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Will we run out of BGP AS numbers?

In the “internet meltdown” post I’ve described the main reason for the routing problems we’re experiencing in the Internet: everyone wants to be truly multihomed. All these end-customers obviously need their own AS number and it’s no wonder the experts predict we’ll run out of AS numbers in two to three years.

There’s no need to panic: the technical solution (four byte AS numbers) has been ready for several years … but it’s not implemented yet in majority of Cisco IOS-based platforms. Does that mean we’ll experience Internet-wide problems when the regional registries start allocating AS numbers larger than 65536 in a few months? Luckily, the answer is NO, the new BGP standards are completely backward-compatible … but if you’re a Service Provider, you have to start thinking about the upgrade path.

You can find more answers on this topic
in the article I wrote for SearchTelecom.

The list of all articles I wrote for SearchTelecom is available in the CT3 wiki.


  1. Backward-compatibility is ok for routers but the ecosystem should upgrade to. I particularly think about monitoring and management tools. As a security fanatic I also lost my thought to integer overflow exploitation in the worst case.
  2. I'd need to check but IOS-XR supports 4-byte AS.
  3. @Francois: You're absolutely correct. All provisioning and network management tools have to be checked ... not to mention AS-based Netflow :)

    @Wolruf: IOS-XR should support 4-byte AS (I didn't want to power up the box to check it).
  4. Call me an ignorant, but how many devices are supporting IOS XR? From what I know very few...
    The problem with 4 byte AS is the same like IPv6. If harder to change the mentality of the users than to implement it.
    The solution is out there, but as long as IPv4 will be tricked to be enough and the current scheme of AS numbering, no one will try to implement the new ones.
  5. @Calin: only the high-end devices (CRS, GSR) run IOS XR.

    The problem with 4-byte AS is probably worse (but less noticed) than the IPv6; while we can cheat around a while with NAT (and thus try to avoid IPv6), it's pretty hard to be truly multihomed without a public AS number.

    Fortunately, 4-byte AS numbers are seamlessly compatible with the 2-byte AS numbers.
  6. Any idea about whether new IOS release to cater this need for 7200 series router ?
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