[Bridge] STP bug, loop not detetcted

[Francesco Dolcini]

Dylan Hall wrote:
> Most Cisco switches will generate a loopback packet (ethertype 0x9000)
> at fairly regular intervals.  Both the src and dst mac addresses are set
> to the interface mac of the port emitting the packets.  In the event a
> loop exists in the network that hasn't been dealt with by (R)STP the
> switch will receive the packet back on another of it's interfaces.  In
> the event one of the loopback packets does make it back to the switch
> the port is error-disabled (I can't remember if it's the sending or
> receiving port that is disabled).  This mechanism is independent of
> STP. 
> 
> UDLD also seems to have a mechanism to detect a port that is looped back
> on itself.  Again, independent of STP.
> 
you can also add cisco loop guard to the list, but this is an 
improvement to standard STP, not a different protocol

> Rather than modifying STP would it not be better to implement something
> like the above as a standalone daemon? 
I agree, but it is also true that if we can improve STP without breaking 
compatibility there is no reason to not do it.