Understanding Linux Network Internals

Bridge and Port IDs

The selection of the root bridge and the port state and roles depends on a set of parameters. Each parameter is assigned a default value that can be changed by user configuration. Here are the main parameters:

Bridge ID

Each bridge is assigned an ID, called the bridge ID, that is defined as an 8-byte value split into two components. The lowest six bytes are assigned the Ethernet MAC address of one of the bridge ports (see Chapter 16), and the highest two bytes are a configurable priority, called the bridge priority . The bridge ID is the field used by the root bridge selection algorithm (see the section "Root Bridge Selection").

Port ID

Each port is assigned an ID. A portion of the ID represents a unique identifier called the port number. The way the port number is assigned is implementation dependent, and its value is meaningful only locally on the bridge. For example, the number can reflect the sequence in which ports were created: the first port is assigned 1, the second port 2, etc. Another approach could use the physical location of the port: for example, the first port on the bus is assigned 1, etc. It is desirable to have the port number assignments be deterministic and consistent across reboots so that the system administrator does not need to change the bridge configuration to reflect the changes after a reboot.

Another portion of the ID, called the port priority , is used to assign a priority to the port (where a lower value means a higher priority). ...