Root Port Election
The root port is. the port that leads back to the root. Continuing with Figure
3-1, once A is acknowledged as the root, the remaining bridges sort out their
lowest cost path back to the A.
■ Switch B—Uses the link to A with a cost of 19 (link speed of 100
Mbps).
■ Switch C—The connected link has a cost of 100 (Ethernet), the link
through B has a path cost of 38 (two 100 Mbps links), and so B is
chosen.
■ Switch D—The link through B has a path cost of 119, the path cost
through C to A is 119, the path through C then B is 57, so C is chosen.
■ Switch E—The lowest path cost is the same for both ports (76 through
D to C to B to A). Next check sender BID—sender for both ports is D,
so that it does not break the tie. Next check sender Port ID. Assuming
default port priority, the PID for 0/1 is lower than the PID for 0/2, so
the port on the left is the root port.