Spanning Tree ProtocolSpanning Tree Protocol

STPSTP

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TopicsTopics

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• REDUNDANT LINKSREDUNDANT LINKS

• Issues (layer2 loop - broadcast storm - Duplicate Unicast Issues (layer2 loop - broadcast storm - Duplicate Unicast frame)frame)

• Spanning tree algorithmSpanning tree algorithm

• Choose the root bridgeChoose the root bridge

• Select root portsSelect root ports

REDUNDANT LINKSREDUNDANT LINKS

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layer2 looplayer2 loop

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broadcast stormbroadcast storm

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A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed.

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Duplicate Unicast frameDuplicate Unicast frame

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BPDUBPDU

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3276932769 000A00222222000A00222222

BRIDGE PROTOCOL DATA BRIDGE PROTOCOL DATA UNITUNIT

Choose the root bridgeChoose the root bridge

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• Each switch has a bridge ID (BID) of priority value followed by MAC address

• Switches exchange Bridge Protocol Data Units (BPDUs) to compare bridge IDs

• The switch with the lowest bridge ID becomes the root bridge

• Administrator can set the priority to fix the selection

Spanning tree algorithmSpanning tree algorithm

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The switches use this algorithm to decide which ports should be shut down.

1. Choose one switch to be “root bridge”

2. Choose a “root port” on each other switch

3. Choose a “designated port” on each segment.

4. Close down all other ports.

• Every non-root bridge (Switch) selects a root port

• This is the port with the lowest cost path to the root bridge

Select root portsSelect root ports

Set by IEEE

Choose the port on the switch with the lower bridge ID.

Suppose this is switch B.

If the cost is same and Close down If the cost is same and Close down redundant linksredundant links

Best pathBest path

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Video

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https://www.youtube.com/watch?v=ihF_78oIaDI