Cisco AP – basic STP information

This post gives basic information about STP settings on Cisco Aironet AP.

Quick notes

  1. By default no STP settings are configured on Cisco Aironet AP. – No STP instance is running on the AP.
  2. In most cases the STP is not needed. Usually AP is connected to LAN switch on the one side (via ethernet) and to WLAN clients on the other side (via Radio[s]). – There is no place for any loop so STP is not needed.
  3. STP might be needed if you configure your AP to act as wireless bridge. For example you can configure your APs to create a wireless connection between two LAN switches. In this case the path can possibly create a loop in you network.

Verify STP configuration

If you did not change default STP configuration then no STP instance is running on the AP:

ap#show spanning-tree

No spanning tree instances are running.
ap#

If for some reason STP was configured then you can see similar output:

ap#show spanning-tree

Bridge group 1 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, address 0017.59fa.c770
  Configured hello time 2, max age 20, forward delay 15
  We are the root of the spanning tree
  Topology change flag not set, detected flag not set
  Number of topology changes 7 last change occurred 00:17:55 ago
          from FastEthernet0.1
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0, aging 300

 Port 7 (Dot11Radio0.1) of Bridge group 1 is forwarding
   Port path cost 33, Port priority 128, Port Identifier 128.7.
   Designated root has priority 32768, address 0017.59fa.c770
   Designated bridge has priority 32768, address 0017.59fa.c770
   Designated port id is 128.7, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 5
   BPDU: sent 884, received 0

 Port 8 (FastEthernet0.1) of Bridge group 1 is forwarding
   Port path cost 19, Port priority 128, Port Identifier 128.8.
   Designated root has priority 32768, address 0017.59fa.c770
   Designated bridge has priority 32768, address 0017.59fa.c770
   Designated port id is 128.8, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   BPDU: sent 553, received 0

Note that STP is configured (enabled) on bridge groups (in this case on bridge group 1). Interfaces are configured to operate in specific bridge groups (in this case Dot11Radio0.1 and FastEthernet0.1 was bound to group bridge 1). The subinterfaces are used because the VLANs were configured on that AP; otherwise you would use physical interface instead a subinterface.

If you want to check quickly if all ports are in the STP forwarding state then you can run the following command:

ap#show spanning-tree | include Bridge group
 Bridge group 1 is executing the ieee compatible Spanning Tree protocol
 Port 7 (Dot11Radio0.1) of Bridge group 1 is forwarding
 Port 8 (FastEthernet0.1) of Bridge group 1 is forwarding

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