© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—1-1

Configuring Catalyst Switch Operations

Introducing Spanning Tree Protocol

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—1-2

Outline

• Overview

• Spanning Tree Protocol

• Spanning-Tree Operation

• Root Bridge Selection

• Spanning-Tree Port States

• Spanning-Tree Path Costs

• Spanning-Tree Recalculation

• Rapid Spanning Tree Protocol

• Summary

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—1-3

• Provides a loop-free redundant network topology by placing certain ports in the blocking state

Spanning Tree Protocol

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Spanning-Tree Operation

• One root bridge per broadcast domain

• One root port per nonroot bridge

• One designated port per segment

• Nondesignated ports are unused

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—1-5

• BPDU (default = sent every two seconds)

• Root bridge = bridge with the lowest bridge ID

• Bridge ID =

In this example, which switch has the lowest bridge ID?

Spanning Tree Protocol Root Bridge Selection

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• Spanning tree transits each port through several different states:

Spanning-Tree Port States

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Spanning-Tree Port States (Cont.)

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Spanning-Tree Operation

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Spanning-Tree Path Cost

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Spanning-Tree Recalculation

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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 STP, which disrupts user traffic.

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Rapid Spanning-Tree Protocol

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Rapid Transition to Forwarding

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Summary

• STP is a bridge-to-bridge protocol used to maintain a loop-free network.

• To maintain a loop-free network topology, STP establishes a root bridge, a root port, and designated ports.

• With STP, the root bridge has the lowest BID, which is made up of the bridge priority and the MAC address.

• When STP is enabled, every bridge in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, the ports then stabilize to the forwarding or blocking state.

• If the network topology changes, 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.

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—1-15