BGP 路由属性 BGP状态机

[Docs] [txt|pdf] [draft-ietf-idr-au...] [Diff1] [Diff2] BEST CURRENT PRACTICE Network Working Group J. Hawkinson Request for Comments: 1930 BBN Planet BCP: 6 T. Bates Category: Best Current Practice MCI March 1996 Guidelines for creation, selection, and registration of an Autonomous System (AS) Status of this Memo This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited. Abstract This memo discusses when it is appropriate to register and utilize an Autonomous System (AS), and lists criteria for such. ASes are the unit of routing policy in the modern world of exterior routing, and are specifically applicable to protocols like EGP (Exterior Gateway Protocol, now at historical status; see [EGP]), BGP (Border Gateway Protocol, the current de facto standard for inter-AS routing; see [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the Internet is expected to adopt when BGP becomes obsolete; see [IDRP]). It should be noted that the IDRP equivalent of an AS is the RDI, or Routing Domain Identifier. Table of Contents 1. Introduction ............................................ 2 2. Motivation .............................................. 2 3. Definitions ............................................. 2 4. Common errors in allocating ASes ........................ 5 5. Criteria for the decision -- do I need an AS? .......... 5 5.1 Sample Cases ........................................... 6 5.2 Other Factors .......................................... 7 6. Speculation ............................................. 7 7. One prefix, one origin AS ............................... 8 8. IGP issues .............................................. 8 9. AS Space exhaustion ..................................... 8 10. Reserved AS Numbers .................................... 9 11. Security Considerations ................................ 9 12. Acknowledgments ........................................ 9 13. References ............................................. 9 14. Authors' Addresses ..................................... 10 Hawkinson & Bates Best Current Practice [Page 1] RFC 1930 Guidelines for creation of an AS March 1996 1. Introduction This memo discusses when it is appropriate to register and utilize an Autonomous System (AS), and lists criteria for such. ASes are the unit of routing policy in the modern world of exterior routing, and are specifically applicable to protocols like EGP (Exterior Gateway Protocol, now at historical status; see [EGP]), BGP (Border Gateway Protocol, the current de facto standard for inter-AS routing; see [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the Internet is expected to adopt when BGP becomes obsolete; see [IDRP]). It should be noted that the IDRP equivalent of an AS is the RDI, or Routing Domain Identifier. 2. Motivation This memo is aimed at network operators and service providers who need to understand under what circumstances they should make use of an AS. It is expected that the reader is familiar with routing protocols and will be someone who configures and operates Internet networks. Unfortunately, there is a great deal of confusion in how ASes should be used today; this memo attempts to clear up some of this confusion, as well as acting as a simple guide to today's exterior routing. 3. Definitions This document refers to the term "prefix" throughout. In the current classless Internet (see [CIDR]), a block of class A, B, or C networks may be referred to by merely a prefix and a mask, so long as such a block of networks begins and ends on a power-of-two boundary. For example, the networks: 192.168.0.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 can be simply referred to as: 192.168.0.0/22 The term "prefix" as it is used here is equivalent to "CIDR block", and in simple terms may be thought of as a group of one or more networks. We use the term "network" to mean classful network, or "A, B, C network". The definition of AS has been unclear and ambiguous for some time. [BGP-4] states: Hawkinson & Bates Best Current Practice [Page 2] RFC 1930 Guidelines for creation of an AS March 1996 The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other ASes. Since this classic definition was developed, it has become common for a single AS to use several interior gateway protocols and sometimes several sets of metrics within an AS. The use of the term Autonomous System here stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it. To rephrase succinctly: An AS is a connected group of one or more IP prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy. Routing policy here is defined as how routing decisions are made in the Internet today. It is the exchange of routing information between ASes that is subject to routing policies. Consider the case of two ASes, X and Y exchanging routing information: NET1 ...... ASX <---> ASY ....... NET2 ASX knows how to reach a prefix called NET1. It does not matter whether NET1 belongs to ASX or to some other AS which exchanges routing information with ASX, either directly or indirectly; we just assume that ASX knows how to direct packets towards NET1. Likewise ASY knows how to reach NET2. In order for traffic from NET2 to NET1 to flow between ASX and ASY, ASX has to announce NET1 to ASY using an exterior routing protocol; this means that ASX is willing to accept traffic directed to NET1 from ASY. Policy comes into play when ASX decides to announce NET1 to ASY. For traffic to flow, ASY has to accept this routing information and use it. It is ASY's privilege to either use or disregard the information that it receives from ASX about NET1's reachability. ASY might decide not to use this information if it does not want to send traffic to NET1 at all or if it considers another route more appropriate to reach NET1. In order for traffic in the direction of NET1 to flow between ASX and ASY, ASX must announce that route to ASY and ASY must accept it from ASX: Hawkinson & Bates Best Current Practice [Page 3] RFC 1930 Guidelines for creation of an AS March 1996 resulting packet flo