Exchanging Routing Information Across Provider Boundaries in the CIDR Environment
RFC 1520

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Network Working Group                                         Y. Rekhter
Request for Comments: 1520        T.J. Watson Research Center, IBM Corp.
Category: Informational                                      C. Topolcic
                                                                    CNRI
                                                          September 1993

       Exchanging Routing Information Across Provider Boundaries
                        in the CIDR Environment

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

1.  Introduction

   Classless Inter-Domain Routing (CIDR) has been adopted as a solution
   to the scaling problem in the Internet. The overall CIDR architecture
   is described in [1]. The architecture for IP address assignment with
   CIDR is covered in [2] and [3]. The inter-domain routing protocols
   that are capable of supporting CIDR are covered in [4], [5], and [6].

   The purpose of this document is twofold. First, it describes various
   alternatives for exchanging inter-domain routing information across
   domain boundaries, where one of the peering domain is CIDR-capable
   and another is not.  Second, it addresses the implications of running
   CIDR-capable inter-domain routing protocols (e.g., BGP-4, IDRP) on
   intra-domain routing.

   This document is not intended to cover all the cases (either real or
   imaginable). Rather, it focuses on what are viewed to be the most
   common cases.  We expect that individual service providers will use
   this document as guidelines in establishing their specific
   operational plans for the transition to CIDR.

   The concepts of "network service provider" and "network service
   subscriber" were introduced in [3]. For the sake of brevity, we will
   use the term "provider"  or "service provider" here to mean either
   "network service provider" or "network service subscriber", since for
   the most part, the distinction is not important to this discussion.
   Furthermore, we use the same terms to refer to the network and to the
   organization that operates the network. We feel that the context
   makes it amply clear whether we are talking about hardware or people,
   and defining different terms would only make this paper harder to
   read.

Rekhter & Topolcic                                              [Page 1]
RFC 1520           CIDR Provider Information Exchange     September 1993

   This document defines a CIDR-capable provider as the provider that
   can perform correct IP packet forwarding (both internally and to
   other adjacent providers) when the inter-domain routing information
   acquired by the provider is expressed solely in terms of IP address
   prefixes (with no distinction between A/B/C class of addresses).

   This document defines CIDR-capable forwarding as the ability of a
   router to maintain its forwarding table and to perform correct
   forwarding of IP packets without making any assumptions about the
   class of IP addresses.

   This document defines CIDR reachability information as reachability
   information that may violate any assumptions about the class of IP
   addresses. For instance, a contiguous block of class C networks
   expressed as a single IP address prefix constitutes CIDR reachability
   information.

2.  Taxonomy of Service Providers

   For the purpose of this document we partition all service providers
   into the following categories, based on the type and volume of
   inter-domain routing information a provider needs to acquire in order
   to meet its service requirements:

      - Requirements imposed on a service provider preclude it from
        using Default inter-domain route(s) -- we'll refer to such a
        pqrovider as a Type 1 provider.

      - Requirements imposed on a service provider allow it to rely on
        using one or more Default routes for inter-domain routing, but
        this information must be supplemented by requiring the provider
        to acquire a large percentage of total Internet routing
        information -- we'll refer to such a provider as a Type 2
        provider.

      - Requirements imposed on a service provider allow it to rely on
        using one or more Default routes for inter-domain routing;
        however, to meet its service requirements the provider must
        supplement Default route(s) by acquiring a small percentage of
        total Internet routing information -- we'll refer to such a
        provider as a Type 3 provider.

      - Requirements imposed on a service provider allow it to rely
        solely on using one or more Default routes for inter-domain
        routing; no other inter-domain routing information need to be
        acquired -- we'll refer to such a provider as a Type 4 provider.

Rekhter & Topolcic                                              [Page 2]
RFC 1520           CIDR Provider Information Exchange     September 1993