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STP Questions 3

November 21st, 2019 in SWITCH 300-115 Go to comments

Question 1

Explanation

The command “spanning-tree port priority 0” is better than the command “spanning-tree port priority 16” as interface G1/0 is preferred over other interfaces with lowest priority port ID. So answer E is better than B.

Question 2

Question 3

Explanation

If you configure “spanning-tree portfast” then it will only takes effect on access ports (portfast is still disabled on trunk ports). To enable portfast on a trunk port you need the trunk keyword (“spanning-tree portfast trunk”).

Note: The command “spanning-tree portfast default” can only be configured under global configuration mode but this question requires to “apply on the interfaces”.

Question 4

Explanation

The full macro command is “spanning-tree vlan <vlan-id> root primary”. This command will adjust its priority to a lower value (than the value of the current root bridge), so it will become the new root bridge for the specified VLAN, so this command basically generates a macro. But this keyword does not adjust STP timers so maybe this requirement does not exist in the exam.

If the question asks about “automatically adjust STP timers” only then the answer should be “diameter”:

Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.

Reference:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_50_se/configuration/guide/scg/swstp.html

Question 5

Question 6

Explanation

From the second command output (show spanning-tree mst) we learn that MST1 includes VLANs 10 & 20. Therefore if we want DSW1 to become root bridge for these VLANs we need to set the MST 1 region to root -> The command “spanning-tree mst 1 root primary” can do the trick. In fact, this command runs a macro and sets the priority lower than the current root.

Also we can see the current root bridge for these VLANs has the priority of 32769 (default value + sysid) so we can set the priority of DSW1 to a specific lower value. But notice that the priority must be a multiple of 4096. Therefore D is a correct answer.

Question 7

Explanation

By calculating and assigning the port cost of the switch ports, you can ensure that the shortest (lowest cost) distance to the root switch is used to transmit data. You can calculate and assign lower path cost values (port costs) to higher bandwidth ports by using either the short method (which is the default) or the long method. The short method uses a 16-bit format that yields values from 1-65535. The long method uses a 32-bit format that yields values from 1-200,000,000.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html

The port cost depends on the port speed; the faster interface speeds indicate smaller costs. MST always uses long path costs.

Reference: http://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/spanning-tree-mst-cost.html

Question 8

Question 9

Question 10

Explanation

We suppose there is no Etherchannel configured between ASW1 and DSW1.

For STP, the cost of 10Gbps is 2 while the cost of 1Gbps is 4 so the paths from ASW1 -> DSW1 -> DSW2 and ASW1 -> DSW2 have the same accumulate STP Path cost (= 4). If multiple paths are available to reach the Root Bridge (Root Switch) with the same accumulated Spanning Tree Path Cost in a Non-Root Switch, select the port connected to the neighbor switch which has the lowest Switch ID value as the Root Port. Therefore the 1Gbps port on ASW1 will be chosen as Root port (as DSW2) has the lowest Switch ID value -> traffic will go from ASW1 -> DSW2 -> core. 

Question 11

Explanation

From the output we see DSW2 is running in RSTP mode (in fact Rapid PVST+ mode as Cisco does not support RSTP alone). When a new switch running PVST+ mode is added to the topology, they keep running the old STP instances as RSTP (in fact Rapid PVST+) is compatible with PVST+.

Question 12

Question 13

Explanation

Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_50_se/configuration/guide/scg/swstp.html

Comments
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  1. lalaversa
    May 16th, 2018

    Q2. Correct answer is A (PVST+).
    PVST+ is based on the IEEE 802.1D standard and Cisco proprietary extensions. It is the default spanning-tree mode used on all Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum supported, ensuring that each has a loop-free path through the network.
    N.B.: some hw platform should have Rapid-PVST+ or MST as default STP (ex. Packet Tracer has PVST as default STP…), but in this Q&A surely A is the correct answer.

  2. lalaversa
    May 16th, 2018

    Q5. Fake question..
    the diagram is missing, it is not specified if the bridge priority is default for all the switches and then asks for Vlan10 only while there is no answer for Vlan10 alone.
    The answer is to trust and according to a certain logic it should be A and B.

  3. patzen
    May 24th, 2018

    Q10 I think we could assume the 2 10Gb links are not in ether channel, after all this is STP question.

    If that’s the case one of the links is going to be in blocking state, DSW2 is the root, lowest priority 24576

    cost to DSW2 from ASW1 is 4,
    cost to DSW2 via DSW1 is 4.

    Now this is STP tie breaker when it comes to selecting port roles

    -Lowest Sender Bridge ID – this is us, DSW2 has lower ID from the one of DSW1, 28672<24576
    -Lowest Sender Port Priority
    -Lowest Sender Port Number

    B is correct

  4. kingkong123123555
    May 30th, 2018

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  5. YF
    June 12th, 2018
  6. nataliya
    June 19th, 2018

    @certprepare
    Q5 definitely needs diagram, the question state is noncence

  7. Anonymous
    August 15th, 2018

    Q.5
    Surely E is not correct the command does not exist.
    B SURELY IS CORRECT BECAUSE they mentioned that is the lowest.
    D is correct because if B is correct because of the word (4096 THE LOWEST), so zero is less than 4096 and is a valid value.

