Question about STP

[tinhnho123]

Hello All,

On our distribution switch 6505 model, I have:

spanning-tree vlan 1,111,115,121,123,125 priority 28672
spanning-tree vlan 112,116,122,124,126 priority 24576

Can someone please explain what do these commands do?

Which one have higher priority?

Thanks.

[srprasaad_nj]

 Your disturbution switch 6509 will be the root bridge for the VLAN's with priority 24576.

 The same distribution switch will be secondary root bridge for VLAN's with priorty 28672.

These commnads will ensure that a switch is primary root bridge for vlan's which are configured with better priority in the network. This provides better adminstrative control in network.

[tinhnho123]

Without those 2 lines above, does STP still work?

[Jon Marshall · Accepted solution]

Yes, STP still works.

Without those lines any of the switches interconnected with each other could become STP root or secondary.

You generally pick your distribution or core/distribution if using a collapsed core to be the STP root and secondary switchches as these tend to be the switches with most resources in terms of CPU and memory.

Be aware that if you removed those lines then your whole network would need to reconverge which would result in a temporary outage while the switches worked out which ones should be root and secondary.

Jon

[tinhnho123]

Thanks Jon , I understand better now. 

Last couple questions and hope you or anyone can help me to answer:

How do these switches define who is the root and secondary if there aren't 2 lines above?

Is root primary and secondary selection same deal with root bridge election one?

Thanks!

[Joseph W. Doherty]

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Posting

Actually primary and secondary roots are relative, not absolute.

With STP priorities, lower is better.

Root bridge is active switch with lowest bridge ID, which is priority + MAC.

Secondary bridge is active switch with next lowest bridge ID.

In your original posting, often you'll want to chose which bridge should be root, and perhaps which to take its place if it fails (the secondary).  When using multiple VLANs, you might, want different roots (and secondaries) for different VLANs (this allows load distribution).

[Jon Marshall · Accepted solution]

As Joseph mentions if you haven't explicitly set the priority on any switches then they all use the same default priority.

So it then comes down to the mac address.

The switch with the lowest mac address becomes STP root and the switch with the second lowest mac address becomes STP secondary.

Is root primary and secondary selection same deal with root bridge election one?

Not sure what you mean by this ?

Jon

[tinhnho123]

Hi Jon,

Forget about the second question, I was confused about the STP root bridge. Thanks,