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STP Questions 3

September 11th, 2017 in SWITCH 300-115 Go to comments

Question 1


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. But maybe there is something different in the exam.

Question 2

Question 3


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


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.

Question 5

Question 6


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


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


In this question DSW2 is the root bridge because of lowest priority value (24576) so all traffic must pass through it. 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). Therefore the 1Gbps port on ASW1 will be chosen as Root port -> traffic will go from ASW1 -> DSW2 -> core.

Note: 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.

Question 11

Question 12

  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


    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.


  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 .


  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.

  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

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