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CCNA – STP Questions

February 1st, 2011 Go to comments

Here you will find answers to Spanning Tree Protocol Questions

Note: If you are not sure how STP and RSTP work, please read my STP tutorial and RSTP tutorial.

Question 1

Which three statements about RSTP are true? (choose three)

A. RSTP significantly reduces topology reconverging time after a link failure.
B. RSTP expends the STP port roles by adding the alternate and backup roles.
C. RSTP port states are blocking, discarding, learning, or forwarding.
D. RSTP also uses the STP proposal-agreement sequence.
E. RSTP use the same timer-based process as STP on point-to-point links.
F. RSTP provides a faster transition to the forwarding state on point-to-point links than STP does.


Answer: A B F

Question 2

Which two states are the port states when RSTP has converged? (choose two)

A. blocking
B. learning
C. disabled
D. forwarding
E. listening


Answer: A D

Explanation

RSTP only has 3 port states that are discarding, learning and forwarding. When RSTP has converged there are only 2 port states left: discarding and forwarding but the answers don’t mention about discarding state so blocking state (answer A) may be considered the best alternative answer.

Question 3

Which command enables RSTP on a switch?

A. spanning-tree mode rapid-pvst
B. spanning-tree uplinkfast
C. spanning-tree backbonefast
D. spanning-tree mode mst


Answer: A

Question 4

At which layer of the OSI model is RSTP used to prevent loops?

A. data link
B. network
C. physical
D. transport


Answer: A

Question 5

Refer to the exhibit. Given the output shown from this Cisco Catalyst 2950, what is the most likely reason that interface FastEthernet 0/10 is not the root port for VLAN 2?

Switch# show spanning-tree interface fastethernet0/10

STP_show_spanning_tree_interface.jpg

A. This switch has more than one interface connected to the root network segment in VLAN 2.
B. This switch is running RSTP while the elected designated switch is running 802.1d Spanning Tree.
C. This switch interface has a higher path cost to the root bridge than another in the topology.
D. This switch has a lower bridge ID for VLAN 2 than the elected designated switch.


Answer: C

Question 6

Which two of these statements regarding RSTP are correct? (Choose two)

A. RSTP cannot operate with PVST+.
B. RSTP defines new port roles.
C. RSTP defines no new port states.
D. RSTP is a proprietary implementation of IEEE 802.1D STP.
E. RSTP is compatible with the original IEEE 802.1D STP.


Answer: B E

Question 7

Refer to the exhibit. Each of these four switches has been configured with a hostname, as well as being configured to run RSTP. No other configuration changes have been made. Which three of these show the correct RSTP port roles for the indicated switches and interfaces? (Choose three)

RSPT_port_states.jpg

A. SwitchA, Fa0/2, designated
B. SwitchA, Fa0/1, root
C. SwitchB, Gi0/2, root
D. SwitchB, Gi0/1, designated
E. SwitchC, Fa0/2, root
F. SwitchD, Gi0/2, root


Answer: A B F

Explanation

The question says “no other configuration changes have been made” so we can understand these switches have the same bridge priority. Switch C has lowest MAC address so it will become root bridge and 2 of its ports (Fa0/1 & Fa0/2) will be designated ports -> E is incorrect.

Because SwitchC is the root bridge so the 2 ports nearest SwitchC on SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root ports -> B and F are correct.

Now we come to the most difficult part of this question: SwitchB must have a root port so which port will it choose? To answer this question we need to know about STP cost and port cost.

In general, “cost” is calculated based on bandwidth of the link. The higher the bandwidth on a link, the lower the value of its cost. Below are the cost values you should memorize:

Link speed Cost
10Mbps 100
100Mbps 19
1 Gbps 4

SwitchB will choose the interface with lower cost to the root bridge as the root port so we must calculate the cost on interface Gi0/1 & Gi0/2 of SwitchB to the root bridge. This can be calculated from the “cost to the root bridge” of each switch because a switch always advertises its cost to the root bridge in its BPDU. The receiving switch will add its local port cost value to the cost in the BPDU.

