In CCIE wireless lab version 2.0 equipment list Cisco 6506-E listed with 12.2 SXI IP Services feature set. Why this is important when configuring Spanning Tree Protocol (STP) ? From this software code onwards Cisco has changed terminology slightly & you need to be familiar with configuration syntax.

In this post we will see how to configure basic STP features & in the next post look at how to configure optional STP features like Portfast, BPDU Guard, BPDU Filter, Root Guard, etc.

STP(IEEE 802.1D) is a Layer 2 link-management protocol that provides path redundancy while preventing undesirable loops in the network.In an extension known as per-VLAN spanning tree (PVST), Layer 2 Ethernet ports can use STP on all VLANs. By default, a single instance of STP runs on each configured VLAN. The RSTP (802.1w) provides rapid convergence of the spanning tree by assigning port roles and by learning the active topology. The RSTP builds upon the 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value)

STP defines a tree with a root bridge and a loop-free path from the root to all network devices in the Layer 2 network. STP forces redundant data paths into a standby (blocked) state. If a network segment in the spanning tree fails and a redundant path exists, the STP algorithm recalculates the spanning tree topology and activates the standby path.

Here is the default STP configuration settings when it enabled.Basic-STP-01

Spanning Tree will be enabled for all vlans you created on the switch (by default enable it on vlan 1 as well). You can enable/disable it per VLAN level & below show the basic configurations to do that.”default spanning-tree vlan vlan_ID” command will set all STP parameters to default value for that vlan.

6506(config)#spanning-tree vlan vlan_ID
6506(config)#default spanning-tree vlan vlan_ID
6506(config)#no spanning-tree vlan vlan_ID
6506#show spanning-tree vlan vlan_ID

If you want to configure extended range vlans (1024 – 4096) you have to enable extended system id for STP. “spanning-tree extend system-id” global configuration commands will do this. “show spanning-tree summary” command can be used to verify this setting.

6506#sh spanning-tree summary 
Switch is in rapid-pvst mode
Root bridge for: VLAN0001, VLAN0160, VLAN0182, VLAN0199, VLAN0438, VLAN0585
  VLAN0756, VLAN0783, VLAN0870, VLAN0888-VLAN0889, VLAN0990
EtherChannel misconfig guard            is enabled
Extended system ID                      is enabled
Portfast Default                        is disabled
Portfast Edge BPDU Guard Default        is disabled
Portfast Edge BPDU Filter Default       is disabled
Loopguard Default                       is disabled
Platform PVST Simulation                is enabled
PVST Simulation Default                 is enabled but inactive in rapid-pvst mode
Bridge Assurance                        is enabled
UplinkFast                              is disabled
BackboneFast                            is disabled
Pathcost method used                    is short

You can configure STP mode as follows & above command can be used to verify.

6506(config)#spanning-tree mode ?
  mst         Multiple spanning tree mode
  pvst        Per-Vlan spanning tree mode
  rapid-pvst  Per-Vlan rapid spanning tree mode

You can configure a switch to become STP root bridge by changing the bridge priority values which is multiplier of 4096 (0,4096,8192…..,32768,…..,61440).If you want to ensure no other switches become a root, you have set bridge priority to 0 for a given switch for a vlan or vlans. If you want to make another switch to become secondary root, you have to give next lower priority (which is 4096) and leave all other switches default priority (which is 32768) or any other value higher that 4096.


You can also use “spanning tree vlan root primary or secondary” command to do this as well. These commands actualy change the bridge priority value for those in order to make that switch STP primary root (24576) or secondary root (28672) in a given topology. Diameter keyword in the given syntax use to specify the max hops in layer 2 network STP BPDU expect to travel. Once you configure this switch will set optimal BPDU hello time suitable to that topology (I would not change these unless asked to do so)

6506(config)# spanning-tree vlan <vlan_ID or Vlan_Range> priority <Priority_Value>
6506(config)# spanning-tree vlan vlan_ID root primary [diameter hops [hello-time seconds]]
6506(config)# spanning-tree vlan vlan_ID root secondary [diameter hops [hello-time seconds]]

Here is sample configuration if I do this unless asked to do this any otherway (eg. Do not use spanning-tree priority command to do this). Lets say 6506-A should become STP root & 6506-B to be secondary for all vlans.

6506-A(config)#spanning-tree vlan 1-4094 priority 0
6506-B(config)#spanning-tree vlan 1-4094 priority 4096

If a loop occurs, STP considers port priority when selecting a LAN port to put into the forwarding state. You can assign higher priority values to LAN ports that you want STP to select first and lower priority values to LAN ports that you want STP to select last. If all LAN ports have the same priority value, STP puts the LAN port with the lowest LAN port number in the forwarding state and blocks other LAN ports. The possible priority range is 0 through 240 (default 128), configurable in increments of 16.Cisco IOS uses the port priority value when the LAN port is configured as an access port and uses VLAN port priority values when the LAN port is configured as a trunk port.

6506(config)# interface g6/2
6506(config-if)# spanning-tree port-priority port_priority
6506(config-if)#spanning-tree port-priority ?
  <0-240>  port priority in increments of 16

6506(config-if)#spanning-tree vlan 999 port-priority ?
  <0-240>  port priority in increments of 16

6506#sh spanning-tree interface g6/2
Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0199            Desg FWD 2         128.144  P2p

The STP port path cost default value is determined from the media speed of a LAN interface. If a loop occurs, STP considers port cost when selecting a LAN interface to put into the forwarding state. You can assign lower cost values to LAN interfaces that you want STP to select first and higher cost values to LAN interfaces that you want STP to select last. If all LAN interfaces have the same cost value, STP puts the LAN interface with the lowest LAN interface number in the forwarding state and blocks other LAN interfaces. The possible cost range is 0 through 200000000 (the default is media specific)

6506(config-if)#spanning-tree cost ?
  <1-200000000>  port path cost

6506(config-if)#spanning-tree vlan 999 ?
  cost           Change an interface's per VLAN spanning tree path cost
  port-priority  Change an interface's spanning tree port priority

6506(config-if)#spanning-tree vlan 999 cost ?
  <1-200000000>  Change an interface's per VLAN spanning tree path cost

Timer values (hello, forwarding-delay,max-age) for STP can be changed by using following configuration commands.

6506(config)# spanning-tree vlan vlan_ID hello-time hello_time(1-10s)  <- Default 2s
6506(config)# spanning-tree vlan vlan_ID max-age max_age (6-40s)  <- Default 20s
6506(config)# spanning-tree vlan vlan_ID forward-time forward_time (4-30s)  <- Default 15s
6506#sh spanning-tree vlan 199 bridge 
                                                   Hello  Max  Fwd
Vlan                         Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
VLAN0199            8391 (8192,199) 0014.1bf6.0140    2    20   15  rstp

That covers the  basic STP configuration. Did you notice any specific syntax to 12.2SXI ? No, these specific syntax is required when you configure optional STP features which will look into next few posts.

You can read Cisco IOS Software Configuration Guide-Release 12.2(33)SXH and Later Releases.(Chapter 28) for more detail.

Related Posts

1. Configuring STP-Portfast
2. Configuring STP-BPDU Guard & Filter
3. Configuring STP-Root Guard & Loop Guard
4. STP –