Given the partial outputs displayed in the graphic, which switch device would be the spanning tree
root bridge for a network of only these four devices?
A.
Miami
B.
Cairo
C.
London
D.
Tampa
Explanation:
Cairo is the correct answer because it has the lowest Bridge priority. The default priority value
is 32768 (same as Tampa ), and the bridge with the lowest priority will become the root bridge.
A root bridge is chosen based on the results of the BPDU process between the switches. Initially,
every switch considers itself the root bridge. When a switch first powers up on the network, it
sends out a BPDU with its own BID as the root BID. When the other switches receive the BPDU,
they compare the BID to the one they already have stored as the root BID. If the new root BID has
a lower value, they replace the saved one. But if the saved root BID is lower, a BPDU is sent to
the new switch with this BID as the root BID. When the new switch receives the BPDU, it realizes
that it is not the root bridge and replaces the root BID in its table with the one it just received. The
result is that the switch that has the lowest BID is elected by the other switches as the root bridge.