Elements
Our network above
consists of 4 switches
We will investigate
spanning tree election and will try to modify its parameters to influence the
selection
Configuration
SW1,
SW2, SW3, SW4
conf
t
vlan
10
We
will configure all the links to be access ports in vlan 10 created above
conf
t
int
f1/1
switchport
mode access
switchport
access vlan 10
speed
100
duplex
full
Verification
Now let us check what the result of the selection process is as we left all values to its default
SW1#show spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address c201.3602.0001
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c201.3602.0001
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/1 128.42 128 19 FWD 0 32768 c201.3602.0001 128.42
FastEthernet1/2 128.43 128 19 FWD 0 32768 c201.3602.0001 128.43
As can be seen, SW1 elects itself as the root bridge, why? Because the priority value on all switches are left to default (32768) which leave the MAC address to handle the process , as SW1 appears to be the root bridge that means it has the lowest MAC address and as we can see all of its ports are in FWD state
SW2#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address c201.3602.0001
Cost 19
Port 42 (FastEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c202.3602.0001
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/1 128.42 128 19 FWD 0 32768 c201.3602.0001 128.42
FastEthernet1/2 128.43 128 19 FWD 19 32768 c202.3602.0001 128.43
FastEthernet1/4 128.45 128 19 FWD 19 32768 c202.3602.0001 128.45
SW3#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address c201.3602.0001
Cost 19
Port 42 (FastEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c203.3602.0001
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/1 128.42 128 19 FWD 0 32768 c201.3602.0001 128.43
FastEthernet1/2 128.43 128 19 BLK 19 32768 c202.3602.0001 128.43
R4#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address c201.3602.0001
Cost 38
Port 45 (FastEthernet1/4)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c204.3602.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/4 128.45 128 19 FWD 19 32768 c202.3602.0001 128.45
As can be seen from the output above, R3 is the least preferred switch to be elected as the root bridge which can be confirmed from the BLK status of its F1/2 interface which was chosen to be blocked because it has higher priority than F1/1 interface (128.43), R4 has a higher MAC address but it’s not part of the loop
Now, what we want to do is to make SW4 the root bridge, we have two commands that we can configure to do that:
spanning-tree vlan 10 priority <0-65535> (we can choose any value less than 32768 in decrements of 4096)
spanning-tree vlan 10 root primary (which automatically will give the switch a value of 24576 which is less than the default and will be preferred to be the root bridge)
SW4
conf t
spanning-tree vlan 10 root primary
VLAN 10 bridge priority set to 8192
VLAN 10 bridge max aging time unchanged at 20
VLAN 10 bridge hello time unchanged at 2
VLAN 10 bridge forward delay unchanged at 15
Now, let us check the outputs again
SW1#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address c204.3602.0000
Cost 38
Port 42 (FastEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c201.3602.0001
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/1 128.42 128 19 FWD 19 32768 c202.3602.0001 128.42
FastEthernet1/2 128.43 128 19 FWD 38 32768 c201.3602.0001 128.43
As can be seen SW1 is no longer the root bridge . c204 represents SW4
SW2#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address c204.3602.0000
Cost 19
Port 45 (FastEthernet1/4)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c202.3602.0001
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/1 128.42 128 19 FWD 19 32768 c202.3602.0001 128.42
FastEthernet1/2 128.43 128 19 FWD 19 32768 c202.3602.0001 128.43
FastEthernet1/4 128.45 128 19 FWD 0 8192 c204.3602.0000 128.45
SW3#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address c204.3602.0000
Cost 38
Port 43 (FastEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address c203.3602.0001
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/1 128.42 128 19 BLK 38 32768 c201.3602.0001 128.43
FastEthernet1/2 128.43 128 19 FWD 19 32768 c202.3602.0001 128.43
SW4#sh spanning-tree vlan 10 brief
VLAN10
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address c204.3602.0000
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 8192
Address c204.3602.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/4 128.45 128 19 FWD 0 8192 c204.3602.0000 128.45
What we can notice is that SW3 F1/1 is now blocked, why? because the factor that precede the port priority is the path cost to the root bridge , F1/1 cost is 38 (default value for FastEthernet connection is 19 , which means two links) , F1/2 cost is 19 which means it’s preferred and its role will be root port and F1/2 will be in BLK state
