RFC 2270

Using a Dedicated AS for Sites Homed to a Single Provider

📅 published January 1998, from ietf

RFC 2270: Using a Dedicated AS for Sites Homed to a Single Provider 💖

okay bestie, this one is the tea 🫖 It's RFC 2270, filed as Informational in January 1998. The authors are John Stewart (ISI) and Tony Bates, Ravi Chandra, and Enke Chen (all Cisco). It doesn't say whether it obsoletes or updates anything, so we're not guessing 🙈

Status of This Memo

this memo gives info to the Internet community 📚 it doesn't specify an Internet standard of any kind, and distribution is unlimited ✨

Copyright Notice

Abstract

ngl the Internet has been growing SO fast, and tons more customers are using BGP4 now 📈 RFC1930 gives guidelines for when you actually need an AS (Autonomous System). but here's the catch: the customer and the ISP end up with a problem 😭 by those guidelines, you don't need an allocated AS, yet sometimes BGP4 is honestly the most practical (maybe even the only) way to connect a customer who's homed to a single ISP. this paper proposes a fix that still lines up with what RFC1930 recommends 💅

1. Problems

okay so the Internet keeps growing, and more customers are using BGP4 [1],[2] 🌱 RFC1930 [4] lays out guidelines for when you need and should use an AS. the customer and the service provider (ISP) are left with a problem though: under those guidelines there's no need for an allocated AS, but certain conditions make BGP4 a super pragmatic, maybe only way to connect a customer homed to a single ISP. the conditions are:

Customers multi-homed to single provider

picture Figure 1 below 👀

                        +-------+      +-------+
                           +----+       |      |       |
                +------+   |    | ISP A +------+ ISP B |
                | Cust.+---+    |       |      |       |
                |   X  +--------+       |      |       |
                +------+        ++-----++\     +-------+
                                 |     |  \
                                 |     |   \  +--------+
                                ++-----++   +-|        |
                                | Cust. |     |  ISP C |
                                |   Y   |     |        |
                                +-------+     +--------+

Figure 1: Customers multi-home to a single provider 🧚

both customer X and customer Y are multi-homed to one provider, ISP A 🫶 because those multiple connections are "localized" between ISP A and its customers, the rest of the routing system (ISP B and ISP C here) doesn't need to see routing info for one multi-homed customer any differently than a singly-homed one. ISP A has the same routing policy relative to ISP B and ISP C, so it's giving "singly-homed" to the whole outside world 💁‍♀️ the messy multiple connections don't matter globally. AS numbers are identifiers used in routing protocols, and they're needed by routing domains as part of the global routing system. but as [4] correctly points out, orgs with the same routing policy as their upstream provider don't need an AS 🙌

here's the problem though: lots of ISPs can only handle the load-sharing and reliability needs of a multi-homed customer if that customer swaps routing info using BGP-4, and BGP-4 requires an AS as part of the protocol 😩

Singly-homed customers requiring dynamic advertisement of NLRI's

this isn't super common, since static routing usually handles it 🙈 but if a big pile of NLRI's needs to go from the customer to the ISP, it's often easier admin-wise to advertise those prefixes with a dynamic routing protocol. right now, the only exterior gateway protocol (EGP) that can do that is BGP. so this lands you in the same problem as condition 1 above 💀

so yeah, there's clearly a problem with the recommendations in [4] and with using BGP4 in the scenarios above 😭 Section 2 proposes a solution, and the sections after it cover what the solution implies and how to apply it ✨

also, if a customer is multi-homed to more than one ISP, they're advised to get an official allocated AS from their allocation registry 🏆

2. Solution

the solution we're proposing is that all BGP customers homed to the same single ISP use a single, dedicated AS specified by that ISP 🎀

logically, this means an ISP ends up with lots of peers sharing the same AS, even though that AS exists in "islands" that are totally disconnected from each other 🏝️

there are some practical implications, which we'll cover in the next section 💅

3. Implications

3.1 Full Routing Table Announcement

this solution means a BGP customer on the dedicated AS can't receive 100% of the full routes 😭 BGP speakers reject routes that have their own AS number in the AS path, because of routing loop detection on AS path. picture Customer X and Customer Y both peering with Provider A using AS number N. then Customer X won't be able to receive routes from Customer Y. we don't think this is a problem for Customer X though 🙌 both X and Y are stub networks, so default routing is good enough, and missing a very small chunk of the full routing table is unlikely to noticeably change traffic patterns guided by MEDs received.

a BGP customer on the dedicated AS must carry a default route (preferably receiving it from its provider via BGP) 🛣️

3.2 Change of External Connectivity

the dedicated AS a provider specifies is purely for peering between its customers and the provider 🤝 if a customer on the dedicated AS changes its external connectivity, it may be necessary to reconfigure their network to use a different AS number. that could be a globally unique one if they're homed to multiple providers, or a dedicated AS from a different provider 🔄

3.3 Aggregation

since BGP customers on this dedicated AS are only homed to one ISP, their routes (allocated from the provider's CIDR block) don't need to be announced upstream by that provider, because the provider is already originating the larger block 🧠 [6].

3.4 Routing Registries

providers use the Internet Routing Registry (IRR) [5] to generate route filtering lists 📋 those lists are mainly derived from the "origin" attribute of route objects. the "origin" is the AS that originates the route. with multiple customers on the same AS, you need finer granularity to generate the right route filtering 🔍 for example, the "mntner" attribute or the "community" attribute of a route object can be used along with the "origin" attribute to build the filtering lists 💜

4. Practice

the AS number a provider specifies can be either an AS from the private AS space (64512 - 65535) [4], or an AS previously allocated to the provider 🍓 with the former, the dedicated AS, like all other private AS's, should be stripped from its AS path while the route is propagated to the rest of the Internet routing system 🧹

5. Security Considerations

the usage of AS numbers described in this document has no effective security impact 🛡️ how to accept and filter AS numbers from customers is handled in other documents 📄

6. Acknowledgments

big thank yous to Roy Alcala of MCI and Arpakorn Boonkongchuen for their input 🫶 the members of the IDR Working Group also gave helpful comments 🌸

7. References

[1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995.
[2] Rekhter, Y., and P. Gross, "Application of the Border Gateway Protocol in the Internet", RFC 1772, March 1995.
[3] Rekhter, Y., "Routing in a Multi-provider Internet", RFC 1787, April 1995.
[4] Hawkinson, J., and T. Bates, "Guidelines for creation, selection, and registration of an Autonomous System (AS)", RFC 1930, March 1996.
[5] Bates, T., Gerich, E., Joncheray, L., Jouanigot, J-M, Karrenberg, D., Terpstra, M., and J. Yu., "Representation of IP Routing Policies in a Routing Registry (ripe-81++)", RFC 1786, March 1995.
[6] Chen, E., and J. Stewart., "A Framework for Inter-Domain Route Aggregation", Work in Progress.

8. Authors' Addresses

John Stewart
USC/ISI
4350 North Fairfax Drive
Suite 620
Arlington, VA  22203

EMail: jstewart@isi.edu


Tony Bates
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134

EMail: tbates@cisco.com


Ravi Chandra
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134

EMail: rchandra@cisco.com


Enke Chen
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134

EMail: enkechen@cisco.com

9. Full Copyright Statement

this document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. however, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards (in which case the procedures for copyrights defined in the Internet Standards process must be followed), or as required to translate it into languages other than English.

the limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.

this document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 🌈

📦 where this page comes from

ietf: rfc2270 1998-01
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