Network Considerations for a Network Which Does Not Support 4-Octet AS
Source: Cisco Systems
This paper will assume that an 'OLD' BGP speaker is supporting only a 2-octet AS number, and that that a BGP speaker that supports the new 4-octet AS number is a 'NEW' BGP speaker. RFC4893 (BGP Support for Four-octet AS Number Space) describes the compatibility between OLD and NEW BGP speakers and allows a gradual transition from a 2-octet supporting router, towards 4-octet supporting router. Even though there is a high degree of compatibility, it is desirable to understand some specific considerations when operating OLD BGP speakers in a world where there may be either directly connected NEW BGP speakers or BGP prefixes containing 4-octet Autonomous System numbers.