RFC 1656

RFC 1656: BGP-4 Protocol Document Roadmap and Implementation Experience 🦋

okay bestie, here's the title block 🎀

• Network Working Group 💌
• Request for Comments: 1656
• Author: Paul Traina (P. Traina), cisco Systems
• Category: Informational
• Date: July 1994
• It has no Obsoletes or Updates lines in its title block 🙈

Status of This Memo ✨

This memo provides information for the Internet community. It does not specify an Internet standard of any kind, so it's not a standard, not even a little bit 🙈. Distribution of this memo is unlimited 📣.

Introduction 💖

okay so Border Gateway Protocol v4 (BGP-4) [1] is an inter-Autonomous System routing protocol, which means it's the thing that lets different networks (Autonomous Systems, or ASs) talk about routes to each other 💅. It's built on what everyone learned from BGP as defined in RFC-1267 [2] and from BGP usage in the connected Internet as described in RFC-1268 [3]. It's giving experience-based ✨

The main job of a BGP speaking system is to swap network reachability information with other BGP systems 🦋. That info includes the list of Autonomous Systems (ASs) the reachability information travels through. That's enough to build a graph of AS connectivity, which means routing loops can get pruned and some policy decisions at the AS level may be enforced 🧚.

BGP-4 brings a whole new set of tools for classless inter-domain routing 🌈. These include support for advertising an IP prefix, and it totally gets rid of the concept of network "class" within BGP, which is so iconic 👑. BGP-4 also adds ways to aggregate routes, including aggregating AS paths. These changes support the proposed supernetting scheme [4] 💜.

The management information base has been defined [5], and security considerations are discussed in the protocol definition document [1] 🔒.

Applicability Statement for BGP-4 🎀

BGP-4 is explicitly designed for carrying reachability information between Autonomous Systems 🌸. It is not intended to replace interior gateway protocols like OSPF [7] or RIP [6], so don't get it twisted 🙅‍♀️.

Implementations 🧚

Four vendors had developed independent implementations at the time of this memo 🫶:

• ANS (gated)
• Europanet
• 3COM
• cisco

The complete interoperability matrix for all known implementations of the various BGP versions is available under separate cover [9] 📚.

Implementation Testing 🌈

One implementation got extensively tested in a network designed to mirror the complex connectivity you find at many major Internet borders 🔥. This network has multiple BGP-3 and BGP-4 speakers carrying full routing information injected from Alternet, EBone, Sprint, CERFnet, and cisco. In lots of cases, extra AS adjacencies are simulated with IP over IP tunnels to make the routing topology even messier 🌀.

The primary feature of BGP-4 is that it can carry network reachability information without regard to classfull routing 💅. Along with the canonical routing information, CIDR prefixes (both supernets and subnets) are being injected from IGP information and aggregated using the methods described in BGP-4. AS set aggregation and policy decisions based upon AS sets have also been tested ✨.

The secondary extensions in version 4 of this protocol are seriously cute 🎀. They include enhancements to how the INTER_AS_METRIC (now called MULTI_EXIT_DISC) gets used, the addition of a LOCAL_PREF parameter to influence route selection within an AS, and a specified method of damping route fluctuations 🦋. All of these features have been tested in at least one implementation 💖.

Observations 👑

All implementations are able to carry and exchange network reachability information 💅.

Not all implementations can generate aggregate information based on the existence of more specific routes 🙈.

No implementation supports automatic deaggregation. That's the process of enumerating all networks in an aggregate block, for backwards compatibility with routing protocols that don't carry mask information (for example, BGP-3) 😭. However, most implementations do allow for statically configured, controlled deaggregation, which gives minimal backwards compatibility with non-CIDR capable routers 🧸.

At least one implementation that can run earlier versions of BGP deliberately does not automatically negotiate down to those earlier versions 🛑. Connections to BGP-4 peers must be explicitly configured as such 🔐.

Conclusions 💀

The ability to carry and inject natural networks and CIDR supernets is the immediate requirement for BGP-4 🚨. The ability to carry subnet information (useful when reassigning parts of class A networks to organizations with different routing policies) is more of a secondary concern 🌷.

The ability to conditionally aggregate routing information can be worked around by injecting static or IGP network information into BGP. Or aggregation can be performed by an upstream router that is capable 🔁.

Deaggregation is dangerous. It leads to information loss and, unless it is tightly controlled by a manual mechanism, it will create a routing information explosion 💥.

Automatic version negotiation is dangerous because of its state-less nature. Given packet losses or spontaneous restarts, it is possible for two BGP peers that are capable of BGP-4 to negotiate a BGP-3 or BGP-2 connection. That connection can't carry super/subnet reachability information or AS set information 😱💀.

Acknowledgments 🫶

The author would like to acknowledge Yakov Rekhter (IBM) and Tony Li (cisco) for their advice, encouragement, and insightful comments 💜🌸.

References 📚

• [1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4), RFC 1654, cisco Systems, T.J. Watson Research Center, IBM Corp., July 1994.
• [2] Lougheed K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM Corp., October 1991.
• [3] Gross P., and Y. Rekhter, "Application of the Border Gateway Protocol in the Internet", RFC 1268, T.J. Watson Research Center, IBM Corp., ANS, October 1991.
• [4] Fuller V., Li. T, Yu J., and K. Varadhan, "Supernetting: an Address Assignment and Aggregation Strategy", Work in Progress. [Note: This is an expired draft, and is also referred to in BGP4.6.]
• [5] Willis S., Burruss J., and J. Chu, "Definitions of Managed Objects for the Border Gateway Protocol (Version 4) using SMIv2", RFC 1657, Wellfleet Communications Inc., IBM Corp., July 1994.
• [6] Hedrick, C., "Routing Information Protocol", RFC 1058, Rutgers University, June 1988.
• [7] Moy J., "Open Shortest Path First Routing Protocol (Version 2)", RFC 1583, Proteon, March 1994.
• [8] Varadhan, K., Hares S., and Y. Rekhter, "BGP4/IDRP for IP---OSPF Interaction", Work in Progress, September 1993.
• [9] Li, T., and P. Traina, "BGP Interoperabilty Matrix", Work in Progress, November 1993.

Security Considerations 🔥

Security issues are not discussed in this memo 😬. Keep that in mind, bestie, because nothing in here covers them 🙈⚠️.

Author's Address 🌸

Paul Traina
cisco Systems, Inc.
1525 O'Brien Drive
Menlo Park, CA 94025

EMail: pst@cisco.com