Re: unswitched behavior of a switched network...

[Jan Rottschaefer <jrott () online de>]

On Friday 13 October 2006 18:32, Jon Hart wrote:

hi jon,

i had a similar situation in a switched environment. certain frames to a 
particular server where to be seen at every port on all switches within the 
same vlan. the reason was that the server was attached with several cards for 
loadbalancing. arp request for the virtual address where answered by each 
server card but when the client send ip packets using the learned virtual mac 
the server cards replied using their physical address which is stupid since 
the vmac was never used as a source and so it could not be learned by the 
switches. as a result frames that had the vmac as a destination where always 
flooded...also a nice example on how to turn expensive network equipment into 
a hub :)

regards

jan

> Greetings,
>
> I've got a situation here that I can't quite figure out.  It is well
> known that it is possible to cause a switched network to act like an
> unswitched network by flooding the CAM table.  There are countless tools
> and documents out there that cover the offensive and defensive measures
> related to this issue.
>
> While this isn't Cisco's official documentation on this issue,
> http://xrl.us/r8k7 says:
>
>    "Content-addressable memory (CAM) overflow: A CAM table is used to
>    determine where to direct incoming frames depending on which port the
>    incoming MAC address came from. When the CAM receives a frame with an
>    unknown destination, the proper procedure is to flood frames within
>    the acceptable Layer 2 domain (the proper VLAN). Hardware and
>    software tools are available (some for free), that can flood a switch
>    with MAC addresses. Once the CAM table limit is exceeded, switches
>    behave differently depending on the brand of the switch."
>
> My question is, has anyone seen a situation where the same broadcast
> behavior occurs, but the CAM table itself is not overloaded and there is
> no good reason for entries to be expiring?  Furthermore, even if the
> entries were expired, has anyone encountered situations (malicious or
> otherwise), where a given port will receive traffic outside of its own
> L2?
>
> Thanks,
>
> -jon
>
>
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