Objectives

• Observe the behavior of a separate spanning tree instance per VLAN.

• Change spanning tree mode to rapid spanning tree.

Background

Four switches have just been installed. The distribution layer switches are Catalyst 3560s, and the access layer switches are Catalyst 2960s. There are redundant uplinks between the access layer and distribution layer. Because of the possibility of bridging loops, spanning tree logically removes any redundant links. In this lab, you will see what happens when spanning tree is configured differently for different VLANs.

Note: This lab uses Cisco WS-C2960-24TT -L switches with the Cisco IOS image c2960-lanbasek9-mz.122-46.SE.bin and Catalyst 3560-24PS with the Cisco IOS image c3560-advipservicesk9-mz.122-46.SE.bin. Other switches (such as a 2950 or 3550), and Cisco IOS Software versions can be used if they have comparable capabilities and features. Depending on the switch model and Cisco IOS Software version, the commands available and output produced might vary from what is shown in this lab.

Required Resources

• 2 switches (Cisco 2960 with the Cisco IOS Release 12.2(46)SE C2960-LANBASEK9-M image or comparable)

• 2 switches (Cisco 3560 with the Cisco IOS Release 12.2(46)SE C3560-ADVIPSERVICESK9-M image or comparable)

• Ethernet and console cables

Step 1: Prepare the switches for the lab.

a. Delete the vlan.dat file, erase the startup configuration, and reload the switches.

b. Give each switch a hostname according to the topology diagram.

c. Configure ports Fa0/7 through Fa0/12 on all switches to be trunks. On the 3560s, first set the trunk encapsulation to dot1q. On the 2960s, only dot1q is supported, therefore the switchport trunk encapsulation command is unavailable, but the mode still needs to be changed to trunk. If you do not set the mode of the ports to trunk, they will negotiate the operational mode according to their default DTP settings.

Note: The default mode on a 3560 or 2960 is dynamic auto; the default mode on a 3550 or 2950 is dynamic desirable.

DLS1 example:

DLS1(config)# interface range fastEthernet 0/7 - 12

DLS1(config-if-range)# switchport trunk encapsulation dot1q
DLS1(config-if-range)# switchport mode trunk

Step 2: Configure VTP.

a. Configure all switches with VTP mode transparent and VTP domain CISCO. Add VLAN 10 and 20 to all of them. Use the show vlan brief command to view the VLAN configurations.

DLS1 example:

DLS1(config)# vtp mode transparent

Setting device to VTP TRANSPARENT mode.

DLS1(config)# vtp domain CISCO CISCO
Changing VTP domain name from NULL to
DLS1(config)# vlan 10,20
DLS1(config-vlan)# end
DLS1# show vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/9, Fa0/10
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Fa0/24
10 VLAN0010 active Gi0/1, Gi0/2

20 VLAN0020 active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup






b. Issue the show spanning-tree command on any of the four switches. Notice that instead of just one VLAN there are three non-reserved VLANs. VLANs 1002-1005 are reserved for internal switch usage. All other VLANs shown are non-reserved.

Note: By default Cisco switches use PVST+, a Cisco-proprietary IEEE 802.1Q-compatible per-VLAN spanning tree protocol.

DLS1# show spanning-tree


VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

VLAN0010 tree enabled protocol ieee
Spanning
Root ID Priority 32778
Address 000a.b8a9.d680
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

VLAN0020




Spanning tree enabled protocol ieee
Root ID Priority 32788
Address 000a.b8a9.d680
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ----------------------------
Fa0/7 Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

Step 3: Assign a root switch for each VLAN.

Notice that all the ports have identical spanning tree behavior for each VLAN. This is because all VLANs are running spanning tree with the default behavior. However, you can modify the default spanning tree behavior on a per-VLAN basis. The default priority is 32768. Configuring a switch with a lower priority value for a given VLAN makes it the root bridge for that VLAN. For this lab, we assign DLS1 as the root bridge for VLAN 10, and DLS2 for VLAN 20.

a. To change the priority for a given VLAN, use the spanning-tree vlan number priority number command. Configure DLS1 with priority 4096 for VLAN 10. Configure DLS2 similarly for VLAN 20.

DLS1(config)# spanning-tree vlan 10 priority 4096

DLS2(config)# spanning-tree vlan 20 priority 4096


b. If you look at the output of show spanning-tree on the four switches, you see that the port states and root switches vary on a per VLAN basis.

