CCNA prep --> Wrong DIKTA answer?? Spanning Tree

Unanswered Question
Dec 3rd, 2007

Hi there,

I just started off with my CCNA prep with CCNA ICND2 Official Certification guide. There's an answer to a question of the DIKTA section of chapter 2 that confuses me. It's quesion 10 and it goes like this:

Switch SW3 is receiving only two Hello BPDUs, both from the same root switch, received on the two interfaces listed as follows:

Port Status VLAN Duplex Speed Type

Fa0/13 conn. 3 a-half a-100 10/100

G0/1 conn. 1 a-full a-1000 1000B

SW3 has no STP-related configuration commands. The Hello received on Fa0/13 lists cost 10 and the Hello received on G0/1 list cost 20. Which of the following is true about STP on SW3?

a. SW3 will choose Fa0/13 as its root port.

b. SW3 will choose G0/1 as its root port.

c. SW3 Fa0/13 will become designated port

d. SW3 G0/1 will become designated port.

According to the book answer b is correct, as 24 is the lower cost of the two paths.

In my opinion answer a is also correct as the question says "SW3 has no STP-related configuration commands". This means switch SW3 uses spanning-tree mode pvst (default), ergo Fa0/13 is root port for VLAN3, G0/1 is root port for VLAN1. Anything wrong with my assumption? Appreciate any comments.


I have this problem too.
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Francois Tallet Mon, 12/03/2007 - 17:18

I agree with you. Now, you don't know if the ports are trunking, if the BPDUs received are tagged, which vlan(s) the question is applicable to...

Depending on that, c and d could also be valid;-) That's so many unknown variables that I would not be surprised if any answer could be considered as correct.



bvsnarayana03 Mon, 12/03/2007 - 20:17

When no conditions are given in the question, then default mode operation is considered. Here 2 ports are connecting to the root. The root port is one with the lowest path cost to the root bridge. Since gig port has the lowest path cost it will be the root.

Francois Tallet Mon, 12/03/2007 - 23:10

Yes, but as the original poster noticed, the two ports seem to be in different vlans. I say seem, because of the unknown variables mentioned earlier.


diplomarbeit001... Tue, 12/04/2007 - 00:09


thanks for your replys.

The interface status is derived from a "show interface status" command on the affected switch. I did not mention this in the original posting, sorry. So both port are are access ports (Fa0/13 accesses VLAN 3, G0/1 accesses VLAN 1) and therefore must be root ports!?


Francois Tallet Tue, 12/04/2007 - 09:58

Yes, they should both be root ports then. That's very likely to be a typo though. They would not have mentioned that the BPDUs were coming from the same root if they did not have the intend of having you comparing them.




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