1. Introduction

In the base BGP specification [RFC4271], the Autonomous System (AS) number is encoded as a two-octet entity. To prepare for the anticipated exhaustion of the two-octet AS numbers, this document describes extensions to BGP to carry the AS numbers as four-octet entities.

More specifically, this document defines a BGP capability code, "support for four-octet AS number capability", to be used by a BGP speaker to indicate its support for four-octet AS numbers. Two attributes, AS4_PATH and AS4_AGGREGATOR, are introduced that can be used to propagate four-octet-based AS path information across BGP speakers that do not support four-octet AS numbers. This document also specifies mechanisms for constructing the AS path information from the AS_PATH attribute and the AS4_PATH attribute.

The extensions specified in this document allow a gradual transition from two-octet AS numbers to four-octet AS numbers.

This document obsoletes RFC 4893 and updates RFC 4271. It includes several clarifications and editorial changes, and it specifies the error handling for the new attributes.