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Spanning Tree Protocol

Spanning Tree Protocol. STP. Topics. REDUNDANT LINKS Issues (layer2 loop - broadcast storm - Duplicate Unicast frame) Spanning tree algorithm Choose the root bridge Select root ports. REDUNDANT LINKS. layer2 loop. broadcast storm.

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Spanning Tree Protocol

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  1. Spanning Tree Protocol STP

  2. Topics • REDUNDANT LINKS • Issues (layer2 loop - broadcast storm - Duplicate Unicast frame) • Spanning tree algorithm • Choose the root bridge • Select root ports

  3. REDUNDANT LINKS

  4. layer2 loop

  5. broadcast storm A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. 2 1 2 1 3 1 2 1 2 2 1

  6. Duplicate Unicast frame

  7. BPDU BRIDGE PROTOCOL DATA UNIT 32769 000A00222222

  8. Choose the root bridge • 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

  9. Spanning tree algorithm The switches use this algorithm to decide which ports should be shut down. Choose one switch to be “root bridge” Choose a “root port” on each other switch Choose a “designated port” on each segment. Close down all other ports.

  10. Select root ports • Every non-root bridge (Switch) selects a root port • This is the port with the lowest cost path to the root bridge Set by IEEE

  11. If the cost is same and Close down redundant links Choose the port on the switch with the lower bridge ID. Suppose this is switch B.

  12. Best path

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

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