7. Transition

When an Autonomous System is using a two-octet AS number, then the BGP speakers within that Autonomous System MAY be upgraded to support the four-octet AS number extensions on a piecemeal basis. There is no requirement for a coordinated upgrade of the four-octet AS number capability in this case. However, if an Autonomous System wishes to use a four-octet AS number as its own AS number, then this document assumes that an Autonomous System can use a four-octet AS number only after all the BGP speakers within that Autonomous System have been upgraded to support four-octet AS numbers.

A non-mappable four-octet AS number cannot be used as a "Member AS Number" of a BGP Confederation until all the BGP speakers within the Confederation have transitioned to support four-octet AS numbers.

In an environment where an Autonomous System that has OLD BGP speakers peers with two or more Autonomous Systems that have NEW BGP speakers and use AS_TRANS (rather than having a globally unique mappable AS number), the use of the MULTI_EXIT_DISC attribute [RFC4271] by the Autonomous System with the OLD BGP speakers may result in a situation where the MULTI_EXIT_DISC attribute will influence route selection among the routes that were received from different neighboring Autonomous Systems.

Under certain conditions, it may not be possible to reconstruct all of the AS path information from the AS_PATH and the AS4_PATH attributes of a route. This occurs when two or more routes that carry the AS4_PATH attribute are aggregated by an OLD BGP speaker, and the AS4_PATH attribute of at least one of these routes carries at least one four-octet AS number (as opposed to a two-octet AS number that is encoded in 4 octets). When such aggregation results in creating a route that is less specific than any of the component routes (routes whose Network Layer Reachability Information (NLRI) covers the NLRI of all the component routes), loss of the AS path information does not create the risk of a routing loop. In all other cases, loss of the AS path information does create the risk of a routing loop.