STP Performance Tuning and Troubleshooting
Spanning Tree Protocol Revisited
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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
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- 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?
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.