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BGP Autonomous Systems transition: The 10 biggest concerns

As the supply of BGP Autonomous Systems (AS) 2-byte numbers continues to decline, the Internet community is preparing for the transition to 4-byte numbers. Find out what you need to do now, what to do later, and how your vendors need to support you.

Recently I had a meeting with a visibly nervous enterprise customer who got extremely upset by an alarming announcement from his regional Internet registry (RIR) explaining that the Autonomous System (AS) number format used by Border Gateway Protocol (BGP), one of the Internet's core protocols, will change in a few months. The customer is multi-homed to two Internet Service Providers (ISPs), and his two obvious questions were: What do I have to do about it, and is the equipment I bought supporting this change?

Yes, it's true that the pool of BGP Autonomous System numbers is being depleted.

The existing 2-byte AS numbers pool is predicted to be exhausted in 2011. A worldwide tested production-grade infrastructure [must exist] well before that.
Ivan Pepelnjak
IP expert
The Internet community has decided to replace 2-byte AS numbers with 4-byte AS numbers, while at the same time guaranteeing indefinite backward compatibility with old equipment. In two years, you or your customers will probably not be able to get a 2-byte AS number, and you should start preparing for that.

In most cases, you will have to upgrade your routers and your network management software before you can support 4-byte AS numbers. But most equipment vendors aren't ready for the change, so use whatever leverage you have to push them to implement it before it's too late.

If you're going to have a conversation about the changes with a customer or your boss in the near future, here are some of the typical questions raised during a transition like this and answers that will probably help you calm anxious execs during the impending transition to a new Autonomous System number format.

  1. Is this change important?
    Absolutely. The current numbering scheme allows for up to 64,500 AS numbers, and with everyone trying to get multi-homed (connected to more than one ISP), we're running out of AS numbers. The existing 2-byte AS numbers pool is predicted to be exhausted in 2011. We have to have a worldwide tested production-grade infrastructure well before that.
  2. Do I need to panic about this?
    Absolutely not. The changes to BGP, described in RFC 4893, ensure that the "old" BGP routers can interoperate with the "new" BGP routers indefinitely.
  3. How is interoperability achieved?
    Whenever an AS number higher than 65535 (the upper limit of the 2-byte AS numbers) is encountered in the BGP attributes -- for example, in the AS path -- which would have to be sent to the "old" BGP routers, it's replaced with AS 23456. The 4-byte BGP attributes are transparently propagated across the "old" BGP world, assuming the vendors did not stray too far from the BGP standard in their implementation.
  4. Do I have to do something if I'm an existing end-customer?
    Not at all. You can run the "old" BGP forever. You might experience minor inconveniences if you use AS-path filters, as you will not be able to match 4-byte AS numbers in the AS path.
  5. Can I use a new 4-byte AS number assigned to me with old equipment?
    Related BGP articles
    Introduction to Border Gateway Protocol (BGP)

    5 essential reasons for BGP in your IP network

    Designing large-scale BGP networks

    Improving BGP services and security

    Scale your backbone with core MPLS, BGP on the edge
    No. Your equipment has to support the 4-byte AS numbers, although you can use your 4-byte AS number to peer with "old" BGP routers.
  6. Do I have to do something if I'm an ISP?
    In the short term, no. You can even connect a "new" BGP customer if you use AS 23456 as their AS number, regardless of the actual 4-byte AS number they got from RIR. In the long term, however, you will have to upgrade.
  7. Will it hurt me in the long run?
    Yes. Without the support for 4-byte AS numbers, you will not be able to enforce routing policies based on AS path -- for example, you will not be able to enforce non-transit contractual rules on your customers. You may also experience problems with your BGP peers if you need to send them 4-byte BGP communities, which you can't do from an "old" BGP router.
  8. Can I upgrade my routers now?
    If you've decided to buy from Juniper, you may not even need to upgrade. If you've bought boxes from other vendors, you may not even have an upgrade option today. Check the 4-byte AS numbers Operational resources page to find out what your upgrade options are.
  9. What else do I have to do?
    Even if your routers support 4-byte AS numbers, check all the network management software you use in your deployment/operations. If you use AS numbers anywhere in your network management software -- to generate AS-path filters automatically, for example -- you may have to modify or upgrade your software.
  10. Have the vendors dropped the ball?
    No doubt. As of Oct. 1, 2008, the only major vendor supporting this feature is Juniper. Cisco supports it only in the IOS XR, which requires a CRS-only network, and IOS NX (quite useless for building the service provider networks; Nexus is primarily a data center box). All other Cisco customers currently have no upgrade options. Even vendors with a mouthful of "open source networking" marketing messages haven't done much better. It should also be noted that the policy regarding the allocation of 4-byte AS numbers by RIRs was created in December 2005 and took effect on Jan. 1, 2007 -- two years before the deadline.
About the author: Ivan Pepelnjak, CCIE No. 1354, is a 25-year veteran of the networking industry. He has more than 10 years of experience in designing, installing, troubleshooting and operating large service provider and enterprise WAN and LAN networks and is currently chief technology advisor at NIL Data Communications, focusing on advanced IP-based networks and Web technologies. His books include MPLS and VPN Architectures and EIGRP Network Design. Check out Ivan's blog " Cisco IOS hints and tricks" for more on telecom topics.
This was last published in October 2008

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