Spanning Tree Basics
Fixed Switch Configuration Guide 15-11
that port will be selected as root. In the case of no single port having a lowest port priority, the root
port is selected based upon the overall port ID value. Figure 15-5 on page 15-11 presents a root
port configuration for Bridge B determined by the port priority setting. If there is still a tie, these
ports are connected via a shared medium. The final tie breaker is the receiving port ID.
Figure 15-5 Root Port Selection Based On Lowest Port ID
After selecting the root port, the bridge’s cost to root is the total of the root port’s designated cost
as advertised in the received BPDU, plus the path cost associated with that port. In a hierarchically
designed network, the designated cost for ports attached to the next higher level will be less than
the bridge’s cost to the root. Ports attached to bridges on the same level will have designated costs
equal to the bridge’s cost to the root. Likewise, this bridge’s port will advertise the bridge’s cost to
the root. Thus one port connected to the LAN will be the designated port and the other(s) will be
alternate. The port on the bridge with the lower ID will be the designated port.
Cost 10
Port Priority=128
Alternate
Blocked Port
Cost 10
Port Priority=112
Root
Bridge A
Bridge B
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