DLS1# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ----------------------------
Fa0/7 Desg FWD 19 128.9 P2p




Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4106
Address 000a.b8a9.d780
This bridge is the root Age 20 sec Forward Delay 15 sec
Hello Time 2 sec Max
Bridge ID Priority 4106 (priority 4096 sys-id-ext 10)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 4116
Address 000a.b8a9.d680
Cost 19
Port 13 (FastEthernet0/11) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

DLS2# show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee

Root ID Priority 32769
Address 000a.b8a9.d680


This bridge is the root Age 20 sec Forward Delay 15 sec
Hello Time 2 sec Max
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8a9.d680 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4106
Address 000a.b8a9.d780
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 000a.b8a9.d680 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 4116
Address 000a.b8a9.d680
This bridge is the root Age 20 sec Forward Delay 15 sec
Hello Time 2 sec Max
Bridge ID Priority 4116 (priority 4096 sys-id-ext 20)
Address 000a.b8a9.d680 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p



Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p
ALS1# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19
Port 11 (FastEthernet0/9) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0019.0635.5780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Altn BLK 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Root FWD 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4106
Address 000a.b8a9.d780
Cost 19
Port 9 (FastEthernet0/7) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0019.0635.5780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Root FWD 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Altn BLK 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p

VLAN0020

Spanning tree enabled protocol ieee
Root ID Priority 4116
Address 000a.b8a9.d680
Cost 19



Port 11 (FastEthernet0/9) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 0019.0635.5780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Altn BLK 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Root FWD 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p
ALS2# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19
Port 9 (FastEthernet0/7) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0019.068d.6980 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Root FWD 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Altn BLK 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Altn BLK 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4106
Address 000a.b8a9.d780
Cost 19 (FastEthernet0/9)
Port 11 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0019.068d.6980 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ----------------------------


Fa0/7 Altn BLK 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Root FWD 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Altn BLK 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 4116
Address 000a.b8a9.d680
Cost 19
Port 9 (FastEthernet0/7) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 0019.068d.6980 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Root FWD 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Altn BLK 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Altn BLK 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

Step 4: Configure RSTP.

Other spanning tree modes besides PVST+ are available. One of these is RSTP (rapid spanning tree protocol), which greatly reduces the time for a port to transition to forwarding state while still preventing bridging loops. Cisco-proprietary per-VLAN rapid spanning tree (PVRST+) combines the functionality of RSTP and PVST.

Note: You can use the clear spanning -tree detected-protocols command after configuring different spanning tree modes. This can avoid a mutual deadlock between two switches when they consider themselves as 802.1D legacy bridges when in fact they were configured for RSTP.

a. To change the spanning tree mode to PVRST+, use the global configuration command spanning-tree mode rapid-pvst. Configure this on all four switches. During the transition period, rapid spanning tree falls back to 802.1D spanning tree on the links that have 802.1D spanning tree configured on one side.

DLS1(config)# spanning-tree mode rapid-pvst


b. After configuring all four switches with this command, use the show spanning-tree command to verify the configuration:

DLS1# show spanning-tree

VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19




Port 13 (FastEthernet0/11) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID  Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8a9.d780 sec Forward Delay 15 sec
Hello Time 2 sec Max Age 20
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 4106
Address 000a.b8a9.d780
This bridge is the root Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 4106 (priority 4096 sys-id-ext 10)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ----------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p
VLAN0020
Spanning tree enabled protocol rstp
Root ID Priority 4116
Address 000a.b8a9.d680
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 000a.b8a9.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ----------------------------
Fa0/7 Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p




Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

STP Crash Course

Objective

• Observe the default behavior of STP.

Background

Four switches have just been installed. The distribution layer switches are Catalyst 3560s, and the access layer switches are Catalyst 2960s. There are redundant uplinks between the access layer and distribution layer. Because of the possibility of bridging loops, spanning tree logically removes any redundant links. In this lab, you will observe what spanning tree does and why.

Note: This lab uses Cisco WS-C2960-24TT- L switches with the Cisco IOS image c2960-lanbasek9-mz.122-46.SE.bin and Catalyst 3560-24PS switches with the Cisco IOS image c3560- advipservicesk9-mz.122-46.SE.bin. Other switches (such as a 2950 or 3550), and Cisco IOS Software versions can be used if they have comparable capabilities and features. Depending on the switch model and Cisco IOS Software version, the commands available and output produced might vary from what is shown in this lab.

Required Resources

• 2 switches (Cisco 2960 with the Cisco IOS Release 12.2(46)SE C2960-LANBASEK9-M image or comparable)

• 2 switches (Cisco 3560 with the Cisco IOS Release 12.2(46)SE C3560-ADVIPSERVICESK9-M image or comparable)

• Ethernet and console cables

Step 1: Prepare the switches for the lab.

Refer to Lab 1-1 Clearing a Switch and Lab 1-2 Clearing a Switch Connected to a Larger Network to prepare all four switches for this lab. Cable the equipment as shown. If you are accessing your equipment remotely, ask your instructor for instructions on how to do this.

Step 2: Configure basic switch parameters.

a. Configure the four switches as shown in the diagram with a hostname.

