BGP made simple: announcing your prefixes and controlling your routing

BGP (Border Gateway Protocol) is the routing protocol that holds the Internet together. It lets autonomous systems — the large networks that make up the Internet — exchange information about which paths to take to reach each block of IP addresses. Without BGP, no router would know how to reach a network it isn't directly connected to.

The principle: announcing paths

Each network announces to its neighbours the prefixes (blocks of IP addresses) it can reach, along with the AS path to get there. Hop by hop, this information propagates, and every router builds a view of the available routes. When several paths exist, BGP picks one according to a series of criteria — the shortest AS path being the best known, but not the only one.

Do I need to run BGP?

Running BGP "on your side" assumes two things: having your own AS number and a block of IP addresses (obtained from an RIR such as the RIPE NCC in Europe). It becomes relevant when you want to:

• Be multi-homed: announce your prefixes through several providers so you stay reachable if one fails.
• Control your routing: influence where your traffic enters and leaves, for performance or cost.
• Keep your IP addresses: retain the same block whatever the operator, with no renumbering.

"Announcing your prefixes", concretely

In practice you establish a BGP session with your operator (an adjacency between your routers), then you announce your prefixes to it. The operator in turn propagates them to the rest of the Internet, usually after filtering what you are allowed to announce — an essential security point to avoid route leaks and hijacks.

You can also fine-tune your announcements: artificially lengthen a path (AS-path prepending) to make a route less attractive, or use BGP communities to tell your operator how to handle your announcements.

BGP communities, an underrated tool

BGP communities are tags attached to your announcements that let you request a specific behaviour from the operator: don't export a prefix to a given neighbour, change its preference, trigger a blackhole during an attack… It is a powerful lever to steer your traffic without manual intervention.

At Connect-IX, our IPv4/IPv6 BGP sessions are provisioned quickly (including over GRE tunnels), and we publish the full list of our BGP communities. You can also view our routes in real time in the Looking Glass. For connectivity itself, see our IP transit offering.