Advanced Spanning Tree
Share this page:root primary - sets the priority to:
if ROOT > 24576 - sets to 24576 (priority 24576 sys-id-ext 12)
if ROOT =< 24576 - sets to 4096
root secondary - sets the priority to 28762
GREAT COMMAND:
#show spanning-tree bridge <- See the MAC address of the Switch
**#show version | i Base **
Cat-1#show spanning-tree vlan 12
VLAN0012
Spanning tree enabled protocol ieee
Root ID Priority 24588 <— ABOUT THE ROOT BRIDGE, 24588 = 32768 + 12 (vlan 12) - 8192
Address ec44.768a.6d80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24588 (priority 24576 sys-id-ext 12) <— ABOUT THIS SWITCH (LOCAL Bridge)
Address ec44.768a.6d80 <– ON ROOT BridgeID and RootID have the same MAC
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type **<— ABOUT INTERFACES IN THIS VLAN **
-—————— —- — ——— ——– ——
Gi3/0/19 Desg FWD 4 128.127 P2p <— COST IS 4 CAUSE THIS IS GigabitEthernet Port
Gi3/0/20 Desg FWD 4 128.128 P2p (on FastEth is would be 19)
Great command to check the ROOT>
#show spanning-tree root
** Root Hello Max Fwd**
**Vlan Root ID Cost Time Age Dly Root Port**
**-————— ——————– ——— —– — — ————**
**VLAN0001 32769 aabb.cc00.0600 200 2 20 15 Et2/2**
**VLAN0100 24676 aabb.cc00.0600 200 2 20 15 Et2/2**
**VLAN0200 24776 aabb.cc00.0700 100 2 20 15 Et2/2**
**VLAN0300 24876 aabb.cc00.0800 100 2 20 15 Et3/1**
**VLAN0400 24976 aabb.cc00.0900 0 2 20 15 <— COST TO ROOT IS 0, SO I’m the ROOT!!!**
BEST PRACTICE:
Change the COST on the interface level to change the PATH
Change the PORT PRIORITY to influence ONLY the NEIGHBORING SWITCH