Exploring STP behaviors – Part 1

Lets explore the behavior of STP (802.1D) on our new device “Switch1”.  We plug it in, power it up and connect to the console with our console cable.  We quickly enter enable mode and run show ver and note our base MAC address.

We can run a few more commands to see what is going on with spanning tree on the switch.
Switch1#show spanning-tree
Switch1#show spanning-tree active 
Switch1#show spanning-tree detail
Switch1#show spanning-tree vlan 1

The commands above will all return the output:  No spanning tree instance exists.

The commands show spanning-tree inconsistentports  will return limited output.  The command show spanning-tree summary will give us a summary of features, most importantly at this point is that it lets us know that the switch is running STP per vlan by default.


Connecting the switch to a shutdown router interface will initiate the STP instance.  Once the router interface is powered on or a device with an active interface is connected, we can observe the spanning tree instance in action.

Switch1#sh spanning-tree


  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     00 01.6376.BDED

             This bridge is the root

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     0001.6376.BDED

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

—————- —- — ——— ——– ——————————–

Fa0/1            Desg LSN 19        128.1    P2p

The switch will quickly establish himself as the root / begins BPDU exchange and  the interface will begin transitioning through the STP States:

Power On / Initialize


Listening state

Learning state

Forwarding state


add summarization of port state above.


SWITCH1 will send out BPDUs to multicast address(es) 01:80:C2:00:00:00 – 01:80:C2:00:00:10  every 2 seconds according to the default timer avobe.   By default the BPDU’s will carry the default priority of 32769 (about half of 2^16).

BPDU will contain a Root ID (8 bytes – initially self) and a Bridge ID (8 bytes – initially self) and other fields such as the STP version, the timers and BPDU type.  For now lets understand the Bridge ID
Again – the Bridge ID is 8 bytes.  Its composed of the MAC address of the switch/bridge and the priority.  the mac address in this case is 0001.6376.BDED  that’s 12 nybles, or 6 bytes the remaining 2 bytes are used for the Priority.

The details above while wordy are not unimportant.  The root election when other switches are plugged in will depend entirely on the contents of the Bridge ID in the BPDU exchange.

The show spanning-tree details command will now give a more verbose explanation of the spanning tree elements, including very valuable detail regarding topology changes and their source.

Switch1#show spanning-tree detail 

VLAN0001 is executing the ieee compatible Spanning Tree Protocol

  Bridge Identifier has priority of 32768, sysid 1, 0001.6376.BDED

  Configured hello time 2, max age 20, forward delay 15

  Current root has priority 32769

  Topology change flag not set, detected flag not set

  Number of topology changes 0 last change occurred 00:00:00 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 1 (FastEthernet0/1) of VLAN0001 is designated forwarding

  Port path cost 19, Port priority 128, Port Identifier 128.1

  Designated bridge has priority 32769, address 0001.6376.BDED

  Designated port id is 128.1, designated path cost 19

  Timers: message age 16, forward delay 0, hold 0

  Number of transitions to forwarding state: 1

  Link type is point-to-point by default

Here are some of the other commands and their output:

Switch1#show spanning-tree summary 

Switch is in pvst mode

Root bridge for: default

Extended system ID           is enabled

Portfast Default             is disabled

PortFast BPDU Guard Default  is disabled

Portfast BPDU Filter Default is disabled

Loopguard Default            is disabled

EtherChannel misconfig guard is disabled

UplinkFast                   is disabled

BackboneFast                 is disabled

Configured Pathcost method used is short

Name                   Blocking Listening Learning Forwarding STP Active

———————- ——– ——— ——– ———- ———-

VLAN0001                     0         0        0          1          1

———————- ——– ——— ——– ———- ———-

1 vlans                      0         0        0          1          1

Switch1#show spanning-tree interface fastEthernet 0/1

Vlan             Role Sts Cost      Prio.Nbr Type

—————- —- — ——— ——– ——————————–

VLAN0001         Desg FWD 19        128.1     P2p

The output above lets us know that f0/1 on Switch1 is participating in Spanning Tree for VLAN1.  Which has converged (we know this because the port is “Designated” and has transitioned to FWD state.   I will continue this next time with the effects of directly connecting a 2nd and 3rd switch to Switch 1.

That’s all for today.  Later gaters!


Gabe @ networkdojo.net