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VTP Lab 2

March 9th, 2017 in Lab Sim, LabSim Go to comments


Answer and Explanation

1) “CORE should be the root bridge for VLAN 20; however, DSW1 is currently the root bridge for VLAN 20” -> We need to make CORE switch the root bridge for VLAN 20.

By using the “show spanning-tree” command as shown above, we learned that DSW1 is the root bridge for VLAN 20 (notice the line “This bridge is the root”).

DSW1#show spanning-tree


To determine the root bridge, switches send and compare their priorities and MAC addresses with each other. The switch with the lowest priority value will have highest priority and become the root bridge. Therefore, we can deduce that the priority of DSW1 switch is lower than the priority of the CORE switch so it becomes the root bridge. To make the CORE the root bridge we need to increase the DSW1’s priority value, the best value should be 61440 because it is the biggest value allowed to assign and it will surely greater than of CORE switch. (You can use another value but make sure it is greater than the CORE priority value by checking if the CORE becomes the root bridge or not; and that value must be in increments of 4096).

(Notice that the terms bridge and switch are used interchangeably when discussing STP)

DSW1#configure terminal
DSW1(config)#spanning-tree vlan 20 priority 61440

2) “Traffic for VLAN 30 should be forwarding over the gig 1/0/6 trunk port between DSW1 and DSW2. However VLAN 30 is currently using gig 1/0/5”

DSW1 is the root bridge for VLAN 30 (you can re-check with the show spanning-tree command as above), so all the ports are in forwarding state for VLAN 30. But the question said that VLAN 30 is currently using Gig1/0/5 so we can guess that port Gig1/0/6 on DSW2 is in blocking state (for VLAN 30 only), therefore all traffic for VLAN 30 will go through port Gig1/0/5.


The root bridge for VLAN 30, DSW1, originates the Bridge Protocol Data Units (BPDUs) and switch DSW2 receives these BPDUS on both Gig1/0/5 and Gig1/0/6 ports. It compares the two BPDUs received, both have the same bridge-id so it checks the port cost, which depends on the bandwidth of the link. In this case both have the same bandwidth so it continues to check the sender’s port id (includes port priority and the port number of the sending interface). The lower port-id value will be preferred so the interface which received this port-id will be the root and the other interface (higher port-id value) will be blocked.

In this case port Gig1/0/6 of DSW2 received a Priority Number of 128.6 (means that port priority is 128 and port number is 6) and it is greater than the value received on port Gig1/0/5 (with a Priority Number of 128.5) so port Gig1/0/6 will be blocked. You can check again with the “show spanning-tree” command. Below is the output (notice this command is issued on DSW1 – this is the value DSW2 received and used to compare).


Therefore, all we need to do is to change the priority of port Gig1/0/6 to a lower value so the neighboring port will be in forwarding state. Notice that we only need to change this value for VLAN 30, not for all VLANs.

DSW1(config)#interface g1/0/6
DSW1(config-if)#spanning-tree vlan 30 port-priority 64

3) “Traffic for VLAN 40 should be forwarding over the gig 1/0/5 trunk port between DSW1 and DSW2. However VLAN 40 is currently using gig 1/0/6”

Next we need to make sure traffic for VLAN 40 should be forwarding over Gig1/0/5 ports. It is a similar job, right? But wait, we are not allowed to make any configurations on DSW2, how can we change its port-priority for VLAN 40? There is another solution for this…

Besides port-priority parameter, there is another value we can change: the Cost value (or Root Path Cost). Although it depends on the bandwidth of the link but a network administrator can change the cost of a spanning tree, if necessary, by altering the configuration parameter in such a way as to affect the choice of the root of the spanning tree.

Notice that the Root Path Cost is the cost calculated by adding the cost in the received hello to the cost of the interface the hello BPDU was received. Therefore if you change the cost on an interface of DSW1 then only DSW1 will learn the change.

By default, the cost of a 100Mbps link is 19 but we can change this value to make sure that VLAN 40 will use interface Gig1/0/5.

DSW1(config)#interface g1/0/5
DSW1(config-if)#spanning-tree vlan 40 cost 1


You should re-check to see if everything was configured correctly:

DSW1#show spanning-tree

Save the configuration:

DSW1#copy running-config startup-config

(Notice: Many reports said the copy running-config startup-config didn’t work but they still got the full mark)

Remember these facts about Spanning-tree:

Path Selection:

1) Prefer the neighbor advertising the lowest root ID
2) Prefer the neighbor advertising the lowest cost to root
3) Prefer the neighbor with the lowest bridge ID
4) Prefer the lowest sender port ID

Spanning-tree cost:


Other good resource for reference:



Other lab-sims in this site:

LACP with STP Sim
Spanning Tree Lab Sim
AAAdot1x Lab Sim


Comment pages
1 2 3 48
  1. cl
    August 30th, 2017

    For Edwin: since DSW1 is not the root for vlan 40, modifying the port-priority won’t help. The port-id of DSW2 is used as a tiebreaker (not its own port-id).
    Simpler said, you must modify port-priority to influence neighbor port election but use cost to influence own ports selection.

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    September 6th, 2017

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  5. CertPrep Nickname
    September 9th, 2017

    Why the heck is this called VTP Lab 2? What has this got to do with VTP?

  6. CertPrep Nickname
    September 9th, 2017

    Also, it seems everyone is blind in the comments.

    Can anyone who has taken the exam recently confirm if this lab ever comes up?

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  24. jey2539
    December 19th, 2017

    do i need to do the configuration on both switches DSW1 and DSW2. on PT practice after changing both ports port-priority i notice the port state changing from blk to fwd

  25. Marcus
    December 19th, 2017

    What about inverse solutions to questions 2 and 3? Instead of REDUCING the priority on gi1/0/6 in the second question, we can INCREASE the priority on port gi1/0/5 and get the same result. Similarly, in the third question, we can INCREASE the cost on port gi1/0/6 instead of REDUCING it on gi1/0/5 and it also will be appropriate solution. Or not?

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  32. Hamid
    February 21st, 2018

    Can some please tell me why this lab is named as VTP LAB 2? It should be STP lab, isn’t it?

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    March 2nd, 2018


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  37. Confuse guy
    March 28th, 2018


    I agree with you.

  38. Confuse guy
    March 28th, 2018


    to add on we need to watch out for the cost value prior to playing with the port priority. :)

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    March 30th, 2018

    For CCNP Switch exam in March 2018, should we prepare for this ?

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  47. Remlin
    September 30th, 2018

    why cant we just apply spanning-tree vlan 40 port-priority 64 for vlan 40 instead of spanning-tree vlan 40 cost 1?


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  49. PeteSeguro
    October 10th, 2018

    we are not allowed to do any type of config on DSW2, and port-priority is only relevant on the upstream switch not locally, in case we change the port priority on DSW1 for interface g1/0/5 the choosen path would still be the same. Thus by artificially dcreasing the path cost on g1/0/5 it will become the forwarding port.

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