Home > ICND2 – STP Questions 2

ICND2 – STP Questions 2

October 27th, 2017 in ICND2 200-105 Go to comments

Question 1


SPT must performs three steps to provide a loop-free network topology:

1. Elects one root bridge
2. Select one root port per nonroot bridge
3. Select one designated port on each network segment -> Answer B is correct.

Question 2


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 3


Spanning Tree Protocol convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. When layer 2 is converged, root bridge is elected and all port roles (Root, Designated and Non-Designated) in all switches are selected.

Question 4


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 5


As we can see from the output above, the priority of the root bridge is 20481 while that of the local bridge is 32769.

Question 6


From the output, we see that all ports are in Designated role (forwarding state) -> A and E are correct.

The command “show spanning-tree vlan 30″ only shows us information about VLAN 30. We don’t know how many VLAN exists in this switch -> B is not correct.

The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768 -> C is correct.

All three interfaces on this switch have the connection type “p2p”, which means Point-to-point environment – not a shared media -> D is not correct.

The only thing we can specify is this switch is the root bridge for VLAN 30 but we can not guarantee it is also the root bridge for other VLANs -> F is not correct.

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.

Question 8

Question 9


Only non-root bridge can have root port. Fa0/11 is the root port so we can confirm this switch is not the root bridge -> A is not correct.

From the output we learn this switch is running Rapid STP, not PVST -> B is not correct.

0017.596d.1580 is the MAC address of this switch, not of the root bridge. The MAC address of the root bridge is 0017.596d.2a00 -> C is not correct.

All of the interface roles of the root bridge are designated. SwitchA has one Root port and 1 Alternative port so it is not the root bridge -> D is correct.

Question 10

Question 11

Question 12

Question 13

Comments (1) Comments
  1. AC/DC
    December 11th, 2017

    Q13 imho is wrong, the only correct answer is E. The A is wrong cause it’s the Priority (2 bytes) part of the Bridge ID which is composed by a 4bit Bridge priority and a 12bit system ID extension