One more thing to notice is that a root bridge always advertises the cost to the root bridge (itself) with an initial value of 0.

Now let’s have a look at the topology again

RSPT_port_states_explanation.jpg

SwitchC advertises its cost to the root bridge with a value of 0. Switch D adds 4 (the cost value of 1Gbps link) and advertises this value (4) to SwitchB. SwitchB adds another 4 and learns that it can reach SwitchC via Gi0/1 port with a total cost of 8. The same process happens for SwitchA and SwitchB learns that it can reach SwitchC via Gi0/2 with a total cost of 23 -> Switch B chooses Gi0/1 as its root port -> D is not correct.

Now our last task is to identify the port roles of the ports between SwitchA & SwitchB. It is rather easy as the MAC address of SwitchA is lower than that of SwitchB so Fa0/2 of SwitchA will be designated port while Gi0/2 of SwitchB will be alternative port -> A is correct but C is not correct.

Below summaries all the port roles of these switches:

RSPT_port_roles.jpg

+ DP: Designated Port (forwarding state)
+ RP: Root Port (forwarding state)
+ AP: Alternative Port (blocking state)

Question 8

Which two protocols are used by bridges and/or switches to prevent loops in a layer 2 network? (Choose two)

A. 802.1d
B. VTP
C. 802.1q
D. STP
E. SAP


Answer: A D

Question 9

Which switch would STP choose to become the root bridge in the selection process?

A. 32768: 11-22-33-44-55-66
B. 32768: 22-33-44-55-66-77
C. 32769: 11-22-33-44-55-65
D. 32769: 22-33-44-55-66-78


Answer: A

Question 10

Refer to the topology shown in the exhibit. Which ports will be STP designated ports if all the links are operating at the same bandwidth? (Choose three)

STP_designated_port.jpg

A. Switch A – Fa0/0
B. Switch A – Fa0/1
C. Switch B – Fa0/0
D. Switch B – Fa0/1
E. Switch C – Fa0/0
F. Switch C – Fa0/1


Answer: B C D

Explanation

First by comparing their MAC addresses we learn that switch B will be root bridge as it has lowest MAC. Therefore all of its ports are designated ports -> C & D are correct.

On the link between switch A & switch C there must have one designated port and one non-designated (blocked) port. We can figure out which port is designated port by comparing their MAC address again. A has lower MAC so Fa0/1 of switch A will be designated port while Fa0/1 of switch C will be blocked -> B is correct.

Comments (50) Comments
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  1. RaviS
    May 17th, 2012

    Q1 was in my exam today

  2. 9tut
    May 18th, 2012

    @Samyak: There is no “disable” state in RSTP because the 802.1D disabled, blocking, and listening states are merged into a unique 802.1w discarding state. You can read it here:

    http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

    But I think there is something wrong in this question. Maybe you will see slightly different answers in the exam.

  3. puneet
    May 20th, 2012

    Hi all, I am taking CCNA 640-802 exam first time on 30/05/2012. Could anyone please send me latest dumps which are valid for UK? My e-mail address is puneet_gill84@yahoo.co.uk. Many thanks.

  4. Abdul Fathah MK
    May 23rd, 2012

    crisp and to the point….gr8 explanation.

  5. OCSIC
    May 25th, 2012

    Hey guys, i have 1 question to confirm regarding STP from Collisio dumps..

    In which circumstances are multiple copies of the same unicast frame likely to be transmitted in a switched LAN.

    A. After broken links are re-established
    B. In an improperly implemented redundant topology
    C. When upper-layer protocols require high reliabilty
    D. During high traffic periods
    E. When dual ring topology is in use

    The correct answer should be ‘B’ but collisio dumps’ answer is ‘E’.

  6. MUHAMMAD RASHID
    May 28th, 2012

    I PASSED CCNA EXAM TODAY THANKS TO ALL MIGHTY ALLAH
    960/1000

  7. Mike M.
    June 5th, 2012

    @ OSCIC
    I’m also seeing a discrepency on that same question. Also I see differences with this question:

    Question 2

    Which two states are the port states when RSTP has converged? (choose two)

    A. blocking
    B. learning
    C. disabled
    D. forwarding
    E. listening

    Answer: A D

    Collisio says D and C

    Can anyone tell us which is correct of these two questions (mine and OSIC’s)

  8. Mike M.
    June 5th, 2012

    Sorry, I see 9tut answered this just above. There is no disabled state.

