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STP Questions 2

May 20th, 2017 Go to comments

Note: If you are not sure about Spanning Tree Protocol, please read our Spanning Tree Protocol STP Tutorial.

Question 1


When you enable PortFast on the switch, spanning tree places ports in the forwarding state immediately, instead of going through the listening, learning, and forwarding states.

Question 2

Question 3


Root port is the port on non-root bridge which is closest to the root bridge (in term of cost). Therefore when a port receives the best BPDU on a non-root bridge it will become the root port.

Question 4


We can configure the port priority to change the preferred forwarding interface as we wish. For example we can change the port priority of an interface for VLAN 20 to 64 as follows:

Switch(config-if)#spanning-tree vlan 20 port-priority 64

Question 5


Per VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It means a switch can be the root bridge of a VLAN while another switch can be the root bridge of other VLANs in a common topology. For example, Switch 1 can be the root bridge for Voice data while Switch 2 can be the root bridge for Video data. If designed correctly, it can optimize the network traffic.

Question 6

Question 7

Question 8


PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port.

In a valid configuration, PortFast-configured interfaces do not receive BPDUs (because PortFast should only be configured on interfaces which are connected to hosts). If a PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service.

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

Question 9

Question 10


First, the question asks what switch services the printers, so it can be Switch 3 or Switch 4 which is connected directly to the Printers.

Next, by comparing the MAC address of Switch 3 and Switch 4 we found that the MAC of Switch 3 is smaller. Therefore the interface connected to the Printers of Switch 3 will become designated interface and the interface of Switch 4 will be blocked. The picture below shows the roles of all ports:


DP: Designated Port
RP: Root Port
BP: Blocked Port

(Please notice that Switch 1 will become the root bridge because of its lowest priority, not Switch 3)

Question 11


PortFast causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states.

Question 12

Question 13

Question 14


Answer A is not correct as we can choose which switch to become root bridge by configuring bridge priority. The switch with lowest bridge priority (value) would become the root bridge.

For answer B, this paragraph from Cisco confirms it is the correct answer:

“When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. Each VLAN must have its own root bridge because each VLAN is a separate broadcast domain. The roots for the different VLANs can all reside in a single switch or in various switches.”

Reference: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

The meaning of answer C is not clear but maybe it means “every VLAN must use the same root bridge” which is not correct as Sw1 can be the root bridge for VLANs 1, 3, 5 but Sw2 can be the root bridge for VLAN 2, 4, 6…

From the quote above we can say answer D is not correct.

Comments (13) Comments
  1. Joel
    June 13th, 2017

    Found this VCE with the CCNA questions, can anyone confirm if this are valid https://drive.google.com/open?id=0B5mAFqgydmCza1ViTmFlNkQwYzg

  2. Lanny
    June 19th, 2017

    Thanks for sharing Joel, the dumps are valid.

  3. Anonymous
    June 24th, 2017

    Cannot open, please help

  4. Anonymous
    July 6th, 2017

    Or to be rude but q3 is bit confusing with the question be asked

  5. Anonymous
    July 6th, 2017


  6. Mike
    July 27th, 2017

    Question 9
    Ans: 802.1Q and STP

  7. Anonymous
    August 6th, 2017

    @Mike, 802.1D is the IEEE MAC Bridges standard which includes Bridging, Spanning Tree and others. IEEE 802.1Q is the networking standard that supports virtual LANs (VLANs) on an Ethernet network.

  8. Anonymous
    September 2nd, 2017

    Why does question 14 and 15 have different answers but asking for the same thing

  9. Anonymous
    September 4th, 2017

    Regarding Q14, I believe the correct answer is B, according to the official documentation from Cisco:

  10. 9tut
    September 4th, 2017

    @Anonymous: Yes, answer B is the correct answer. It is very difficult to understand when it stands alone. Thanks for your contribution!

  11. anon
    September 9th, 2017

    I question the explanation and answer on 14.

    Just because you lower the priority does not mean the election process halts right then. the process has to find which one has the lowest priority first. Once that is confirmed then it would stop.

    So I think A is a valid answer.

  12. johnyb
    November 9th, 2017

    Which statement about spanning-tree root-bridge election is true?
    A. It is always performed automatically
    B. Each VLAN must have its own root bridge
    C. Each VLAN must use the same root bridge
    D. Each root bridge must reside on the same root switch

    What is the correct answer in this question??A or C??

  13. Dave
    November 11th, 2017

    @johnyb the correct answer is B

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