STP Written Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.


*** Theory ***

  • PVST – Per Vlan Spanning Tree use to make ISL key but 802.d adopted it. It allows you to use the blocked redundant links in spanning tree for vlans you specify, kind of load balancing. 1 core switch can be the root for vlan 10 and 20 and 2nd core switch can be the root for vlan 30 and 40.
  • Port fast – disables STP on that port. Only use for host not switches etc so that DHCP etc doesn’t time out because port is inactive/blocked. Also use with BPDU guard
    • Config-if# spanning-tree portfast /portfast
  • BPDU GUARD – Disables port if switch is connected to a host / portfast port
    • Config# spanning-tree portfast bpduguard
  • Uplink Fast – If a switch as uplink to 2 or more switch and the root port goes down pointing to SW1, then a rootport to SW2 is made in less than 5 seconds rather than waiting for 50 seconds
  • RSTP – 802.1w – Fewer port states (only discarding, learning and forwarding)adds new port roles



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