MikroTik strikes again ?

Alexander Harrowell a.harrowell at gmail.com
Mon May 3 05:48:42 CDT 2010


On Monday 03 May 2010 11:25:45 Bret Clark wrote:
> Uhm....okay...but why does anyone prepend their ASN that much? Are you 
> saying the Mikrotik did that on purpose?
> 

There was a well-known routing incident last year in which a difference 
between the Mikrotik and Cisco CLIs caused the propagation of extremely long 
AS-PATH attributes, which caused certain Cisco routers to crash.

Basically, someone remembered their Cisco IOS syntax and typed "bgp-prepend 
47868" into a Mikrotik; the correct syntax would have been "bgp-prepend x 
47868" where x is an integer between 0 and 16 representing the desired number 
of prepends. The Mikrotik correctly tried to prepend 47868 47868 times, but 
had only one byte to store this value and therefore produced 255 prepends.

Some Cisco machines, it turned out, had a bug that caused path lengths close 
to 255 to crash them. Fun and games ensued.

The Renesys blog has much, much more: 
http://www.renesys.com/blog/2009/02/longer-is-not-better.shtml
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