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STP Questions

March 23rd, 2017 in ICND2 200-105 Go to comments

Question 1

Question 2


Cisco developed PVST+ to allow strolling numerous STP instances, even over an 802.1Q network via the use of a tunneling mechanism. PVST+ utilizes Cisco gadgets to hook up with a Mono Spanning Tree area to a PVST+ region. No particular configuration is needed to attain this. PVST+ affords assist for 802.1Q trunks and the mapping of a couple of spanning trees to the single spanning tree of popular 802.1Q switches strolling Mono Spanning Tree.

Question 3


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 4


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 5


Rapid PVST+—This spanning-tree mode is the same as PVST+ except that is uses a rapid convergence based on the IEEE 802.1w standard. To provide rapid convergence, the rapid PVST+ immediately deletes dynamically learned MAC address entries on a per-port basis upon receiving a topology change. By contrast, PVST+ uses a short aging time for dynamically learned MAC address entries.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_53_se/configuration/guide/2960scg/swstp.html

Question 6


PVST+ is based on IEEE802.1D Spanning Tree Protocol (STP). But PVST+ has only 3 port states (discarding, learning and forwarding) while STP has 5 port states (blocking, listening, learning, forwarding and disabled). So discarding is a new port state in PVST+.

Question 7


This is a tricky question. We know the switch with lowest value of priority is elected the root switch. Therefore in this question the switches with priority of 4096, 8192, 12288 (which are lower than the current value of the root bridge 16384) are not joining the root bridge election by somehow. The only suitable answer is the switch with priority 20480 will become the root bridge.

Comments (7) Comments
  1. RCC
    March 26th, 2017

    Question 5> Correct me if I’m wrong, but isn’t the correct answer (D) since the question is referring to RSTP not STP. I was under the impression Rapid PVST got rid of timers?

  2. 9tut
    March 27th, 2017

    @RCC: Yes, the answer of Q5 should be D. Thanks for your detection. We have just fixed it!

  3. Kumo
    April 23rd, 2017

    Hi shouldn’t the answer for Q1 be D ? because the priority of fa0/10 to vlan 2 is 128.2 so not the best value. therefore it doesn’t have the best bridge id for vlan 2, and hence wasn’t elected as root for vlan 2?

  4. CR
    May 2nd, 2017

    Could someone please explain the why the answer to Q 7 is “A”. To me C feels right.

  5. gotthatpma
    May 11th, 2017

    I think the idea for question 7 is that the election was ALREADY held, and the switches for B, C, and D lost for some reason (or else C would already be the root switch). Therefore, only choice A makes sense.

  6. Nirbad
    May 17th, 2017

    You have configured your switches with the spanning-tree vlan x root primary and spanning-tree vlan x root secondary commands. Which of the following tertiary switch will
    take over if both switches fail?

    A. A switch with priority 4096
    B. A switch with priority 8192
    C. A switch with priority 12288
    D. A switch with priority 20480

  7. Atef
    May 26th, 2017

    i dont find questions… just the Explanation part is here… where are all the Questions anyway?