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STP Questions 3

September 11th, 2017 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”).

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

Question 12

Question 13

Question 14

Explanation

The Portfast feature can be configured on both access or trunk port. This feature instructs the port to skip listening and learning state and move to forwarding state immediately.

Question 15

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
  1. BoZZ
    July 10th, 2017

    Please update the link to the questions

  2. Dee
    July 16th, 2017

    can someone kindly explain question 4 and 8?

  3. mikeSWE
    July 19th, 2017

    @Dee I have a seen different dumps saying diameter is correct.

    I am not sure on that one @Cetprepare anyone please help.

  4. CCNP boy
    July 24th, 2017

    In Q9 the answer is A ( 128 ) but if this value increase in 32 ( 0-32-64-128….) then why 0 value answer C is not correct ?

  5. jonathan
    July 30th, 2017

    On Question 6 why A. spanning-tree mstp 1 priority 0 is not correct answer

  6. Peter Pan
    August 20th, 2017

    Q5. why not D instead of B ? 0 is the lowest priority which can be configured in cisco switches.

  7. Peter Pan
    August 20th, 2017

    @ jonathan “mstp” instead of “mst” ?

  8. BoZZ
    August 24th, 2017

    Q4 explanation – Macro commands are a set of ordinary CLI commands combined together. Think of it as an alias or shortcut for a script – applying the macro command runs those normal commands in one go.

    Example Macro Command that changes priority to make switch the root as well as changes the timers:

    SWITCH> (enable) set spantree root 8 diameter 4 hello 2
    VLAN 8 bridge priority set to 8192.
    VLAN 8 bridge max aging time set to 14.
    VLAN 8 bridge hello time set to 2.
    VLAN 8 bridge forward delay set to 10.
    Switch is now the root switch for active VLAN 8.

  9. BoZZ
    August 24th, 2017

    Q8 is simply asking which commands can be used to configure LoopGuard.

    A will do it for all ports.
    B will do it for individual port.

    Other options are incorrect.

  10. CaCol
    September 9th, 2017

    Q4.

    Answer : Diameter.

    -You can configure the root bridge with the network diameter for a specific VLAN, and the timer values are computed accordingly. Cisco recommends that, if you must make changes, only configure the diameter and optional hello time parameters on the root bridge for the VLAN.

    spanning-tree vlan vlan-id [root {primary | secondary}] [diameter diameter-value [hello hello-time]]

    !— This command needs to be on one line.

    This macro makes the switch root for the specified VLAN, computes new timer values on the basis of the diameter and hello time specified, and propagates this information in configuration BPDUs to all other switches in the topology.

    https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html

  11. Anonymous
    October 3rd, 2017

    Q1.. The priority values are 0, 32, 64, 96, 128, 160, 192, and 224. All other values are rejected.(cisco) ..so definitely E .

    https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/n5k/commands/spanning-tree-port-priority.html

  12. Anonymous
    October 7th, 2017

    Can someone explain Q1 to me please?
    I must be misunderstanding the topology.
    I thought port priority should be set on the up-stream switch.
    Thanks.

  13. Marc
    October 30th, 2017

    Q5 and Q6: Why choose priority 4096 instead of 0? 0 is a valid priority value and is numerical lower therefor superior to 4096. Or did I miss something?

  14. Mago
    November 15th, 2017

    I think Q2 is B)

    Default STP mode for switches is currently Rapid PVST+

  15. Alan
    November 20th, 2017

    Does anyone know why in Q5 and Q6 a priority of 4096 is prefered to a priority of 0 ?

  16. Marcus
    December 7th, 2017

    What about Q10 and link between DSW1 and ASW1? We can see TWO red lines there. Could we assume an etherchannel there? In this case STP will take into account the portchannel path cost which MUST be lower than cost of single 10G link. So, preferred path will include DSW1 and answer D is correct, isn’t it?

