2.2 - Spanning Tree Protocol

Loop protection

• Connect two switches to each other

– They’ll send traffic back and forth forever

– There’s no “counting” mechanism at the MAC layer

• This is an easy way to bring down a network

– And somewhat difficult to troubleshoot

– Relatively easy to resolve

• IEEE standard 802.1D to prevent loops

in bridged (switched) networks (1990)

STP port states

• Blocking - Not forwarding to prevent a loop

• Listening - Not forwarding and cleaning the MAC table

• Learning - Not forwarding and adding to the MAC table

• Forwarding - Data passes through and is fully operational

• Disabled - Administrator has turned off the port

RSTP (802.1w)

Rapid Spanning Tree Protocol (802.1w)

– A much-needed updated of STP

– This is the latest standard

• Faster convergence - From 30 to 50 seconds to 6 seconds

• Backwards-compatible with 802.1D STP

– You can mix both in your network

• Very similar process - An update, not a wholesale change

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