ALS1 example:

Switch> enable
Switch# configure terminal

Switch(config)# hostname ALS1


b. Optionally, configure an enable secret password and console security. Configure the console line with logging synchronous and no timeout.

ALS1 example:

ALS1(config)# enable secret class

ALS1(config)# line console 0
ALS1(config-line)# logging synchronous

ALS1(config-line)# exec-timeout 0 0
ALS1(config-line)# password cisco

ALS1(config-line)# login


Note: After the cables are connected spanning tree is initiated and the switch detects the redundant links.

By default, spanning tree runs on every port. When a new link becomes active, the port goes through the IEEE 802.1D spanning tree listening and learning states before transitioning to forwarding state. During this period, the switch discovers if it is connected to another switch or an end-user device.

One of the switches is elected as the root bridge for the tree. Then an agreement is established as to which links to keep active and which links to logically remove from the spanning tree (disable) if multiple links exist.


The results in this lab will vary. Spanning tree operation is based on the MAC addresses of the switches.

c. Observe the LEDs on the switch to check the status of the link. For access ports a bright green light indicates an active link. An amber light indicates an inactive link.

Step 3: Display default spanning tree information for all switches.

a. Verify IEEE 802.1D STP with the show spanning-tree command on DLS1.

Note: Your output may differ, based on the root bridge selected in your topology. The sample output below may also differ from those in your lab, because they were generated with a different set of switches.




DLS1# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19 (FastEthernet0/11)
Port 13 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8b3.d780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---- --- --------- -------- ------------------------------
Fa0/7 Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

b. Verify STP with the show spanning-tree command on DLS2.

DLS2# show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
This bridge is the root Age 20 sec Forward Delay 15 sec
Hello Time 2 sec Max
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8a9.d680 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---- --- --------- -------- ------------------------------
Fa0/7 Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p

c. Verify STP with the show spanning-tree command on ALS1.

ALS1# show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769


Address 000a.b8a9.d680
Cost 19 (FastEthernet0/9)
Port 11 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0019.0635.5780 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---- --- --------- -------- ------------------------------
Fa0/7 Altn BLK 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Root FWD 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p

Fa0/11 Altn BLK 19 128.11 P2p
Fa0/12 Altn BLK 19 128.12 P2p
d.  Verify STP with the show spanning-tree command on ALS2.
ALS2# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
Cost 19
Port 9 (FastEthernet0/7) Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0019.068d.6980 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ------------------------------


Fa0/7 Root FWD 19 128.9 P2p
Fa0/8 Altn BLK 19 128.10 P2p
Fa0/9 Altn BLK 19 128.11 P2p
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Altn BLK 19 128.13 P2p
Fa0/12 Altn BLK 19 128.14 P2p

Notice that between each pair of switches, at least one of the two ports is blocking. Blocking can occur on the access layer switch or the distribution layer switch. If all ports have their default setting, the higher interface number of the two ports will block.

A port is placed in the blocking state because the switch detects two links between the same switches. A bridging loop would result if one of the switches did not logically disable a redundant link.

e. Display the spanning tree information for DLS2 again.

DLS2# show spanning-tree


VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.b8a9.d680
This bridge is the root Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b8a9.d680 Forward Delay 15 sec
Hello Time 2 sec Max Age 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
----------------Fa0/7 ---- --- --------- -------- ------------------------------
Desg FWD 19 128.9 P2p
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/9 Desg FWD 19 128.11 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/11 Desg FWD 19 128.13 P2p
Fa0/12 Desg FWD 19 128.14 P2p



f. Another useful STP command is show spanning-tree root. This command displays a summary listing of the VLANs defined, the Root (bridge) ID for each one, the Root Cost and the Root Port that the switch uses to reach the root bridge. In this lab the only active VLAN is default VLAN 1. Issue the show spanning-tree root command on ALS1. The output shows the priority and MAC address of DLS2 as the Root ID for VLAN 1. The Root Cost is 19 and ALS1 uses port Fa0/9 to reach DLS2.

ALS1# show spanning-tree root
Vlan Root ID Root Hello Max Fwd Root Port
Cost Time Age Dly
VLAN0001---------------- -------------------- --------- ----- --- --- ------------
32769 0017.5a53.a380 19 2 20 15 Fa0/9


g. Issue the show spanning-tree root command on DLS2. The output shows the priority and MAC address of DLS2 as the Root ID for VLAN 1. The Root Cost is 0 and there is no Root Port listed because DLS2 is the root bridge.

DSL2# show spanning-tree root
Vlan Root ID Root Hello Max Fwd Root Port
Cost Time Age Dly
VLAN0001---------------- -------------------- --------- ----- --- --- ------------
32769 0017.5a53.a380 0 2 20 15