2. Discussion

It may be useful to subdivide autonomous systems with a very large number of BGP speakers into smaller domains for purposes of controlling routing policy via information contained in the BGP AS_PATH attribute. For example, one may choose to consider all BGP speakers in a geographic region as a single entity.

In addition to potential improvements in routing policy control, if techniques such as those presented here or in [RFC4456] are not employed, [BGP-4] requires BGP speakers in the same autonomous system to establish a full mesh of TCP connections among all speakers for the purpose of exchanging exterior routing information. In autonomous systems, the number of intra-domain connections that need to be maintained by each border router can become significant.

Subdividing a large autonomous system allows a significant reduction in the total number of intra-domain BGP connections, as the connectivity requirements simplify to the model used for inter-domain connections.

Unfortunately, subdividing an autonomous system may increase the complexity of routing policy based on AS_PATH information for all members of the Internet. Additionally, this division increases the maintenance overhead of coordinating external peering when the internal topology of this collection of autonomous systems is modified.

Therefore, division of an autonomous system into separate systems may adversely affect optimal routing of packets through the Internet.

However, there is usually no need to expose the internal topology of this divided autonomous system, which means it is possible to regard a collection of autonomous systems under a common administration as a single entity or autonomous system, when viewed from outside the confines of the confederation of autonomous systems itself.