  9. flower
    June 7th, 2012

    @9tut
    Q1: if the priority & mac addr are the same,so we should look at the “port ID” what is port ID??

    Q2: in the examples above, how can we know if Gi0/1, fa0/1…etc their cost are 2, 4, 19 or 100?? because above they didn’t mention if it’s 10Gbps ,1Gbps ,100mbps or 10mpbs..they just write Gi0/1 or fa0/1.

    Thank you so much!

  10. xallax
    June 7th, 2012

    @flower
    the mac address can not be the same

    gi0/1 is 1 gigabit (look at its name)
    fa0/1 is fastethernet, 100 mb
    et0/1 is ethernet, 10 mb

  11. flower
    June 7th, 2012

    @xallax
    you’re so good in explaining! are you working in 9tut team?? coz I saw you reply almost for everyone’s question..
    thanks a lot ,keep it going :)

  12. scikron
    June 11th, 2012

    Q7 was explained wonderfully, had serious issues with the concepts…

  13. God’s Grace
    June 21st, 2012

    I passed my ccna exam today Praise be to God! Thank you Jesus! and thanks to 9TUT for the tutorials and explanations, great site and thanks to xallax for your explanations to questions and thanks to http://www.examcollection.com for the dumps. Pls guys lets donate and help to keep this site up!

    48 ques for exams including 3 simulation, I had EIGRP, Acesslist2 and VTP. Make sure the practice the simulation, use packet tracer or gns3. Best wishes to all!

  14. Anonymous
    June 23rd, 2012

    Which two states are the port states when RSTP has converged? (choose two)

    A. blocking
    B. learning
    C. disabled
    D. forwarding
    E. listening

    #here the C option is not meant to be disabled , it is discarding infact!!!

  15. ajaz
    June 23rd, 2012

    Which two states are the port states when RSTP has converged? (choose two)

    A. blocking
    B. learning
    C. discarding
    D. forwarding
    E. listening
    # and the answer shall be C,D (the logic behind the answer is right as mentioned in the question no 2)

  16. Moe
    June 24th, 2012

    Regarding question 8. I do understand the answers but STP = 802.1d! this is like selecting STP and STP again! correct me if I am wrong

  17. seren
    June 24th, 2012

    @Moe
    “Spanning Tree Protocol (STP) is standardized as IEEE 802.1D”
    http://en.wikipedia.org/wiki/Spanning_tree_protocol

    Hope this helps.

  18. bonzo
    June 26th, 2012

    Question 7: Is Q7 correct ? Choice of DP between switch A and B. 9tut has chosen fa0/2 as DP based on it’s lower MAC ID. I was taught that it should be that bridge costs should only be used when root costs tie, which they do not. Cost to root GI0/2 =8:; Cost to root fa0/2 = 19. Shouldn’t Gi0/2 be the DP?!!!

  19. nelson
    June 26th, 2012

    @bonzo
    I had the same doubt, could someone please clarify on that point.

  20. nelson
    June 26th, 2012

    @bonzo

    By default if one side is 100Mbps (Fa) and the other side is 1Gbps(Gi) the link will be negotiated to 100Mbps.

    Now using the cost table and the above statement you will get all the right answers.

    @9tut: please correct me if I’am wrong

  21. nelson
    June 26th, 2012

    Also the ‘auto negotiation’ feature on the switch causes the negotiation between different speeds and even resolve’s duplex mismatches.

  22. Aram
    June 26th, 2012

    I agree on QUESTION 7. Cost to root on SwitchB is lower than cost to root on SwitchA, so why is SwitchB Gi0/2 the AP??

    When choosing port roles between 2 switches on a segment, such as the segment between SwitchA and SwitchB, the switch that advertises the lowest cost path wins, if a tie, then it goes by mac address… this is what I have learned.

