RFC 2270 (rfc2270) - Page 2 of 6
Using a Dedicated AS for Sites Homed to a Single Provider
Alternative Format: Original Text Document
RFC 2270 Dedicated AS January 1998 Consider the scenario outlined in Figure 1 below. +-------+ +-------+ +----+ | | | +------+ | | ISP A +------+ ISP B | | Cust.+---+ | | | | | X +--------+ | | | +------+ ++-----++\ +-------+ | | \ | | \ +--------+ ++-----++ +-| | | Cust. | | ISP C | | Y | | | +-------+ +--------+ Figure 1: Customers multi-home to a single provider Here both customer X and customer Y are multi-homed to a single provider, ISP A. Because these multiple connections are "localized" between the ISP A and its customers, the rest of the routing system (ISP B and ISP C in this case) doesn't need to see routing information for a single multi-homed customer any differently than a singly-homed customer as it has the same routing policy as ISP A relative to ISP B and ISP C. In other words, with respect to the rest of the Internet routing system the organization is singly-homed, so the complexity of the multiple connections is not relevant in a global sense. Autonomous System Numbers (AS) are identifiers used in routing protocols and are needed by routing domains as part of the global routing system. However, as [4] correctly outlines, organizations with the same routing policy as their upstream provider do not need an AS. Despite this fact, a problem exists in that many ISPs can only support the load-sharing and reliability requirements of a multi- homed customer if that customer exchanges routing information using BGP-4 which does require an AS as part of the protocol. 2) Singly-homed customers requiring dynamic advertisement of NLRI's While this is not a common case as static routing is generally used for this purpose, if a large amount of NLRI's need to be advertised from the customer to the ISP it is often administratively easier for these prefixes to be advertised using a dynamic routing protocol. Today, the only exterior gateway protocol (EGP) that is able to do this is BGP. This leads to the same problem outlined in condition 1 above. Stewart, et. al. Informational
