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Configuring Catalyst Switch Operations

Introducing Spanning TreeProtocol

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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

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Provides a loop-free redundant network topology by

placing certain ports in the blocking state

Spanning Tree Protocol

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One root bridge per network

One root port per nonroot bridge

One designated port per segment

Nondesignated ports are unused

Spanning Tree Operation

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BPDU = Bridge Protocol Data Unit

(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 ProtocolRoot Bridge Selection

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Spanning tree transits each port throughseveral 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 andbridge ports have transitioned to either theforwarding 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 aroot bridge, a root port, and designated ports.

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

When STP is enabled, every bridge in the network goesthrough the blocking state and the transitory states oflistening and learning at power up. If properly configured, theports then stabilize to the forwarding or blocking state.

If the network topology changes, STP maintains connectivityby transitioning some blocked ports to the forwarding state.

RSTP significantly speeds the recalculation of the spanningtree when the network topology changes.

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