Defining the Spanning Tree Protocol – STP
Forming an association with the root bridge

Non-root bridges place various ports into their proper roles by listening to BPDUs as they come in on all ports. Receiving BPDUs on multiple ports indicates a redundant path to the root bridge.

The switch looks at these components in the BPDU to determine which switch ports will forward data and which switch ports will block data:

  • Lowest path cost
  • Lowest Sender BID
  • Lowest Local port ID

The switch looks at the path cost first to determine which port is receiving the lowest cost path. The path is calculated based on the link speed and the number of links the BPDU traversed. If a port has the lowest cost, that port is eligible to be placed in forwarding mode. All other ports that are receiving BPDUs continue in blocking mode.

If the path cost and Sender BID are equal, as with parallel links between two switches, the switch goes to the port ID as a "tiebreaker." The port with the lowest port ID forwards data frames, and all other ports continue to block data frames.

Path Cost
The spanning tree path cost is a value advertised in the BPDU by each bridge. This is a value representing the cumulative cost of all the links from the root bridge to the switch sending the BPDU. Path cost value is used by the receiving switch to determine the best path to the root bridge. The lowest cost is considered to be the best path.

Post cost values per link are shown above under the Revised IEEE Spec with the lower values being associated with higher bandwidth, and therefore being the more desirable paths. This new specification uses a nonlinear scale with port cost values as shown. In the previous IEEE specification, the cost value was calculated based on Gigabit Ethernet being the maximum Ethernet bandwidth, with an associated value of 1, from which all other values were derived in a linear manner.

Selecting the Root Port
Switch Y receives a BPDU from the root bridge (switch X) on its switch port on the Fast Ethernet segment and another BPDU on its switch port on the Ethernet segment. The root path cost in both cases is zero. The local path cost on the Fast Ethernet switch port is 19, while the local path cost on the Ethernet switch port is 100. As a result, the switch port on the Fast Ethernet segment has the lowest path cost to the root bridge and is elected the root port for switch Y.

Selecting the Designated Port
STP selects one designated port per segment to forward traffic. Other switch ports on the segment become nondesignated ports and continue blocking. The nondesignated ports receive BPDUs but do not forward data traffic to prevent loops. The switch port on the segment with the lowest path cost to the root bridge is elected the designated port. If multiple switch ports on the same segment have the same cost, the switch port with the lowest port ID becomes the designated port.

Because ports on the root bridge all have a root path cost of zero, all ports on the root bridge are designated ports.