Understanding Linux Network Internals

Bridge Protocol Data Units (BPDUs)

Bridges exchange protocol frames, called BPDUs, that include enough information for them to agree on who is the root bridge, and to decide on the roles and states for their local ports. There are two kinds of BPDUs:

Configuration BPDU

Used to define the loop-free topology. You will see in the section "When to Transmit Configuration BPDUs" what conditions trigger the transmission of these BPDUs.

Topology Change Notification (TCN) BPDU

Used by a bridge to notify the root bridge about a detected topology change. See the section "Topology Changes."

Figure 15-8 shows the format of both BPDUs. Note that the two types share the same first three fields and can be distinguished by the BPDU type parameter.

Figure 15-8. a) Configuration BPDU; b) BPDU

Table 15-3 shows the combinations of protocol ID and protocol version used by the three IEEE STPs. In this chapter, we will look only at the basic 802.1D protocol and briefly introduce the other two in the section "Overview of Newer Spanning Tree Protocols."

Table 15-3. BPDU versions

Protocol name	Protocol ID	Protocol version
STP (802.1D-1998)	0	0
RSTP (802.1D-2002 or 802.1w)	0	2
MSTP (802.1Q-2002 or 802.1s)	0	3

Configuration BPDU

Here is the meaning of the fields in the configuration BPDU:

Flags

Only two flags are used: TC (Topology Change) and TCA (Topology Change Acknowledgment). The use of both ...