RFC 2270 (rfc2270) - Page 2 of 6

Using a Dedicated AS for Sites Homed to a Single Provider

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