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7/29/2019 ICND22S01L03
1/15
2004 Cisco Systems, Inc. All rights reserved. ICND v2.21-1
Configuring Catalyst Switch Operations
Introducing Spanning TreeProtocol
7/29/2019 ICND22S01L03
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2004 Cisco Systems, Inc. All rights reserved. ICND v2.21-2
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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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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