providers can vary quite a bit, although most organizations can find them for around $1,000 to $1,500 a month. Most Cisco administrators can handle the setup and installation without additional charges. A one-year total cost of ownership (TCO) is around $51,500—compared to a nonredundant TCO of about $21,500. Subsequent years’ TCO is around $18,000 (the cost of the second T1 line).
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