Bilateral and Multilateral Peering: What’s the Difference?

By Rob Parker, Interconnection Director

These two types of peering relationship may seem similar at first glance, but there are a few considerations you should make to get the most out of your connection to an IX.

When you join an internet exchange, the first thing you’ll need to do after physically connecting is establish some BGP sessions in order to get traffic flowing and to start enjoying the benefits of peering at an IX. There are two types of peering at internet exchanges and although from a BGP configuration point of view there’s very little difference, from an operational point of view there are a few considerations to make.

In this blog, we’ll examine the difference between multilateral and bilateral peering, and how to leverage both for your business.

Multilateral or bilateral?

In a traditional peering relationship, you would identify peers at the internet exchange and reach out to them—likely via contacts listed in PeeringDB—and request a peering session with them. This is bilateral peering.

Peers might have specific requirements before establishing a session, such as traffic volumes, geographic scope of network operations, or some other logic specific to their organization. You need to follow this process for each peer you’d like to exchange traffic with and configure a BGP session for each of them upon their acceptance of your request – but this can quickly become cumbersome and time consuming both initially and over time.

Bilateral peering is ideal for peers you exchange high-value traffic with and where you want to ensure complete control over traffic, but for the “long tail” of smaller peers at the internet exchange, you may find that there are too many to reach out to or maintain configuration with. This is where multilateral peering is ideal.

In summary:

• A multilateral session is one BGP session with many networks facilitated by a route server.
• A bilateral session is one BGP session with one other network (so you’d need one bilateral session for each network you intend to peer with).

Optimize network performance by making sure you and your peers are locally connected.

The benefits of route servers

Because it can be a lot of work arranging and configuring BGP sessions with many (perhaps hundreds of) networks, almost all internet exchanges today offer a route server service that’s configured for you upon joining the internet exchange.

You’ll be able to establish a BGP session to each of the route servers offered (typically that’s four sessions in total: Two each for IPv4 and IPv6, to two diverse route servers to ensure high availability) and in return you’ll receive all prefixes advertised to the route servers by other members who are also using them.

This is known as a multilateral peering relationship – instead of peering with each peer over a bilateral session, you each peer with the route server in a multilateral relationship.

This lets you immediately get traffic flowing across the internet exchange with very little effort – no need to reach out to each peer, just bring up the route server and start enjoying all the benefits of the internet exchange right away. Over time, you can reach out to specific peers that are of high importance to you and arrange bilateral sessions.

It’s important to note that although almost all IX peers will be connected to the route server, not all are, and so there are still some networks you will need to manually request bilateral sessions with. At no time does any of your traffic flow through the route server – it simply acts as a directory for routes and distributes them to you. The traffic still flows between you and your peers directly across the IX.

The looking glass

To aid you in troubleshooting, looking glasses are offered by IXs to allow you to view the route server routing table from the perspective of the route server itself. This includes who’s peering with the route server along with all routes being advertised and received both from your network and other peers.

The looking glass, combined with your flow data, is an ideal way to ensure you’re getting the most out of your connection to the internet exchange.

BGP communities and the route server

You may also wish to retain a little more control of your routes when using the route server - this is possible by using control communities on your advertisements to inform the route server how to handle redistribution of your prefixes to other peers. You can dig into this topic in detail on our documentation page.

Other internet exchanges will have their own list of communities, but they’re relatively standard thanks to the efforts of Euro-IX. These communities allow you to specify certain peers that should not receive certain advertisements, for example, or to adjust preferences or pad your AS path.

Security

Route servers were often not the most secure service from the point of view of route security, but this is a thing of the past in most internet exchanges today.

Strict filtering is in place on every route server session at every MegaIX—filtering by max prefix count, AS-SET, AS path, and RPKI—to ensure that only validated prefixes are originated by the owner of the prefix and that route leaks via the route server are not possible. This is typically more strict filtering than many peers will offer via bilateral sessions, so there is an argument that in some cases the route server may be a better option than bilateral sessions.

Reliability is covered by two route servers located at physically diverse locations, such that an outage or issue with one route server won’t impact your traffic across the IX.

Conclusion

Both bilateral and multilateral sessions have their advantages and trade-offs, but upon joining an internet exchange we highly recommend making use of the route servers to get up and running quickly and easily—and securely—and then over time, identify high-value traffic flows and establish bilateral sessions with those peers. It’s also worth checking which peers do not use the route servers and reaching out to them to establish bilateral sessions as soon as possible.