32-bit ASes at routeviews

Mon Dec 17 11:14:58 UTC 2012

On Sun, Dec 16, 2012 at 11:48:13PM +0100, Iljitsch van Beijnum wrote:
> Looking for 32-bit AS numbers, I get some strange results from routeviews:
> 
> route-views>sh ip bgp regexp _23456_
> BGP table version is 2393809200, local router ID is 128.223.51.103
> Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
>               r RIB-failure, S Stale
> Origin codes: i - IGP, e - EGP, ? - incomplete
> 
>    Network          Next Hop            Metric LocPrf Weight Path
> *  31.177.16.0/22   128.223.253.10                         0 3582 3701 3356 23456 3.1043 i
> *  46.29.72.0/21    129.250.0.11           285             0 2914 12389 12389 12389 12389 23456 3.627 i
> *  46.243.96.0/21   154.11.11.113            0             0 852 174 39704 39704 23456 3.787 i
> *  91.208.62.0/24   154.11.11.113            0             0 852 174 39704 39704 23456 3.787 i
> *  91.217.87.0/24   194.85.40.15                           0 3267 174 23456 3.661 i
> *  91.230.169.0/24  208.51.134.254       13905             0 3549 29152 29152 29152 29152 23456 23456 23456 23456 3.1426 i
> *  91.238.8.0/24    194.85.40.15                           0 3267 8220 23456 3.2040 i
> *  111.235.148.0/22 194.85.40.15                           0 3267 9498 9730 23456 i
> *  141.0.176.0/21   129.250.0.11           285             0 2914 12389 12389 12389 12389 23456 3.627 i
> 
> Unless I missed something, AS 23456 is supposed to show up as a stand-in for 32-bit ASNs on 16-bit BGP implementations, not in _addition_ to 32-bit ASNs. So the penultimate line would make sense if the other lines weren't there and the others don't make sense period.
> 
> Maybe a bug in the IOS they're running?
> 
> route-views>sh ver
> Cisco IOS Software, 7200 Software (C7200P-ADVENTERPRISEK9-M), Version 12.4(24)T2, RELEASE SOFTWARE (fc2)
> 
> Or is something else going on?

This can happen when a old 2-byte only routers are doing prepends with the
neighbor address (4-byte). Then the magic in the 4-byte AS RFC to fix up
ASPATH has no chance to work and you will see 23456.

-- 
:wq Claudio