Lab - Modify Default Spanning Tree Behavior

Modify Default Spanning Tree Behavior

Topology

Objective

  • observe what happens when the default spanning tree behavior is modified.

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 the default spanning tree behavior is modified.

Step 1: Prepare the switches for the lab.

  1. Delete vlan.dat file, erase the startup config, and reload the switches.
  2. Give each switch a hostname according to the topology diagram.
  3. Configure ports f0/7 through f0/12 on all switches to be trunks. On the 3560, 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 stll 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.
All Switches:
interface range f0/7-12
switchport trunk encapsulation dot1q
switchport mode trunk

Step 2: Display default spanning tree information for all switches.

IOU1#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        1 (Ethernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0500
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 100       128.1    Shr
Et0/1               Altn BLK 100       128.2    Shr
Et0/2               Altn BLK 100       128.3    Shr
Et0/3               Altn BLK 100       128.4    Shr
Et1/0               Altn BLK 100       128.5    Shr
Et1/1               Altn BLK 100       128.6    Shr
Et1/2               Desg FWD 100       128.7    Shr
Et1/3               Desg FWD 100       128.8    Shr
Et2/0               Desg FWD 100       128.9    Shr
Et2/1               Desg FWD 100       128.10   Shr
Et2/2               Desg FWD 100       128.11   Shr
Et2/3               Desg FWD 100       128.12   Shr
Et3/0               Desg FWD 100       128.13   Shr
Et3/1               Desg FWD 100       128.14   Shr
Et3/2               Desg FWD 100       128.15   Shr
Et3/3               Desg FWD 100       128.16   Shr 
IOU2(config-if-range)#do show spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             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     aabb.cc00.0100
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr
Et0/1               Desg FWD 100       128.2    Shr
Et0/2               Desg FWD 100       128.3    Shr
Et0/3               Desg FWD 100       128.4    Shr
Et1/0               Desg FWD 100       128.5    Shr
Et1/1               Desg FWD 100       128.6    Shr
Et1/2               Desg FWD 100       128.7    Shr
Et1/3               Desg FWD 100       128.8    Shr
Et2/0               Desg FWD 100       128.9    Shr
Et2/1               Desg FWD 100       128.10   Shr
Et2/2               Desg FWD 100       128.11   Shr
Et2/3               Desg FWD 100       128.12   Shr
Et3/0               Desg FWD 100       128.13   Shr
Et3/1               Desg FWD 100       128.14   Shr
Et3/2               Desg FWD 100       128.15   Shr
Et3/3               Desg FWD 100       128.16   Shr 
IOU3(config-if-range)#do show spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        11 (Ethernet2/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Altn BLK 100       128.1    Shr
Et0/1               Altn BLK 100       128.2    Shr
Et0/2               Desg FWD 100       128.3    Shr
Et0/3               Desg FWD 100       128.4    Shr
Et1/0               Desg FWD 100       128.5    Shr
Et1/1               Desg FWD 100       128.6    Shr
Et1/2               Desg FWD 100       128.7    Shr
Et1/3               Desg FWD 100       128.8    Shr
Et2/0               Desg FWD 100       128.9    Shr
Et2/1               Desg FWD 100       128.10   Shr
Et2/2               Root FWD 100       128.11   Shr
Et2/3               Altn BLK 100       128.12   Shr
Et3/0               Desg FWD 100       128.13   Shr
Et3/1               Desg FWD 100       128.14   Shr
Et3/2               Desg FWD 100       128.15   Shr
Et3/3               Desg FWD 100       128.16   Shr
IOU4(config-if-range)#do show spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        9 (Ethernet2/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr
Et0/1               Desg FWD 100       128.2    Shr
Et0/2               Desg FWD 100       128.3    Shr
Et0/3               Desg FWD 100       128.4    Shr
Et1/0               Desg FWD 100       128.5    Shr
Et1/1               Desg FWD 100       128.6    Shr
Et1/2               Desg FWD 100       128.7    Shr
Et1/3               Desg FWD 100       128.8    Shr
Et2/0               Root FWD 100       128.9    Shr
Et2/1               Altn BLK 100       128.10   Shr
Et2/2               Desg FWD 100       128.11   Shr
Et2/3               Desg FWD 100       128.12   Shr
Et3/0               Desg FWD 100       128.13   Shr
Et3/1               Desg FWD 100       128.14   Shr
Et3/2               Desg FWD 100       128.15   Shr
Et3/3               Desg FWD 100       128.16   Shr

Are BPDUs propagated without trunk links?