  17. RA
    December 12th, 2017

    Q10 is just terribly explained. Can anyone please better explain the answer for Question 10?

  18. wasntme
    December 14th, 2017

    Q10 is rather tricky tbh. are those 2 10G links in a port-channel? if yes, than it should have a cost of 1, to which you add 2 from 10G between distribution, making it better than the 4 between dsw2 and asw1. But if is not, then both have same cost to reach root. This means that we should know the port-priority, then port-number…..I think this diagram is missing info. if anyone has some more info, that will be great.

  19. wasntme
    December 14th, 2017

    Correction: before port id, the tie breaker is bridge id, which again we dont know. for sure info is missing.

  20. CoanX
    December 19th, 2017

    In Q6 “spanning-tree mstp” options are incorrect so the options with “spanning-tree mst” are correct, therefore opt D is correct. Option E is not crrect also because priority 1 is not correct.

    In regards to Q5 option D should be the correct one. Priority value of 0 is the lowest available.

  21. Darla
    January 3rd, 2018

    in q5, why is priority root correct? there is no option to configure this way. should answer be A B

    (config)#spanning-tree vlan 2 root ?
    primary Configure this switch as primary root for this spanning tree
    secondary Configure switch as secondary root

    (config)#spanning-tree vlan 2 priority ?
    bridge priority in increments of 4096

  22. YAB
    January 20th, 2018

    is question 1 correctly written? that Q1 does not make sense.

  23. 1WAY
    February 14th, 2018

    Q1 needs a diagram.

  24. mlefran
    February 18th, 2018

    All questions that explain a diagram should have one instead of trying to explain it

  25. Sergio
    March 11th, 2018

    Q3
    correct answers are not A, D but B, D:

    Switch(config)#spanning-tree portfast ?
    bpdufilter Enable portfast bdpu filter on this switch
    bpduguard Enable portfast bpdu guard on this switch
    default Enable portfast by default on all access ports

  26. Confuse guy
    March 15th, 2018

    @Sergio

    A and D confirmed correct.

    It ask configuration on interface level
    Your posted command is on global level

  27. recertordie
    March 16th, 2018

    Q.94. #show spanning-tree vlan 200 output
    Which two statements about output are true?

    It appears that the root bridge has it’s STP timers changed to 10,20,30. I know the defaults are 2,20,15 which is seen in output of the current switch.

    I also know that the Forward Delay (30) is the combined time spent in the Listen (15) and Learn (15) state, so how could the correct answers be?

    B. The time spent in the listen state will be 30 seconds
    F. BPDU’s sent every 10 seconds

    The correct answer is:
    A: BPDU’s sent every 2 seconds (2 is default so true)
    c: The time spent in the learning state will be 15 seconds (15 is default so true)

  28. Anonymous
    April 4th, 2018

    Q5 I think answer should be A,D
    Why don’t make priority 0 rather than 4096?
    there is no “priority root” command it must be “root primary” so this one must be wrong.
    Making DSW2 priority highest 61440 is another solution, why this is not correct? I will take the exam 2 days later, and confused, please help anyone knows it. thanks.

  29. Anonymous
    April 7th, 2018

    Q6

    Rack1SW1(config)#spanning-tree vlan 1 priority 1
    % Bridge Priority must be in increments of 4096.
    % Allowed values are:
    0 4096 8192 12288 16384 20480 24576 28672
    32768 36864 40960 45056 49152 53248 57344 61440
    https://supportforums.cisco.com/t5/lan-switching-and-routing/stp-priority-values/td-p/891247

  30. lalaversa
    May 5th, 2018

    Q1.
    Fake question… better answer is surely E but if the default port priority is configured on DSW2 (as written on the question, default value is 128) also A could be correct…. ugly and needless question…

  31. 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.

  32. 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.

  33. 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

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    May 30th, 2018

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  35. YF
    June 12th, 2018
  36. nataliya
    June 19th, 2018

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

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