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Spanning-Tree ProtocolOverview

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Objectives

Upon completing this lesson, you will beable to:

Describe the purpose and operation of the STP Explain how to implement STP to solve the

problems associated with redundant switchedor bridged topologies

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Provides a loop-free redundant network topology byplacing 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 the 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 Path Cost

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Spanning-Tree Example

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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 the

Spanning-Tree Protocol, which disrupts usertraffic.

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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 aloop-free network.

STP establishes a root bridge, a root port, and designatedports.

With STP, the root bridge has the lowest bridge ID, whichis made up of the bridges priority and MAC address.

With STP, ports transition through four states: blocking,listening, learning, and forwarding.

If a change occurs to the network topology, STPmaintains connectivity by transitioning some blockedports to the forwarding state.

RSTP significantly speeds the recalculation of thespanning tree when the network topology changes.

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