Understanding Linux Network Internals

Defining the Active Topology

Each bridge, with the help of the local configuration and the information received with the ingress configuration BPDUs, is able to accomplish the following:

• Elect the root bridge

• Select one of its ports as the root port

• For each port, identify the designated bridge and designated port for the LAN to which the port belongs

Those tasks, which I will refer to as a configuration update, are needed every time something changes in the network that may require a change in the topology. For instance:

• A port is either enabled or disabled.

• A port's Message Age timer expires. In this case, the port is restarted (i.e., assigned the designated role).

• The local configuration of a bridge changes.

• A bridge port receives a configuration BPDU with a superior priority vector compared to the one previously received on the same port.

Note that a configuration update is triggered on the bridge where the configuration is changed or where a port changes administrative state. The other bridges will follow (if necessary) upon seeing these changes reflected in the information carried by the BPDUs they receive.

Let's see how the configuration update's tasks are taken care of, one by one.

Root Bridge Selection

We saw in the section "Bridge and Port IDs" how a bridge ID is defined. An algorithm based only on the use of the MAC address for the selection of the root bridge would be sufficient to ensure a deterministic selection, given that MAC addresses are unique worldwide. However, the addition ...