We start by configuring the EtherChannel and trunk links on the C3560 first, on the C2960 and finally on the C2940.
Switch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#hostname SW1
SW1(config)#do show interface status
Port Name Status Vlan Duplex Speed Type
Fa0/1 notconnect 1 auto auto 10/100BaseTX
Fa0/2 notconnect 1 auto auto 10/100BaseTX
Fa0/3 notconnect 1 auto auto 10/100BaseTX
Fa0/4 notconnect 1 auto auto 10/100BaseTX
Fa0/5 notconnect 1 auto auto 10/100BaseTX
Fa0/6 notconnect 1 auto auto 10/100BaseTX
Fa0/7 notconnect 1 auto auto 10/100BaseTX
Fa0/8 notconnect 1 auto auto 10/100BaseTX
Gi0/1 notconnect 1 auto auto Not Present
SW1(config)#interface range fastethernet0/1-2
SW1(config-if-range)#switchport mode trunk
Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
// THIS ERROR HAPPENS TO APPEAR IN 3560
SW1(config-if-range)#switchport trunk encapsulation ?
dot1q Interface uses only 802.1q trunking encapsulation when trunking
isl Interface uses only ISL trunking encapsulation when trunking
negotiate Device will negotiate trunking encapsulation with peer on
interface
SW1(config-if-range)#switchport trunk encapsulation dot1q
SW1(config-if-range)#switchport mode trunk
SW1(config-if-range)#end
----
Switch#configure terminal
*Mar 1 00:02:43.854: %LINK-5-CHANGED: Interface Vlan1, changed state to administratively down
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#hostname SW3
SW3(config)#do show interface status
Port Name Status Vlan Duplex Speed Type
Fa0/1 notconnect 1 auto auto 10/100BaseTX
Fa0/2 notconnect 1 auto auto 10/100BaseTX
Fa0/3 notconnect 1 auto auto 10/100BaseTX
Fa0/4 notconnect 1 auto auto 10/100BaseTX
Fa0/5 notconnect 1 auto auto 10/100BaseTX
Fa0/6 notconnect 1 auto auto 10/100BaseTX
Fa0/7 notconnect 1 auto auto 10/100BaseTX
Fa0/8 notconnect 1 auto auto 10/100BaseTX
Gi0/1 notconnect 1 auto auto 10/100/1000BaseTX
SW3(config)#interface range fastethernet0/1-2
SW3(config-if-range)#switchport mode trunk
SW3(config-if-range)#end
----
Switch#configure terminal
00:02:36: %LINK-5-CHANGED: Interface Vlan1, changed state to administratively down
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#hostname SW2
SW2(config)#do sh int status
Port Name Status Vlan Duplex Speed Type
Fa0/1 notconnect 1 auto auto 10/100BaseTX
Fa0/2 notconnect 1 auto auto 10/100BaseTX
Fa0/3 notconnect 1 auto auto 10/100BaseTX
Fa0/4 notconnect 1 auto auto 10/100BaseTX
Fa0/5 notconnect 1 auto auto 10/100BaseTX
Fa0/6 notconnect 1 auto auto 10/100BaseTX
Fa0/7 notconnect 1 auto auto 10/100BaseTX
Fa0/8 notconnect 1 auto auto 10/100BaseTX
Gi0/1 notconnect 1 auto auto 10/100/1000BaseTX
SW2(config)#interface range fastethernet0/1-2
^
% Invalid input detected at '^' marker. // 2940 NEEDS A SPACE ON INTERFACE RANGE COMMAND
SW2(config)#interface range fastethernet0/1 - 2
SW2(config-if-range)#switchport mode trunk
SW2(config-if-range)#end
STP is enabled by default on switches (for VLAN 1) and we can quickly verify its operation by using the show spanning-tree command. From here we could see the Bridge ID (priority and MAC address) and the elected root bridge. All switches have a default priority of 32768 and the lower value has the highest priority (which in this case a value of 1).
