[isp-bgp] Re: BGP routing loop !

[Jennifer Rexford <jrex@...>]

Hi Kave,

> I have a practical BGP question. I am receiving the following AS
> paths: 2200 5511 702 8342 29651 25515 29369 25515
> 
> With the AS 25515 repeated two times with the AS 29369 in the middle.
> Is it normal?

On the surface, BGP's loop detection mechanism would make such an AS
path impossible.  In practice, though, most existing routers check for
AS loops only by looking for their own AS number in the AS path, rather 
than checking for all possible loops.  So, for example, AS 8342 would
not discard the advertisement upon seeing the path "29651 24414 25515
29369 25515" despite the apparent loop in the path.  The bigger question 
is how did the apparent loop get there in the first place.  One possibility 
is AS prepending.  Perhaps AS 29651 and AS 25515 are owned by the same
company -- I did a

  whois -h whois.ripe.net as29651
  whois -h whois.ripe.net as25515
  whois -h whois.ripe.net as29369

and they all seem to be owned by CenterTelecom Service.  My guess
is that a router in 29369 was configured to do AS prepending using
the AS number 25515, e.g., in Cisco IOS

  set as-path prepend 25515

Perhaps this was done mistakenly -- e.g., because this router used to
be part of AS 25515 and nobody changed the line of configuration when
switching ASes.  

Another way the apparent loops arise is through typographical mistakes
in AS prepending.  Imagine that a router in AS 7081 receives a route
with the path "1239 7018 46", with AS 7018 in the path.  If this router
has been mistakenly configured with

  set as-path prepend 7018 <---- note, 7018 appears rather than 7081

then you'd end up with a path that appears to loop through AS 7018.

See the discussion of AS prepending in Section 3.2.3 and Section 5.2.4 of 

  http://nms.lcs.mit.edu/bgp/rcc/rcc-tr.html

which describes a tool that checks router configuration for this anomaly
and other kinds of mistakes and inconsistencies...

-- Jen


  

  

  






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