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