If all switches have the same priority and extended system ID, the one with the lowest MAC address will be considered the root bridge. Notice that SW1's F0/2 port state is blocked (BLK) since it has the highest MAC address. It also considered this port to be redundant towards the root bridge.
SW2 (C2940) became the root bridge since it has the lowest MAC address compared to the other switches. Also noticed all of its ports are in the forwarding (FWD) state, which is the reward for a root bridge.
SW1#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>
SW1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 001b.0db0.8f00 // MAC ADDRESS OF C2940
Cost 19
Port 2 (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 6416.8dec.a700 // MAC ADDRESS OF C3560
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.2 P2p
Fa0/2 Altn BLK 19 128.3 P2p
SW3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 001b.0db0.8f00
Cost 19
Port 2 (FastEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 001f.9d88.b980 // MAC ADDRESS OF C2960
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 Root FWD 19 128.2 P2p
SW2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 001b.0db0.8f00 // MAC ADDRESS OF C2940
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 001b.0db0.8f00
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 19 128.2 P2p
Since the C2940 has the lowest switching capacity, we could force the C3560 to be the primary root bridge and the C2960 as the secondary root bridge. We do this by issuing the commands spanning-tree vlan 1 root primary and spanning-tree vlan 1 root secondary in global config mode. Notice the priority for primary root and secondary root bridge became 24577 and 28673 respectively (multiples of 4096) and the C2940 F0/2 port went to a blocking state.
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#spanning-tree ?
backbonefast Enable BackboneFast Feature
etherchannel Spanning tree etherchannel specific configuration
extend Spanning Tree 802.1t extensions
logging Enable Spanning tree logging
loopguard Spanning tree loopguard options
mode Spanning tree operating mode
mst Multiple spanning tree configuration
pathcost Spanning tree pathcost options
portfast Spanning tree portfast options
transmit STP transmit parameters
uplinkfast Enable UplinkFast Feature
vlan VLAN Switch Spanning Tree
SW1(config)#spanning-tree vlan ?
WORD vlan range, example: 1,3-5,7,9-11
SW1(config)#spanning-tree vlan 1 ?
forward-time Set the forward delay for the spanning tree
hello-time Set the hello interval for the spanning tree
max-age Set the max age interval for the spanning tree
priority Set the bridge priority for the spanning tree
root Configure switch as root
<cr>
SW1(config)#spanning-tree vlan 1 root ?
primary Configure this switch as primary root for this spanning tree
secondary Configure switch as secondary root
SW1(config)#spanning-tree vlan 1 root primary
SW1(config)#end
00:14:19: %SYS-5-CONFIG_I: Configured from console by console
SW1#show spaning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 6416.8dec.a700
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address 6416.8dec.a700
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.2 P2p
Fa0/2 Desg FWD 19 128.3 P2p
SW3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW3(config)#spanning-tree vlan 1 root ?
primary Configure this switch as primary root for this spanning tree
secondary Configure switch as secondary root
SW3(config)#spanning-tree vlan 1 root secondary
SW3(config)#end
SW3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 6416.8dec.a700
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28673 (priority 28672 sys-id-ext 1)
Address 001f.9d88.b980
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
SW2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 6416.8dec.a700
Cost 19
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 001b.0db0.8f00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p
Just for fun and be able observe an STP loop, I've disabled spanning-tree on all switches using the no spanning-tree vlan 1 command. It could be observed on the last few seconds of the video that the LED lights went fuzzy and CPU utilization shoots up in just a few seconds.
SW1#show processes cpu sorted
CPU utilization for five seconds: 41%/6%; one minute: 21%; five minutes: 8%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
161 12391 22199 558 30.03% 13.42% 3.35% 0 Spanning Tree
155 85 131 648 0.31% 0.07% 0.01% 0 Exec
2 0 1060 0 0.00% 0.00% 0.00% 0 Load Meter
1 0 3 0 0.00% 0.00% 0.00% 0 Chunk Manager
3 0 14 0 0.00% 0.00% 0.00% 0 SpanTree Helper
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