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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 48
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    August 5th, 2016

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  7. Anonymous
    August 19th, 2016

    1 thing to remember – if you want to priority designated port – use spanning-tree vlan xx port-priority, if you want to priority alt/blck port – use spanning-tree vlan xx cost. About sim- its impossible to do it on packet tracer because there is no comand: spanning-tree vlan xx cost

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    October 23rd, 2016

    Does anybody got this lab (VTP Lab 2)?????

    Is it valid???

  13. raj
    November 8th, 2016

    why i am not able to type command DSW1(config-if)#spanning-tree vlan 40 cost 1
    it is not taking the command after vlan 40

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    November 10th, 2016

    Hi Raj, take time to read previous comment and you will find your answer

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  16. zer0
    November 12th, 2016

    About question(2), the below is also OK? If not, why cannot I use “cost” instead of “priority”?

    DSW1(config)#interface g1/0/6
    DSW1(config-if)#spanning-tree vlan 30 cost 1

  17. Jake
    December 26th, 2016

    In the simulator ‘spanning-tree vlan 30 cost 1’ fails but works on exam

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  26. Anonymous
    February 10th, 2017

    men why not change port-priority in case vlan 40

  27. Anonymous
    February 10th, 2017

    men why not change port-priority in case vlan 40 instade of cost

  28. Viktor Zverev
    February 11th, 2017

    Anonymous, because the root for Vlan 40 is DSW2 and it originates BPDUs for Vlan 40. So DSW1 does make a decision which port to use for Vlan 40 according to two pieces of information received on two ports gi1/0/5 and gi1/0/6: 1) originating bridge-id (the same on two ports), 2) Root Path Cost (the same again on two ports and equal to 4). So we alter the cost of the gi1/0/5 by mentioned command and made it equal to 1. So DSW1 will use gi1/0/5 for Vlan 40 and will block gi1/0/6 for Vlan 40.
    In case of Vlan 30 we have DSW1 as root of Vlan 30 (all ports of DSW1 in Vlan 30 are in Forwarding state and have a Desg role), so DSW2 should make a decision which port to use for Vlan 30 and which port to block. So another method have been used. As mentioned in explanation the sequence of make a decision is: sender bridge-id (the same), root path cost (the same and equal to 4), the sender’s port id (different, but not as it is necessary to us). Therefore, we changed the port priority, which is the part of the port id, as it is necessary to us.

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