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