BGP

Chapter 11. Providing Transit Services

Being a multihomed ISP with BGP-speaking customers isn’t very different from just being a multihomed network in most regards, but some things need extra attention. A regular multihomed network sends out just its own routes, which makes for easy outbound filtering: allow out the BGP announcements you’re trying to send, and nothing else. When you’re providing transit services, it gets a bit more complex. You can, of course, filter out any announcements that don’t fit your idea of how your customers should announce their routes. That way, however, you don’t leave any room for traffic engineering or BGP-based anti-DoS measures. A better solution is to accept all reasonable announcements. You should also have a solution for customers with two connections to your network. And expect customers to come to you with questions about IP multicast and IPv6.

The examples in this chapter all use AS 40077 as the local AS number with different customer and upstream ISP connections in each example.

Route Filters

Outbound route filters to upstream networks (peers and transit ISPs) are important to make sure you send only the routes you want to source and those of your customers to your peers and upstream ISPs. You need two types of outbound route filters: AS path and prefix filters. Having both prefix and AS path filters is redundant in theory, but in practice there are many ways in which faulty routes can escape one check, so they must be filtered out by the other. ...