  23. bonzo
    June 28th, 2012

    @Nelson@Aram
    Thanks Nelson. Struth I think you are right, which makes 9tuts explanation wrong. I checked the rule. “When IEEE autonegotiation process is enabled on both devices, both devices agree to the fastest speed supported by both devices” Look at 9tuts costings in his explanation. He has the cost as 4 between switch C and D, not the negotiated cost of 19 you suggest. Using his costings I am right. Also, his explanation is based on MAC cost, which I think is wrong. If YOU are correct about this, 9tut has arrived at the right answer by mistake. Well done (I think)

  24. Helen
    June 28th, 2012

    Thanks 2 God almighty just passed ccna wit 947. Dumps still valid had 48 questions and 3 sim. Eigrp, vtp and Acl..

  25. Adie
    June 28th, 2012

    Great Site and very helpful. Would love it if the answers to the questions could be hidden until a button is pressed as they are so close to the questions and can be seen when scrolling the webpage

  26. MG
    June 30th, 2012

    @bonzo, spanning-tree may not consider bandwidth negotiation (I said “may not” because test on real switches may prove or deny it), it actually will consider the port physically itself, I mean gigabit is gigabit, bandwidth negation is made by IOS, and apparently STP doesn`t care about that. That`s why switch D will have the cost of 4 (advertised 0 by the root + own 4) to the root rather than 19.
    Now as to the question about designated / non-designated ports between A & B, is the previous is true, so switch A cost is 19 and Switch B cost would be 8 (4+4+0). And DR/NON-DR ports are defined on a segment, so switch with a higher cost will have non-dr. In this case Sw A port f0/2 should be blocked. But it is still vague.

  27. MG
    June 30th, 2012

    my previous message mistake: “is the previous is true” -> if the previous is true

  28. Elamin
    July 3rd, 2012

    Question 9

    Which switch would STP choose to become the root bridge in the selection process?

    A. 32768: 11-22-33-44-55-66
    B. 32768: 22-33-44-55-66-77
    C. 32769: 11-22-33-44-55-65
    D. 32769: 22-33-44-55-66-78
    the correct answer is C cuzz it is the lowest one among them but u answered A??????

  29. adi
    July 3rd, 2012

    Q7)can u guys pls comment on why comparison was made between switch A and switch B and why not switch D while choosing dp

  30. bonzo
    July 3rd, 2012

    @MG – Not quite sure why 9tut based decision of DP on MAC address then if costs differ/ segment. That suggests you agree that answer abf incorrect. I will try the packet tracer.

  31. Prakhar
    July 4th, 2012

    @Elamin
    the correct answer is A because it has lowest bridge ID i.e. lowest priority+lowest Mac address

  32. Makh
    July 12th, 2012

    To all who are confused about question 7.
    1) STP chooses DP according to the lowest cost to root on the BPDU advertised on that LAN segment. If a tie occurs it chooses the lower BID of the two switches. That makes the answer to question 7 is wrong as it should be SWb Gi0/2 is the DP on that segment.
    2) Some of you guys went on about the speed negotiation. Of course the bandwidth on that segment will match the slower port.However that should not affect your BPDU cost of root whatsoever. Remember you add the cost of the port that the switch receives the BPDU on and it is always on cost-of-port per switch.
    3) STP ALWAYS considers the actual speed of the interface. So if STP needs to use SWb gi0/2 speed in any calculation it will be dealt with as a fa port which is by default has a cost of port 19.

  33. John Smith
    July 14th, 2012

    Q7

    “On Cisco switches, the STP cost is based on the actual speed of the interface, so if an interface negotiates to use a lower speed, the default STP cost reflects that lower speed. If the interface negotiates to use a different speed, the switch dynamically changes the STP port cost as well.”
    -[CCNA ICND2 640-816 Official Cert Guide, Third Edition By: Wendell Odom] P.89 {Per-VLAN Port Costs}

    In this specific topology, the segment of SWA-SWB has 2 paths to the root:

    1. from SwA-SwC which has a path cost of 19 (C’s fa0/1-A’s fa0/1–>autonegotiate to 100M)

    2. from SwB-SwD-SwC which has a path cost of 23 (C’s fa0/2-D’s gi0/2–>autonegotiate to 100M–>default cost=19)(D’s gi0/1-B’s gi0/1–>autonegotiate to 1G–>default cost=4)

    So SwA wins, the Fa0/2 of the SwA will be the Designate Port. The BID(MAC) contributes nothing in this specific topology when choosing the DP.

