Spanning Tree¶
Spanning Tree Operation
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Determine root bridge – The bridge advertising the lowest bridge ID becomes the root bridge
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Select root port – Each bridge selects its primary port facing the root
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Select designated ports – One designated port is selected per segment
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Block ports with loops – All non-root and non-designated ports are blocked
Port States
Port Roles
Legacy ST
Rapid ST
Legacy ST
Rapid ST
Disabled
Discarding
Root
Root
Blocking
Designated
Designated
Listening
Blocking
Alternate
Learning
Learning
Backup
Forwarding
Forwarding
PVST Enhancements:
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Portfast: Enables immediate transition into the forwarding state
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UplinkFast – Enabled switches to maintain backup paths to root
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BackboneFast – Enables immediate expiration of the “Max Age” timer in the event of an indirect link failure
Spaning-tree Protection
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Root guard – prevents a port from becoming the root port
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BPDU Guard – error-disables a port if a bpdu is received
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Loop guard – prevents a blocked port from transitioning to listening after the max age timer has expired
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BPDU filter – blocks bpdus on an interface (disables STP)
Troubleshooting:
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Show spanning-tree [summary|detail|root]
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Show spanning-tree [interface|vlan]
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Show spanning-tree mst […]