    With extended system IDs, we add 10 (value of the Vlan) to the priority value to obtain the bridge ID. So D is correct.

  8. RED1
    August 15th, 2018

    Q.5
    Surely E is not correct the command does not exist.
    B SURELY IS CORRECT BECAUSE they mentioned that is the lowest.
    D is correct because if B is correct because of the word (4096 THE LOWEST), so zero is less than 4096 and is a valid value.

    With extended system IDs, we add 10 (value of the Vlan) to the priority value to obtain the bridge ID. So D is correct.

  9. RED1
    August 15th, 2018

    Q.10
    B is correct. explanation is very simple, but we suppose that the CORE is not the root bridge, the logic says that no switch is mentioned in the CORE part (it is a smog). So the root bridge is DSW2 because of its lowest priority.
    A.not correct because we have to pass over the root bridge
    C.Is not correct because we attempted the DSW2, the root bridge, we should not go to the DSW1 to attempt the CORE.

    So B or D can be correct.

    The cost is the same, 1G cost is 4, 2*10G cost is 2*2.

    So the first criteria to decide if a port on ASW1 will become root port is the cost.
    Now we have the same cost 4=4.
    The second criteria to decide is the senders bridge ID (BPDU senders bridge ID). The lowest is the winner.
    So the bridge ID of DSW2 is lower because the priority is lower than the DSW1 priority.
    This is why the root port on ASW1 is the port connected to the DSW2 bridge, the other port to the DSW1 will be in blocked state.

  10. WISE FOOL
    October 3rd, 2018

    ALRIGHT GUYS, CHECK THIS OUT!!

    Question 5

    Diagram is: with 1 core, 2 DSW connected with EtherChannel, 2 ALSW, 2 PCs per ALSW. Per diagram current root for VLAN 10 is DSW2. Question is how to make DSW1 the primary root for VLAN 10. (Choose two)
    A. DSW2: change bridge priority to 61440 (highest)
    B. DSW1: change bridge priority to 4096 (lowest)
    C. DSW1: change port priority but in global configuration mode
    D. DSW1: change bridge priority but value is 0
    E. DSW1: change bridge priority but command is priority root

    Answer: B E

    I AGREE ANSWER B COULD BE CORRECT IF THE BRIDGE PRIORITY ON DSW2 IT’S USING DEFAULT PRIORITY OR IS LOWER HIGHER THAN 4096…

    ANSWER A IS CORRECT SINCE IS THE HIGHEST BRIDGE PRIORITY…

    ANSWER E SEEMS AWFULLY INCORRECT SINCE THERE IS NO COMMAND “PRIORITY ROOT” THE ACTUAL COMMAND TO MAKE A SWITCH THE ROOT IS “ROOT PRIMARY” FOR ANY VLAN.

    PLEASE CAN YOU EXPLAIN WHY THESE ANSWERS WERE SELECTED AS RIGHT???

  11. CCNP Question
    October 17th, 2018

    For Question 11
    Refer to the exhibit. Which statement describes the result if a switch that is running PVST+ is added to this network?

    What is the correct answer?
    Is it C?
    In cisco document: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/72836-rapidpvst-mig-config.html#rpvst

    “When any RSTP port receives legacy 802.1D BPDU, it falls back to legacy STP and the inherent fast convergence benefits of 802.1w are lost when it interacts with legacy bridges”

  12. Anonymous
    November 24th, 2018

    Q14 answer should be B and C

  13. Roxy
    January 10th, 2019

    @WISE FOOl
    i think that there is a mistake.
    C: it’s totally wrong
    D: the priority can be written only in multiples of 4096, so 0 isn’t accepeted
    E: this comand doesn’t exist

    The only corretc answers are A and B

  14. Guaedina Saelle
    February 27th, 2019

    Hello I want to comment about the question number 5.

    B & E are the correct answer.

    A. Is incorrect because changing the priority to the highest priority number is not going to put the switch in root state but the contrary it will be the less considered switch to put in root.

    C. Is incorrect the port priority can not be changed in global mode, only in a per port basis

    D. This option says: Change priority but value is 0. The priority value of the DSW1 is not 0, the priority for DSW1 is 28672.

    I hope this analysis that I made help you guys in obtaining your CCNP certification, yours truly,

    G.

  15. Guadeina Saelle
    February 27th, 2019

    Sorry, I have notice that the answer for question 5 is indeed A and B.

    A. Is correct, configuring DSW2 with the highest priority number will make the DSW2 the less considered switch to put as root switch.