  34. iG
    July 20th, 2012

    @John Smith

    “Generally, when a switch port is configured as a designated port, it is based on the BID. However, keep in mind that the first priority is the lowest path cost to the root bridge and that only if the port costs are equal, is the BID of the sender. “, – Network Academy course materials.

    If the cost was the same switches would compare BIDs to define port roles.

  35. MCN
    July 24th, 2012

    what is the right answer of this Q.
    Which two states are the port states when RSTP has converged? (choose two)

    A. blocking
    B. learning
    C. disabled
    D. forwarding
    E. listening

  36. Alboma
    July 26th, 2012

    Hi MCN,
    As above posts mentioned, there is no disabled state in RSTP. (Only STP does) Blocking, listening and disabled are all merged to discarding in RSTP. So the answers should be discarding and forwarding.

  37. Alboma
    July 29th, 2012

    I still have hard time understanding Q5. And guess what, it’s on the exam last Friday!

  38. MG
    July 30th, 2012

    ALBOMA, root port is the “closest” one to the root bridge in terms of total cost. In initial STP there`s one vlan, and root port is chosem in that vlan (vlan 1). But if there`re more than 1 vlan, per-vlan stp or rapit per-vlan stp can be used. Thus in different vlans there can be different root bridges, so path cost to the root bridge differs. And the same port can be a root port in one vlan, and a non-dp port in other vlan simultaneously.

  39. Alboma
    August 3rd, 2012

    Thanks MG!

  40. Alboma
    August 17th, 2012

    Q1 on ICND2 exam today

  41. flexy
    August 22nd, 2012

    Guys can u help me out,

    What do asymmetric switches require?
    A) memory buffers
    B) extra ports
    C) statically assigned ports only
    D) high speed CAM

  42. ramu_acit
    August 24th, 2012

    Question 2
    Which two states are the port states when RSTP has converged? (choose two)
    A. blocking
    B. learning
    C. disabled
    D. forwarding
    E. listening
    Answer: A D

    Its surely A & D y bcoz after convergance takes place ports will be either in forwarding or blocked state,coming to disabled state the port is disabled only when we give shutdown command……if this is wrong plz let me knw.

  43. kamikaze
    August 25th, 2012

    John Smith is 100% correct regarding question 7. I would expect this type of question in the exam. The correct logic (using path cost) and slightly flawed logic (using bridge id) will lead you to the same answer in this instance which is not always the case. Don’t look a gift horse in the mouth.

  44. kamikaze
    August 25th, 2012

    Question 2 should be altered and changed to A) discarding.

  45. mina
    August 28th, 2012

    Can someone pls explain Q5. Thanks.

  46. jacsatx
    August 29th, 2012

    DID IT!!…907…thx 9tut!!!!..CCNA certified….tons of questions from this site…read from top to bottom every question, because you will not know what pops up…q7 and 10 on exam…study concept…good luck to all…can’t stress enough…read EVERY question on this site…

  47. Koopotang
    August 30th, 2012

    Q7 :
    Ithink 9tut is wrong saying cost between switchC and switchD is 4.
    Indeed switch C has fast ethernet and D giga ethernet. The speed will be negotiated to 100M so the cost is 19

  48. sniffer
    September 5th, 2012

    @mina myself i’m lost. dont understand the ans to that question at all. can some one please explain Q5

  49. mina
    September 6th, 2012

    Yes, can someone pls explain answer to Q5.
    Thanks.

  50. xallax
    September 6th, 2012

    @mina
    if the cost was lower then that of the neighboring switch for that segment it would’ve been “Root/FWD” as seen for vlans 1 and 3

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