The switches in the exhibit are configured with the default parameters. What can be determined
from the exhibit?
A.
Switch3 is the root bridge.
B.
Fa0/3 on Switch1 and Fa0/3 on Switch2 are attached to the lowest-cost paths to the root
bridge.
C.
Switch1 is the backup designated root bridge.
D.
Fa0/2 on Switch1 and Fa0/2 on Switch2 are nondesignated ports.
Explanation:
Remember that one root switch is identified per-VLAN. After the root switch identification, the
switches adhere to these rules: STP Rule 1 -All ports of the root switch must be in forwarding
mode.
Next, each switch determines the best path to get to the root. The switches determine this path by
a comparison of the information in all the BPDUs that the switches receive on all ports. The switch
uses the port with the least amount of information in the BPDU in order to get to the root switch;
the port with the least amount of information in the BPDU is the root port. After a switch
determines the root port, the switch proceeds to rule 2. STP Rule 2 -The root port must be set to
forwarding mode.
In addition, the switches on each LAN segment communicate with each other to determine which
switch is best to use in order to move data from that segment to the root bridge. This switch is
called the designated switch. STP Rule 3 -In a single LAN segment, the port of the designated
switch that connects to that LAN segment must be placed in forwarding mode. STP Rule 4 -All the
other ports in all the switches (VLAN-specific) must be placed in blocking mode. The rule only
applies to ports that connect to other bridges or switches. STP does not affect ports that connect
to workstations or PCs. These ports remain forwarded.
Our answer can be found in Rule 1 above. Since none of the ports in SW-3 are blocking, we can
determine that this switch must be the root.
Reference:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009
467c.shtml