What is the process for configuring an outbound BGP routing policy?

1 BGP Basics

BGP is a path-vector protocol that uses TCP as its transport layer. BGP peers, also called neighbors, establish sessions with each other and exchange routing updates. Each update contains a list of network prefixes and their attributes, such as AS path, next hop, local preference, and community. BGP peers can apply various filters and modifications to the updates they receive or send, based on their own policies and preferences. An outbound BGP routing policy is a set of rules that defines how a BGP peer modifies and filters the updates it sends to another peer.

2 Outbound Policy Components

An outbound BGP routing policy consists of three main components: match conditions, actions, and order. Match conditions are criteria that identify which routes or attributes the policy applies to. For example, you can match routes based on their prefix, AS path, community, or other attributes. Actions are operations that the policy performs on the matched routes or attributes. For example, you can modify the AS path, set the local preference, add or remove communities, or accept or reject the routes. Order is the sequence in which the policy evaluates the match conditions and executes the actions. For example, you can use a first-match or a best-match approach.

3 Outbound Policy Objectives

The objectives of an outbound BGP routing policy are based on the needs and requirements of your network. This policy can be used to influence how other ASes reach your network by manipulating the AS path or the MED attributes, control which routes you advertise to different peers with prefix or community filters, implement traffic engineering or load balancing by selectively announcing or withdrawing routes to different peers, and comply with contractual or regulatory obligations by restricting or modifying the routes you advertise to certain peers.

4 Outbound Policy Configuration

The configuration of an outbound BGP routing policy varies depending on the vendor and platform of your router, but the general steps involve defining the match conditions and actions as separate policy statements or clauses, combining them into a policy list or chain to specify the order of evaluation, applying the policy list or chain to the BGP neighbor or group in the outbound direction, and verifying and testing the policy by checking the BGP updates sent to the neighbor or group, as well as their routing tables.

5 Outbound Policy Examples

Outbound BGP routing policies can be used for various scenarios. For example, you can prepend your AS number three times to the AS path of all routes you advertise to a peer with a policy statement that matches all routes and adds your AS number to the AS path attribute. Alternatively, you can advertise only your own routes and the routes of your customers to a peer with a policy statement that matches routes based on their community values and rejects the routes that do not belong to your AS or your customers' ASes. Additionally, you can advertise a more specific route to a peer only when the less specific route is not available by using a policy statement that matches the more specific route and sets its local preference to a lower value than the less specific route.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?