Introduction to Spanning-Tree |

Hi I have one doubt why Sw3 is port priorities are showing 128 + port number, actually the port ID = default port priority + Interface number + 2 right …?

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/14           Desg FWD 19        128.14   P2p 
Fa0/16           Desg FWD 19        128.16   P2p

Hello Networks

The port priority is defined as the port priority + the interface number. The port priority is 128 by default, but can be adjusted anywhere from 0 to 240 in increments of 16 (0, 16, 32, 48 etc). The interface number is composed of 12 bits so this number can be anywhere between 0 and 4096. This means that a switch with up to 4096 ports can be acomodated.

From this information, the port priority is always priority.port. This means that by default, the priority of port FastEthernet 0/14 will always be 128.14.

This is defined in section 9.2.7 Encoding of Port Identifiers on the 802.1D-2004 specification and can also be found in this Cisco command reference.

I hope this has been helpful!