STP Performance Tuning and Troubleshooting

Spanning Tree Protocol Revisited

• Top right becomes root

STP root selection & tuning

• Spanning Tree elects a root bridge
• Elects a root bridge based on bridge ID
  • Lowest priority wins
  • If priority is tied, lowest MAC wins
  • Default priority is 32768
• Spanning tree will break loops
• However, STP should be tuned

• By default the lowest MAC switch is root
• Lowest MAC = oldest switch
• Lowering priority selects root switch
  • Default priority is 32768
  • Increments at 4096
• Which switch should be root?
  • Most central/upstream

Spanning Tree Protocol Tuning Cont'd

• Per-VLAN STP
  • Root primary / root secondary
• Portfast
  • Optimizing ports
  • Basically says, hey switch I want to go ahead and take this port and perform forwarding straight up.
  • Slim port into forwarding stat right away.
• BPDU guard
  • If I get Bridge Protocol Data Unit (BPDU) on this link, then it is not good to do Portfast.
  • We can shutdown the port
• Root guard
  • Allows BPDU
  • But if you see a Superior BPDU
    • Compromise the stability of our network
    • Disruptive, suboptimal, attack
  • If I see one BPDU
    • Shut the port down instantly and contact admin.
    • Drop it just to make sure that the environment is stable.
  • Enforcing resiliency and stability into your environment.
• Loop guard
  • Funny message unique to the switch
  • If the message goes back to the switch then we can detect there is a loop going on.