STP - port identification

When implementing Spanning Tree Protocol (STP), each interface on a device is assigned a port identification number. All interfaces, including physical, virtual, and tunnel interfaces, are assigned such a number. This number is assigned automatically and sequentially by the IOS based on the interfaces that exist on the device upon bootup. As more interfaces are configured (i.e. virtual interfaces, SVIs, tunnels and others), these numbers are assigned sequentially. The number and how it is implemented is also dependent upon if the device in question is modular in nature (with cards that add more physical ports) or fixed.

The port identification value that has been assigned to each interface can be seen in various STP related commands. The output of the show spanning-tree command can be seen below:

SW1#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID    Priority    32769 Address     000f.34ca.1000 Cost        19 Port        19 (FastEthernet0/17) Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1) Address     0011.bb0b.3600 Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec Aging Time 300 Interface           Role Sts Cost      Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/14              Desg FWD 19        128.16   P2p Fa0/17              Root FWD 19        128.19   P2p

The value that appears after the "Port" label indicates the port identification number of the root port. The port identification number is also part of the Prio.Nbr value as well, and is the number that appears after the "." in the 128.16 and 128.19 designation.

The port identification number is also used as a tie-breaker of last resort to determine which port will be set to the blocked state.


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