This document discusses the Spanning Tree Protocol (STP) which provides a loop-free network topology by placing ports into blocking states. It describes how STP elects a root bridge, establishes root and designated ports, and transitions ports between blocking and forwarding states. The document also introduces Rapid Spanning Tree Protocol which speeds up STP's recalculation of the spanning tree when the network topology changes.

1. Configuring Catalyst Switch Operations
Introducing Spanning Tree Protocol
Lecture 6
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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
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3. Spanning Tree Protocol
• Provides a loop-free redundant network topology by
placing certain ports in the blocking state
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4. Spanning-Tree Operation
• One root bridge per broadcast domain
• One root port per nonroot bridge
• One designated port per segment
• Nondesignated ports are unused
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5. Spanning Tree Protocol
Root Bridge Selection
• 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?
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6. Spanning-Tree Port States
• Spanning tree transits each port through several different
states:
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7. Spanning-Tree Port States (Cont.)
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8. Spanning-Tree Operation
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9. Spanning-Tree Path Cost
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10. Spanning-Tree Recalculation
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11. 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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12. Rapid Spanning-Tree Protocol
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13. Rapid Transition to Forwarding
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14. 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—1-14