Working with Open Shortest Path First (OSPF) Routing Protocol
Network Basics: TCP/IP and OSI Network Model Comparisons
Cisco Networking: NetBIOS Extended User Interface

Troubleshooting Spanning Trunk Protocol (STP)

The first part of troubleshooting Spanning Tree Protocol (STP) is to gather additional information about the running protocol. Our good buddy show will get you started. If you have read other chapters on protocols, you will know that the show command has a great deal of information to display, and STP is no different.

Here is a list of the show options available to STP on Cisco switches:

Switch2> enable
Switch2#show spanning-tree ?
  active             Report on active interfaces only
  backbonefast       Show spanning tree backbonefast status
  blockedports       Show blocked ports
  bridge             Status and configuration of this bridge
  detail             Detailed information
  inconsistentports  Show inconsistent ports
  interface          Spanning Tree interface status and configuration
  mst                Multiple spanning trees
  pathcost           Show Spanning pathcost options
  root               Status and configuration of the root bridge
  summary            Summary of port states
  uplinkfast         Show spanning tree uplinkfast status
  vlan               VLAN Switch Spanning Trees
  |                  Output modifiers
  <cr>

Because PVST is the default STP version, it also includes statistical information about the VLANs for which STP is running.

Switch2> enable
Switch2#show spanning-tree summary
Switch is in pvst mode
Root bridge for: none
EtherChannel misconfig guard is enabled
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
UplinkFast                   is disabled
BackboneFast                 is disabled
Pathcost method used         is short
Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001                     1         0        0          2          3
VLAN0002                     1         0        0          1          2
VLAN0005                     1         0        0          1          2
VLAN0010                     1         0        0          1          2
VLAN0015                     1         0        0          1          2
VLAN0020                     1         0        0          1          2
---------------------- -------- --------- -------- ---------- ----------
6 vlans                      6         0        0          7         13
Switch2> enable
Switch2#show spanning-tree root
                                        Root Hello Max Fwd
Vlan                   Root ID          Cost  Time Age Dly  Root Port
---------------- -------------------- ------ ----- --- ---  ----------------
VLAN0001         32769 0006.d6ab.a040     19    2   20  15  Fa0/2               
VLAN0002         32770 0006.d6ab.a040     19    2   20  15  Fa0/2               
VLAN0005         32773 0006.d6ab.a040     19    2   20  15  Fa0/2               
VLAN0010         32778 0006.d6ab.a040     19    2   20  15  Fa0/2               
VLAN0015         32783 0006.d6ab.a040     19    2   20  15  Fa0/2               
VLAN0020         32788 0006.d6ab.a040     19    2   20  15  Fa0/2               

This output shows much more information. To abridge the output, I show only the STP information for VLAN1. Here are some of the major pieces of information in this output:

  • Bridge address: The MAC address of the current switch

  • Address of the Root Bridge: The MAC address of the current Root Bridge

  • Delays and forwarding times: The current configured values of the forwarding and delay timers

  • Port status for that VLAN: Status of the ports configured in the displayed VLAN

In the following output of show spanning-tree from Switch1, note that all listed ports are in a Forwarding (FWD) state. Currently, port 11 and port 12 are connected to Switch2, creating a loop.

Switch1>enable
Switch1#show spanning-tree
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0006.d6ab.a040
             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     0006.d6ab.a040
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 19        128.1    P2p
Fa0/2            Desg FWD 100       128.2    Shr
Fa0/3            Desg FWD 19        128.3    P2p
Fa0/11           Desg FWD 19        128.11   P2p
Fa0/12           Desg FWD 19        128.12   P2p

In the following output, you see what the ports look like on Switch2.

Switch2>enable
Switch2#show spanning-tree
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0006.d6ab.a040
             Cost        19
             Port        11 (FastEthernet0/11)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0006.d6ac.46c0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 15
Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2            Desg FWD 19        128.2    P2p
Fa0/11           Root FWD 19        128.11   P2p
Fa0/12           Altn BLK 19        128.12   P2p

If you examine the state of the ports, you see almost the same output as Switch1, with a few exceptions. Notice that Switch2 knows that it is not the Root Bridge because the Root ID and Bridge ID on this switch do not match. Port Fa0/12 is in a Blocking (BLK) state, and a Priority value is defaulting to 32769.

If you want to force a switch to be the Root Bridge or not be the Root Bridge, you can change this value. A higher value will guarantee that you are not the Root Bridge, while a lower value will ensure that you are the Root Bridge.

The lowest value achieved by adding the priority MAC address will get to the Root Bridge. Therefore, if you change this value, you influence the Root Bridge assignment.

If you have a few core switches that never have a problem with rebooting, adjust the priority so that one of these core switches is your Root Bridge.

Avoid having an edge switch on the far side of a LAN extension as the Root Bridge. An edge switch regularly loses its connection or gets rebooted. When these occur, the entire topology is rebuilt.

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