AS path weirdness

James Aldridge jhma at mcvax.org
Sun Mar 22 07:09:34 CDT 2009


Jason Lewis wrote:
> I'm not entirely sure what I'm looking at. The reserved AS, 65490
> appears in parentheses and I've never seen that in MRT formatted data
> and not sure why it's happening.

That would be a single-element AS_SET, I guess.

> I'm also not clear on why I see 23456 *and* a 32 bit AS in the path.  Is
>  anyone else seeing this or is it something wacky at RRC04?

That's certainly not limited to rrc04.  From one of our routers at the AMS-IX
I see the following (JunOS 9.2, so 32-bit AS numbers are in "ASPLAIN" format
and AS_TRANS is spelt out):

A Destination        P Prf   Metric 1   Metric 2  Next hop        AS path
* 91.201.176.0/22    B 170        100          0 >195.69.144.110  3356 21219 196638 I
                     B 170        100         10 >193.0.0.147     12859 21219 196638 I
                     B 170        100         50 >195.69.144.200  12859 21219 196638 I
                     B 170        100        100 >195.69.144.35   12859 21219 196638 I
                     B 170        100            >195.69.144.178  6320 3327 21219 196638 I
                     B 170        100          0 >195.69.145.47   8218 3257 3356 21219 196638 I
                     B 170        100            >195.69.144.50   1103 3549 3327 21219 196638 I
                     B 170        100            >195.69.144.34   1103 3257 3356 21219 196638 I
                     B 170        100          0 >195.69.145.144  1273 3549 3327 21219 196638 I
                     B 170        100        100 >195.69.144.89   286 3549 3549 3327 21219 196638 I

* 91.207.218.0/23    B 170        100          0 >195.69.144.110  3356 13249 6886 196629 I
                     B 170        100          0 >193.0.0.147     3356 13249 6886 196629 I
                     B 170        100         50 >195.69.144.200  12859 35320 196629 AS_TRANS ?
                     B 170        100        100 >195.69.144.35   12859 35320 196629 AS_TRANS ?
                     B 170        100            >195.69.144.178  6320 2914 35320 196629 AS_TRANS ?
                     B 170        100          0 >195.69.145.144  1273 2914 35320 196629 AS_TRANS ?
                     B 170        100        100 >195.69.144.89   286 2914 35320 196629 AS_TRANS ?
                     B 170        100          0 >195.69.145.47   8218 3257 3356 13249 6886 196629 I
                     B 170        100            >195.69.144.50   1103 3549 3356 13249 6886 196629 I
                     B 170        100            >195.69.144.34   1103 3257 3356 13249 6886 196629 I

* 195.88.52.0/23     B 170        100          0 >195.69.144.110  3356 13249 28761 196665 I
                     B 170        100          0 >193.0.0.147     3356 13249 28761 196665 I
                     B 170        100         50 >195.69.144.200  12859 13249 28761 196665 I
                     B 170        100        100 >195.69.144.35   12859 13249 28761 196665 I
                     B 170        100          0 >195.69.145.144  1273 3356 13249 28761 196665 I
                     B 170        100        100 >195.69.144.89   286 3356 13249 28761 196665 I
                     B 170        100          0 >195.69.145.47   8218 3257 3356 13249 28761 196665 I
                     B 170        100            >195.69.144.50   1103 3549 3356 13249 28761 196665 I
                     B 170        100            >195.69.144.34   1103 3257 3356 13249 28761 196665 I

* 195.128.231.0/24   B 170        100          0 >195.69.144.110  3356 13249 6886 196629 35748 I
                     B 170        100         10 >193.0.0.147     12859 35320 196629 AS_TRANS 35748 I
                     B 170        100         50 >195.69.144.200  12859 35320 196629 AS_TRANS 35748 I
                     B 170        100        100 >195.69.144.35   12859 35320 196629 AS_TRANS 35748 I
                     B 170        100            >195.69.144.178  6320 2914 35320 196629 AS_TRANS 35748 I
                     B 170        100          0 >195.69.145.144  1273 2914 35320 196629 AS_TRANS 35748 I
                     B 170        100        100 >195.69.144.89   286 2914 35320 196629 AS_TRANS 35748 I
                     B 170        100          0 >195.69.145.47   8218 3257 2914 35320 196629 AS_TRANS 35748 I
                     B 170        100            >195.69.144.50   1103 3257 2914 35320 196629 AS_TRANS 35748 I
                     B 170        100            >195.69.144.34   1103 3257 2914 35320 196629 AS_TRANS 35748 I

AS_TRANS and a 32-bit ASN only seem to appear together in the paths through
35320 so it looks as though something odd is going on there.

James




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