Understanding Linux Network Internals

Transmitting Configuration BPDUs

We saw what conditions trigger the transmission of configuration BPDUs in the section "When to Transmit Configuration BPDUs." Regardless of why a configuration BPDU is transmitted, the logic of Figure 15-21 applies.

Figure 15-21. Configuration BPDU transmission logic

The per-port Hold timer enforces a rate limit of one BPDU per second. When a BPDU is transmitted, the timer is started. If another transmission is attempted and the timer is already pending, the BPDU is not transmitted and a flag is set in the bridge port configuration block. When the timer expires, it checks the flag and transmits a configuration BPDU if it finds the flag set.

When the root bridge transmits a BPDU, the timers are initialized to the values configured locally; otherwise, the ones received from the root bridge are used instead. Message age and root path cost are both 0 for the root bridge.

Also, the following is not shown in the figure:

• When the bridge needs to acknowledge the reception of a TCN BPDU, it sets the TCA flag .

• The root bridge sets the TC flag if the Topology Change timer is running.

• Nonroot bridges set the TC flag if the last BPDU received on the root port had the TC flag set.