Question: STP and Choosing a Root Port

Here’s a new CCNA-level question to consider, this time on Spanning Tree Protocol. Before diving in, be warned, it’s meant for study prep, not for assessment. That is, I’m not attempting to make this question exactly like an exam question, but rather, make it useful for forcing you to think about what you learned, apply it in a new way, and really understand this stuff.

In the switched network shown in the figure, consider only the spanning tree for VLAN 2. The switches use some mechanism for STP that creates a separate STP topology per VLAN, for instance, PVST+. The example that follows shows a portion of the output from a show command on switch S1.

Figure 1: Question Topology, Four Switches

Example 1: SW1 Output

S1#show spanning-tree vlan 2 
VLAN0002
Spanning tree enabled protocol ieee
Root ID    Priority    32770  
           Address     0019.e859.5380 
           This bridge is the root 
           Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec 

Bridge ID Priority     32770 (priority 32768 sys-id-ext 2)
          Address      0019.e859.5380
          Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
          Aging Time   15 sec 

! The rest of the output has been omitted

There are no other devices or links in the network. All the links physically work and are enabled (no shutdown). All the links use 802.1Q trunking. Using this description, answer the following questions:

1) Which of the following settings impact a switch’s choice of root port in VLAN 2, on at least one of the switches in this topology?

A) S2’s VLAN 2 STP cost on F0/1

B) S3’s VLAN 2 STP cost on F0/2

C) S4’s VLAN 2 port priority on F0/2

D) S2’s VLAN 2port priority on F0/3

E) S3’s VLAN 2 STP cost on F0/1

F) None of the above