Please help with STP

[erkange005]

Hello People,

I am stumped on a question from the practice quiz from Wendell Odom's book Routing and Switching 200-120 Official Cert Guide Library in the ICND2 section Chapter 2 question 5 which states:

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

SW3# show interfaces status

Port Name Status Vlan Duplex Speed Type

Fa0/13 connected 1 a-half a-100 10/100BaseTX

Gi0/1 connected 1 a-full a-1000 1000BaseTX 

SW3 has no STP-related configuration commands. The hello received on Fa0/13 lists root cost 10, and the hello received on Gi0/1 lists root 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 Gi0/1 as its root port.

c. SW3’s Fa0/13 will become a designated port.

d. SW3’s Gi0/1 will become a designated port.

The answer is B.  Could someone please explain why the Gi0/1 port (at a cost of 20) will be the root port when it's cost is higher than Fa0/13's port at 10.  

2nd question: If Gi0/1 is the root port would that not make Fa0/13 the designated port which would be answer C?  In which case why is there only 1 correct answer?

[Charles Hill · Accepted solution]

If Sw3 receives a hello on f0/13 with a cost of 10 then it adds the cost of that f0/13 port, which is 19.

19 + 10 is 29

If Sw3 receives a hello on G0/1 with a cost of 20 then it adds the cost of G0/1 port, which is 4.

4 + 20 is 24

So if I'm calculating this correctly, G0/1(cost of 24) has the lowest cost and will be the root port.

Please correct me if I'm wrong, but that is my understanding on how the root port would be calculated.
 

Data rate	STP cost (802.1D-1998)	RSTP cost (802.1W-2004, default value)[6]:154
4 Mbit/s	250	5,000,000
10 Mbit/s	100	2,000,000
16 Mbit/s	62	1,250,000
100 Mbit/s	19	200,000
1 Gbit/s	4	20,000
2 Gbit/s	3	10,000
10 Gbit/s	2	2,000

Hope this helps.

[erkange005]

Makes sense to me cehill.  What about my 2nd question?

2nd question: If Gi0/1 is the root port would that not make Fa0/13 the designated port which would be answer C?  In which case why is there only 1 correct answer?

Thank you 

[Charles Hill]

My first thought was the same as yours, that it would be a DP, but Sw3 F0/13 could be placed in the blocking state to provide a loop free topology. 

If this helps, please rate. 

Thank you.

[alisa]

Just taking one part of the question "The Hello received on the Fa0/13 lists cost 10, and the hello received on Gig lists cost 20."

Since the cost of each interface on the root switch to reach itself is 0, therefore I think the cost advertised to the neighbor switch's interface should always be 0 in the root switch interface's Hello BPDUs. Is it possible that the root will advertise the cost of its interface something other than 0? 

If yes then how can it be done? If no then, is it a hypothetical situation given in here just to test the understanding of the STP concepts of interface costs? Not sure of the answer. Any help is appreciated :-)