Specifications

CHAPTER 3
Spanning Tree
Root Port Election
The root port is the port that leads back to the root. Continuing with Figure 3-1, when A is acknowledged as the root, the
remaining bridges sort out their lowest cost path back to the A:
n Switch B: Uses the link to A with a cost of 19 (link speed of 100 Mb/s).
n Switch C: The connected link has a cost of 100 (Ethernet), the link through B has a path cost of 38 (two 100-Mb/s
links), and so B is chosen.
n 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.
n 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.
Designated Port Election
Designated ports are ports that lead away from the root. Obviously, all ports on the root bridge are designated ports (A–B
and A–C in Figure 3-1).
n Segment B–D: B has the lowest path cost to root (19 versus 119), so it is designated for this segment.
n Segment C–D: C has the lowest path cost to the root (100 versus 119), so it is designated for this segment.
n Segment B–C: B has the lowest path cost to the root (19 versus 100), so it is designated for this segment.
n Both segments D–E: D has the lowest cost to the root (57 versus 76), so it is designated for both segments.
Now the looped topology has been turned into a tree with A at the root. Notice that there are no more redundant links.
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue