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QID:BCMSN373
#1
hi, I found following question bound to an incorrect answer (according to my understanding). could anyone please be so kindly confirm ? or explain what is that i am unable to pick here?


QID:BCMSN373) what is the effect of configuring the following command on a switch ?

answer in the test
if BPDUs are receievd by a port configured for portfast then portfast is disabled and the bpdu are processed as normally


correct answer accordin to me
if bpdu are recieved by a port configured for port fast they are ignored and none sent


thanks,
faeza
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#2
I would think the answer given in the test engine is correct, you have to disable portfast to stop any chance of stp topology issues, the bpdu's are then processed normally and the port placed into an stp state.

JC.
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#3
Can anyone please confirm this as I tend to agree with fnagi.

I believe that BPDU filter disables portfast, however, the BPDU does not get processed, instead it is dropped.

The logic behind BPDU filter is that they protect the switch from receiving untrusted superior BPDU. For example a host connected to a port which has portfast and BPDU filter enabled.

A host should not be sending BPDU, so if a BPDU is recieved on this port, it is likely that somebody has replaced the host with a switch and therefore BPDU protects this port from recieving superior BPDU which would preempt the route bridge and compromise the STP setup.

So if the BPDU was processed normally, this would totally go against the very nature of BPDU filter. So the answer is incorrect IMHO.

The correct answer should be that if received on a port fast interface, it is dropped.

Please let me know what your thoughts are on this admin / skilled other.

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#4
BCM320 explanation supports my post above and totally contradicts the answer for BCM373

Admin, can you confirm that this is an error.

Thank you,
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#5
I just noticed this error by how2pass myself. 

The command, ... portfast bpdufilter..., is used to disable STP on a port.  The answer how2pass gives describes the behavior of a port configured just with portfast.  Adding bpdufilter disables STP on the port altogether and it will not disable portfast when the port receives a bpdu.  See  p. 254 of the Official Exam Certification Guide, 4th edition, by David Hucaby. 

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