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| 3.1 |  | Spanning-Tree Protocol Operation |
| 3.1.6 |  | Port ID |

The Port ID is the third parameter used by the spanning-tree algorithm to determine the path to the Root Bridge. The Port ID is a 2-byte STP parameter consisting of an ordered pair of numbers. The first is called the Port Priority, and the second is called the Port Number. On a CatOS switch, the first number is 6 bits and the second number is 10 bits. On an IOS-based switched, both numbers are 8 bits.

Port ID should not be confused with the Port Number. The Port Number is just part of the Port ID. Lower Port IDs are preferred over higher Port IDs in the STP decision process. The order relation for Port IDs is explained in the following example. If (s,t) and (u,v) represent two Port IDs (the first coordinate being the Port Priority and the second coordinate being the Port Number), (s,t)<(u,v) if and only if:

- s<u
- s=u and t<v

Two Port IDs cannot be equal, because Port Numbers uniquely identify the switch ports on a Catalyst switch.

The Port Priority is a configurable STP parameter (unlike the Port Number). The values range from 0 to 255 on an IOS-based switch, with a default value of 128.

Port Numbers are numerical identifiers used by Catalyst switches to enumerate the ports. The Port Number leaves room for 28 (256) ports on an IOS-based switch (Cisco routers also use the 8-bit/8-bit Port ID definition).