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CCNA Semester 3

Chapter 06Chap

ter 06

SPANNING TREESPANNING TREEPROTOCOLPROTOCOL

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ObjectivesObjectives

Define redundancy and its importance innetworking

Describe the spanning tree in a redundant-

path switched network

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Table of ContentTable of Content

1 Redundant topologies2 Spanning tree protocol

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REDUNDANT TOPOLOGIESREDUNDANT TOPOLOGIES

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Redundant switched topologiesRedundant switched topologies

Segment 1

Segment 2

Switch A Switch B

Router YServer/Host X

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Broadcast stormsBroadcast storms

Segment 1

Segment 2

Switch A Switch B

Router YServer/Host X

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Multiple frame transmissionsMultiple frame transmissions

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Media access control database instabilityMedia access control database instability

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SPANNING TREE PROTOCOLSPANNING TREE PROTOCOL

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Spanning Tree ProtocolSpanning Tree Protocol

The ST Algorithm, implemented by the STP,

prevents loops by calculating a stablespanning-tree network topology.

Spanning-tree frames, called bridge protocoldata units (BPDUs), are sent and receivedby all switches in the network at regularintervals and are used to determine thespanning-tree topology.

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Provides a loop-free redundant network topologyplacing certain ports in the blocking state.

Spanning-Tree ProtocolSpanning-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 =

Spanning-Tree Protocol Root Bridge Selection

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Spanning-tree transits each portthrough several different states:

Spanning-Tree Port StatesSpanning-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 RecalculationSpanning-Tree Recalculation

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Spanning-Tree ConvergenceSpanning-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 theSpanning-Tree Protocol, which disrupts usertraffic.

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Rapid Spanning-Tree ProtocolRapid Spanning-Tree Protocol

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Rapid Transition to Forwarding

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SummarySummary

The benefits and risks of a redundant topology

The role of spanning tree in a redundant-pathswitched network

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Q&AQ&A