    B. Is correct, configuring the priority number with 4096 value will set the DSW1 as root

    C. Is incorrect the port priority can not be changed in global mode, only in a per port basis

    D. This option says: “Change priority but value is 0”. The priority value of the DSW1 is not 0, the priority for DSW1 is 28672.

    E. is incorrect, it says that the command syntax is priority root, it is incorrect, the syntax command is root primary.

    I hope this correction helps you in order to achieve the CCNP certification, I am open to discuss any comment that I am making in this forum, kindly regards,

    G.

  16. LuciasDA
    May 31st, 2019

    @Roxy, @WISE FOOL and @CertPrepare
    —————————————————–
    I agree with Roxy on this;
    A: Is correct, as you are forcing DSW2 to have a higher priority. DSW1 has priority 28672 which is lower than those of ALSW 1 and 2.
    B: Is correct, as its lowest multiple of 4096
    C: Is utterly wrong.
    D: As mentioned the entries are in 4096 multiples, which is why B: is correct and D: is not.
    E: does not make sense at all ‘priority root’? the command would have to be “root primary”
    ############
    This is from my lab
    CORE_PRIME(config)#spanning-tree vlan 10 priority root
    ^
    % Invalid input detected at ‘^’ marker.

    CORE_PRIME(config)#spanning-tree vlan 10 root primary

  17. Engr Mark Penaverde
    September 19th, 2019

    Q5 – If we read carefully the question this say that “FOR VLAN 10” alone now if we choose A it will make DSW2 the lowest brigde priority on all switches which is not good in real world scenario and it will also drop his root bridge priority for vlan 20.
    If we now choose B this will also make DSW1 primary root for all vlan since it is now the lowest bridge priority on all switches.
    If we choose C – port priority is only configurable on interface config mode.
    If D – same explanation as B, it will make the lowest bridge priority in all switches so it will act as primary root bridge in all vlan.
    If E- there is no such command as priority root.

    Now the most correct answer will be ; config DWS1 – spanning-tree vlan 10 root primary and spanning-tree vlan 10 priority 20480 or lower the is incremented by 4096.(16384,12288,8192,4096) but this is not included in the options above.

    Hope this help. :)

  18. Dmytro
    October 5th, 2019

    Q3 I think answer “D. spanning-tree portfast trunk” is wrong, A and B are right.

  19. supportdonkey
    October 22nd, 2019

    So IP helper March edition has the same answer for question 5…even though I don’t think it is right…

  20. Burìk
    November 15th, 2019

    Q12 is wrong, the answer is B.

    spanning-tree extend system-id is also known as MAC address reduction.

  21. polleke
    January 11th, 2020

    Q5: Should be A and B.
    A. DSW2: change bridge priority to 61440 (highest)
    B. DSW1: change bridge priority to 4096 (lowest)
    E. “change bridge priority but command is “priority root” is wrong since the command is “spanning-tree vlan 10 root primary”

  22. Ridiculiculous
    January 18th, 2020

    @polleke – I agree about Q5 100%

  23. Anonymous
    January 25th, 2020

    @Cerprepare

    SO many confusions about Question 1 and 5.

    Please help with answers ?

  24. flkang
    February 6th, 2020

    Q3 i think is poorly explained.

    #spannig-tree portfast default —> will be applied in all ports that do not receive BPDUs

    # spanning-tree uplinkfast –> will learn mac address on ALT port to faster convergence in Switch-to-switch links, so it skips the STP timers

    I think this to options would be better, is E option is missing it may be UPLINKFAST

    Question 3
    Your manager asked you to make every port on your switch bypass the normal spanning-tree timers which includes your uplink to other switches. What two commands can you apply on the interfaces? (Choose two)

    A. spanning-tree portfast
    B. spanning-tree portfast default
    C. spanning-tree portfast access
    D. spanning-tree portfast trunk
    E. ?

  25. Q5 wrong answers
    February 8th, 2020

    @Certprepare, please, Could you modify the indicated answers? Should be A and B

  26. Q5 Wrong
    February 9th, 2020

    Answer should be D and E.
    B is incorrect because 4096 is not the lowest. 0 is.

    sw-office(config)#spanning-tree vlan 100 priority ?
    bridge priority in increments of 4096

  27. Q5 Wrong
    February 9th, 2020

    A is correct also correct. as DSW1 would then have the lowest Bridge Priority. While it would work, the down side is, the access switches would be the next highest if DSW1 failed, so it would be a bad life choice.

  28. Romain
    February 10th, 2020

    Q1 This question is stupid, PC1 doesn’t participate in spanning-tree… it’s a computer, so why would you change the spanning-tree priority on those links? This is non-sense…

  29. Peter Pan
    February 19th, 2020

    The only correct answers are A and D.

    A – Priority of 61440 would make it impossible to be the root. (CORRECT)
    B – Priority of 4096 (INCORRECT) The value can be set as “0”.
    C – Totally wrong
    D – Yes, 0 can be set. (CORRECT)
    E – Priority Root – (INCORRECT) NO such command

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