Step 3: Configure specific switches to be primary and secondary root.

you cna use the debug spanning-tree events command to monitor topology changes.

To change the spanning-tree root status, use the global configuration commands spanning-tree vlan vlan_number root primary and spanning-tree vlan vlan_number root secondary.

a. Issue the debug commond on DLS2.

b. Change DLS1 to be the primary root switch.

DLS1(config)#spanning-tree vlan 1 root primary

c. Change ALS1 to the secondary root.

ALS(config)#spanning-tree vlan 1 root secondary

Spanning Tree event debugging is on
IOU2#
*Sep  3 17:12:43.429: STP: VLAN0001 heard root 24577-aabb.cc00.0400 on Et2/0
*Sep  3 17:12:43.429:     supersedes 32769-aabb.cc00.0100
*Sep  3 17:12:43.429: STP: VLAN0001 new root is 24577, aabb.cc00.0400 on port Et2/0, cost 200
*Sep  3 17:12:43.436: STP: VLAN0001 sent Topology Change Notice on Et2/0
*Sep  3 17:12:43.436: STP[1]: Generating TC trap for port Ethernet2/1
*Sep  3 17:12:43.436: STP: VLAN0001 Et2/1 -> blocking
*Sep  3 17:12:43.436: STP: VLAN0001 new root port Et2/3, cost 100
*Sep  3 17:12:43.436: STP: VLAN0001 Et2/1 -> listening
*Sep  3 17:12:43.438: STP: VLAN0001 new root port Et2/2, cost 100
*Sep  3 17:12:43.438: STP[1]: Generating TC trap for port Ethernet2/3
IOU2#
*Sep  3 17:12:43.438: STP: VLAN0001 Et2/3 -> blocking
IOU2#
*Sep  3 17:12:45.446: STP: VLAN0001 sent Topology Change Notice on Et2/2
IOU2#
*Sep  3 17:12:58.445: STP: VLAN0001 Et2/1 -> learning
IOU2#
*Sep  3 17:13:13.452: STP[1]: Generating TC trap for port Ethernet2/1
*Sep  3 17:13:13.453: STP: VLAN0001 sent Topology Change Notice on Et2/2
*Sep  3 17:13:13.453: STP: VLAN0001 Et2/1 -> forwarding
IOU2#
*Sep  3 17:14:21.602: STP: VLAN0001 heard root 20481-aabb.cc00.0500 on Et0/0
*Sep  3 17:14:21.602:     supersedes 24577-aabb.cc00.0400
*Sep  3 17:14:21.602: STP: VLAN0001 new root is 20481, aabb.cc00.0500 on port Et0/0, cost 100
*Sep  3 17:14:21.602: STP: VLAN0001 Et2/3 -> listening
*Sep  3 17:14:21.624: STP: VLAN0001 Et2/3 -> blocking
*Sep  3 17:14:21.624: STP: VLAN0001 sent Topology Change Notice on Et0/0
*Sep  3 17:14:21.625: STP[1]: Generating TC trap for port Ethernet2/2
*Sep  3 17:14:21.625: STP: VLAN0001 Et2/2 -> blocking
*Sep  3 17:14:21.625: STP[1]: Generating TC trap for port Ethernet0/1
*Sep  3 17:14:21.625: STP: VLAN0001 Et0/1 -> blocking
IOU2#
*Sep  3 17:14:44.638: STP: VLAN0001 Et2/3 -> listening
*Sep  3 17:14:44.638: STP: VLAN0001 Et2/2 -> listening
IOU2#
*Sep  3 17:14:51.758: STP: VLAN0001 Et2/3 -> blocking
*Sep  3 17:14:51.758: STP: VLAN0001 Et2/2 -> blocking
时间： 2024-10-11 18:08:58

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