© 2002, Cisco Systems, Inc. All rights reserved.
© 2002, Cisco Systems, Inc. All rights reserved.
2
Spanning-Tree Protocol Overview
Objectives
Upon completing this lesson, you will be able to:
Describe the purpose and operation of the STP
Explain how to implement STP to solve the problems associated with redundant switched
or bridged topologies
Provides a loop-free redundant network topology by
placing certain ports in the blocking state.
Spanning-Tree Protocol

One root bridge per network
One root port per nonroot bridge
One designated port per segment
Nondesignated ports are unused
Spanning-Tree Operation
Bpdu = Bridge Protocol Data Unit
(default = sent every two seconds)
Root bridge = Bridge with the lowest bridge ID
Bridge ID =

In the example, which switch has the lowest bridge ID?
Spanning-Tree Protocol
Root Bridge Selection
Spanning-tree transits each port through several different states:
Spanning-Tree Port States
Spanning-Tree Port States (Cont.)
Spanning-Tree Path Cost
Spanning-Tree Example
Spanning-Tree Recalculation
Spanning-Tree Convergence
Convergence occurs when all the switch and bridge ports have transitioned to either the forwarding or the blocking state.
When the network topology changes, switches and bridges must recompute the Spanning-Tree Protocol, which disrupts user traffic.
Rapid Spanning-Tree Protocol
Rapid Transition to Forwarding
Summary
STP is a bridge-to-bridge protocol used to maintain a loop-free network.
STP establishes a root bridge, a root port, and designated ports.
With STP, the root bridge has the lowest bridge ID, which is made up of the bridge’s priority and MAC address.
With STP, ports transition through four states: blocking, listening, learning, and forwarding.
If a change occurs to the network topology, STP maintains connectivity by transitioning some blocked ports to the forwarding state.
RSTP significantly speeds the recalculation of the spanning tree when the network topology changes.