Bridge ID priority 542

protocol
as needed. To enable or disable spanning tree on an IOS-based switch, use the spanning-tree
vlan vlan_number command or the no spanning-tree vlan vlan_number command. Use the
show spanning-tree command to view the spanning tree status. The following configuration
shows how to enable and disable spanning tree on a 2950 switch:
Terry_2950#conf t
Terry_2950(config)#no spanning-tree vlan 1
Terry_2950(config)#^Z
Terry_2950#show spanning-tree
No spanning tree instances exist.
Terry_2950#conf t
Terry_2950(config)#spanning-tree vlan 1
Terry_2950(config)#^Z
Terry_2950#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 0
Address 00b0.6414.1180
Cost 100
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000b.be53.2c00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/1 128.1 100 LIS 0 0 00b0.6414.1180 128.1
Fa0/24 128.24 100 LIS 100 32769 000b.be53.2c00 128.24
Terry_2950#
498 Chapter 15  Layer 2 Switching and the Spanning Tree Protocol (STP)
Notice that the commands include mandatory references to the VLANs. You will remember
that all ports are in VLAN 1 by default. In the next chapter we will be considering the use of
different spanning trees for each VLAN, and these commands will make a little more sense then.
In the meantime, just trust me and accept that the Cisco IOS demands that you enter a VLAN
number at this time.
To see the spanning tree configuration and whether it is active on a Catalyst 4000 set-based
switch, use the show spantree command as shown here:
Terry_4000> (enable) show spantree
VLAN 1
Spanning tree enabled
Spanning tree type ieee
Designated Root 00-e0-34-88-fc-00
Designated Root Priority 32768
Designated Root Cost 0
Designated Root Port 1/0
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-e0-34-88-fc-00
Bridge ID Priority 32768
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Port Vlan Port-State Cost Priority Fast-Start Group-Method
---- ---- -------------- ----- -------- ---------- ------------
1/1 1 forwarding 19 32 disabled
1/2 1 not-connected 19 32 disabled
2/1 1 not-connected 100 32 disabled
2/2 1 not-connected 100 32 disabled
2/3 1 not-connected 100 32 disabled
2/4 1 not-connected 100 32 disabled
2/5 1 not-connected 100 32 disabled
<Output truncated>
By default, the show spantree command provides information about VLAN 1. You can gather
spanning tree information about other VLANs by using the show spantree vlan# command.
The show spantree command provides you the following information:
Designated root The MAC address of the root bridge.
Designated root priority The priority of the root bridge. All bridges have a default of 32768.
Designated root cost The cost of the shortest path to the root bridge.
Designated root port The port that is chosen as the lowest cost to the root bridge.
Exam Essentials 499
Root timers The timers received from the root bridge.
Bridge ID MAC address This bridge’s ID. This plus the bridge priority make up the bridge ID.
Bridge ID priority The priority set; the preceding bridge output is using the default of 32768.
Bridge timers The timers used by this bridge.
Ports in the spanning tree Not all available ports are displayed in the preceding output. However,
this field does show all ports participating in this spanning tree. It also shows whether they
are forwarding.
Although the command abbreviation show span works on all the switches, you
will get much different output if you use it on the 4000 series. This is because
a SPAN (Switch Port Analyzer) is the port used to connect to a sniffer. On the
4000, abbreviate spantree to no less than spant to avoid this.