Earlier, we discussed how IP addresses and route authorizations work, before we took a break to talk about how the RIRs issue ASNs. As promised, I’ll now cover BGP route selection, how it enables anycasting, and how we can use it to achieve low latency and high availability. We’ll also cover some of the pitfalls of this approach and how it led to an infamous outage.
For those not familiar with the concept, anycasting means the same IP address is shared by devices in multiple locations, with routers sending packets to the “nearest” location. This can result in latency lower than that is possible with the speed of light limitation from a single location1. Although, as you will see later, the routers’ concept of “nearest” may not necessarily be what we expect.
Now, if one location stops announcing the IP address via BGP, then routers will select the next best location, enabling high availability as long as there is one location still available. Somewhat morbidly, I’ve claimed that this website will stay up even if Yellowstone erupts, which is theoretically true since my servers in Europe would still be able to serve traffic to the rest of the world even if every server in North America is down, but I’ve not tested this (and hope it will never be tested).
Side note: AS200351 turns one year old today! 🎂
As you may know, I am currently writing a series on BGP and how the Internet works, from my perspective as the operator of a small autonomous system, AS200351. While we haven’t really exhausted the theoretical material, I think I’ve covered enough to enable readers to set up their own basic autonomous system. Rather than forcing you to do your own research based on outdated and potentially incorrect information on the Internet, or allowing you to fall victim to scams, I think it would be wise to talk about the process of getting your own ASN.
For readers who haven’t read the previous parts of the series and are unfamiliar with why one might want an ASN, here’s a brief explanation:
An autonomous system (AS) is a constituent part of the Internet that can define its own routing to the remainder of the Internet, and ASes exchange routes with each other over Border Gateway Protocol (BGP) to form the Internet itself. By receiving a globally unique identifier, an AS number (ASN), which in my case is 200351, I can exchange routes over BGP with other ASes, announce my own IP addresses to the Internet, and control how traffic flows in and out of my network, as opposed to simply exchanging traffic from a default gateway to reach the Internet with an IP address assigned by my ISP. This comes with several advantages, such as being able to switch upstream ISPs at will (or when such an ISP fails) without changing my IP addresses or breaking a single connection; or to advertise the same IP addresses from multiple locations (anycasting) to allow users to reach my services with lower latency than otherwise permissible by the speed of light with automatic failover.
I will now share what I wish I knew when I impulsively decided to apply for an ASN at 3 a.m. on a cold December night last year, now that I’ve been doing this for a while. I’ll walk through the process as objectively and thoroughly as possible, demystifying the role of any player in this space. I would like you to go into this with full knowledge of the risks and a full understanding of where your money is going. In the end, I will offer some subjective suggestions on providers, but those can be ignored if you would rather do your own research.
In the first part of this series, we had a brief overview of what BGP is. Then last time, we dived into what autonomous systems (ASes) are and the relationships that can exist between them, as well as the existence of Tier 1 networks and Internet Exchange Points (IXPs). That provided a broad overview of the Internet’s structure.
However, so far in this series, we’ve talked about IP addresses—or really, IP prefixes and CIDRs—as if they are something that just exists. This mental model is no longer sufficient. Before we can truly understand routing, we must first understand how IP addresses really work—how they are assigned and who is authorized to announce them.
Last time, we introduced BGP as a protocol, but introduced the concept of autonomous systems (AS for short) and gave some examples. This time, we’ll dive deeper into the concept, which hopefully will give you a better understanding of the various types of ASes and how the Internet is organized on a global level.
But first, let’s start with how the numerical identifiers—the autonomous system numbers (ASNs)—work.
Border Gateway Protocol (often abbreviated BGP) is a critical protocol that makes the modern Internet possible, yet remains one of its most poorly understood parts even among its long-time users. At the same time, it has played a significant role in several high-profile outages on the Internet. As someone who has been running my “own piece of the Internet”—AS200351—for half a year now, I think the time has come to write a piece explaining exactly what BGP is, what AS200351 is, and how the Internet truly functions behind the scenes. We’ll start with the basics.
To understand BGP, we must first understand why it is called the “Internet” in the first place. To simplify greatly, the Internet is called that because it’s an interconnected network of networks (more precisely, autonomous systems, or ASes) glued together by BGP. Since this sounds like a nonsensical sequence of words, let’s dive a bit deeper.