Editing Cisco Commands (section)

== Spanning Tree ==

=== Enable PortFast on an interface ===
Use PortFast on edge/access ports that connect to end devices.

 interface GigabitEthernet0/1
  spanning-tree portfast

=== Enable PortFast globally ===
Enables PortFast on all access ports.

 spanning-tree portfast default

=== Enable PortFast on a trunk (use with caution) ===
Only use when the trunk connects to a single end device (not another switch).

 interface GigabitEthernet0/1
  spanning-tree portfast trunk

=== Verify PortFast status ===

 show spanning-tree interface GigabitEthernet0/1 detail

=== Disable PortFast on an interface ===

 interface GigabitEthernet0/1
  no spanning-tree portfast

=== Best Practice: Enable BPDU Guard with PortFast ===
Shuts down the port if a BPDU is received, protecting against loops.

Per interface:
 interface GigabitEthernet0/1
  spanning-tree portfast
  spanning-tree bpduguard enable

Globally:
 spanning-tree portfast default
 spanning-tree bpduguard default

=== Set the Root Bridge ===
The root bridge is the central switch in the spanning tree topology. It is chosen based on the lowest bridge priority. A lower priority value increases the likelihood of a switch becoming the root bridge.

Set priority to influence root bridge selection:
* The default priority value is **32768** for all switches.
* Priority is set in increments of **4096**, and this value is added to the VLAN ID (e.g., for VLAN 1, the default bridge priority would be 32768).

To influence the root bridge selection, change the priority value:
 interface GigabitEthernet0/1
  spanning-tree vlan 1 priority 4096
  (Sets a lower priority value, increasing the likelihood of this switch becoming the root bridge for VLAN 1)

Set the root bridge for a specific VLAN:
* To make the current switch the root bridge:
 spanning-tree vlan 1 root primary
  (This automatically sets the priority lower than the default value, typically to 24576, to ensure this switch becomes the root bridge)

* To make the current switch the backup root bridge:
 spanning-tree vlan 1 root secondary
  (This sets the priority higher than the primary root, typically to 28672, making it the backup root bridge)

Alternatively, manually set the root bridge priority:
 interface GigabitEthernet0/1
  spanning-tree vlan 1 priority 24576
  (Sets this switch with a higher priority, making it more likely to become the root bridge)

=== Verify Root Bridge ===

 show spanning-tree vlan 1
 (Shows the current root bridge and its priority, along with other STP details)

=== Notes ===
* Do '''not''' enable PortFast on ports connected to other switches (this can cause network loops).
* PortFast should only be used on access/edge ports that connect to end devices like PCs, printers, or phones.
* Incorrect use of PortFast can cause Layer 2 loops.
* The default priority value is **32768**, and it is adjusted in increments of **4096**.
* Lower priority values increase the likelihood of becoming the root bridge.
* The priority value combined with the MAC address (if priorities are equal) is used to determine the root bridge.