32 bits ASN on Cisco

Gary T. Giesen giesen at snickers.org
Sun Apr 11 01:36:01 CDT 2010

You can still request a 16 bit ASN from the registries, so if this is
a concern for a new deployment you're not out of luck (yet). There is
a compatibility mechanism, so you can still receive routes sourced
from (or passing through) 32 bit ASNs, they will simply appear as AS
23456. Even if you have 32-bit ASN-compatible gear, all your direct
peers need to support it as well (more or less), and many providers
still don't have support for this yet (for example, the 7600 platform
only just got support with 12.2(33) SRE0, and no one in their right
mind is running it in production yet).

A great resource for information on 32-bit ASN's is


On Sun, Apr 11, 2010 at 1:33 AM, Franck Martin <franck at genius.com> wrote:
> Sorry if it is not the right forum, but I'm trying to get a grip on 32Bits ASN on Cisco
> I see that feature is available on 12.4(24)T
> http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/data_sheet_C78-521821.html
> Which is available only for "new gear". I don't know how new is the 1800,2800,3800 series, but I know networks still running on 2500 and 2600.
> So does it mean that to get 32bit ASN you need to get new gear?
> Seems to me 12.4(24)T is relatively new, and that this feature is not available on the mainline IOS 12.4?
> So in one hand RIR allocates 32bit ASN, but on the other hand, it is hard to implement it on current equipment? Is that a (un)fair assessment?

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