Understanding Spanning-Tree Port-Priority
Root port selection:
1st -> Lowest cumulative cost to root [ Inverse value based on interface bandwidth]
2nd –> Lowest Upstream BID
3rd —> Lowest Port ID
|–> Lowest port priority (0-255) [default = 128]
|–> Lowest port number 128.X [ X is port number]
Fa 0/1—————————- Fa0/1
Switch 0 Switch 1
Fa0/2 –X————————- Fa0/2
Switch0#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.429A.2814
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 0010.1147.9454
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/2 Altn BLK 19 128.2 P2p
Fa0/1 Root FWD 19 128.1 P2p
Switch1#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.429A.2814
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 0001.429A.2814
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
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First step – lowest cost to root
Both are interfaces are having same cost [19]
Second step – Lowest upstream bridge ID
Both the interfaces will have same bridge ID since neighbor is common.
Third step – Port priority is same [ 128 ] . But Port number is different.
Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/2 Altn BLK 19 128.2 P2p
Fa0/1 Root FWD 19 128.1 P2p
So Fa0/1 is Root port.
Networking Thop 