STP Notes

This is the topology I’m using for these notes.

  • Electing a root bridge for a vlan: in my lab, both switches had the same Bridge priority, which was the default 32768 + the Vlan ID number. So the tie was the MAC address.
  • Now I want to apply “root secondary” without applying “root primary”. Here’s the bridge priority on the target switch:and now I apply “spanning-tree vlan 2 root secondary” on S4:Despite that, S4 became the root bridge for vlan 2!
  • Extended VLAN ID: if the Extended System ID feature is enabled on a switch, we can set bridge priorities only in increments of 4096:
  • setting a priority manually: if the Extended System ID feature is enabled, the real priority that’s set is the one we’ve entered added to the VLAN ID number:


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