Understanding Linux Network Internals

Processing Ingress Frames

We saw in Chapter 14 how a simple bridge handles ingress traffic. Let's now see how a bridge running the STP handles ingress traffic.

Ingress traffic now includes not only data traffic, but BPDUs as well. Bridges handle data traffic the same way, regardless of whether STP is enabled. The only difference is that ports blocked by STP cannot forward any data traffic because they are not considered part of the tree.

Ingress BPDUs

Unlike data traffic, ingress BPDUs are accepted on any port that has not been administratively disabled, including those in the blocking state.

Configuration BPDUs and TCN BPDUs can be distinguished thanks to the BPDU type field, as shown in Figure 15-7. In the section "Letting All Bridges Know About a Topology Change," we already saw how ingress TCN BPDUs are handled. In the next section, we will see how configuration BPDUs are processed.

Ingress Configuration BPDUs

Figure 15-22 shows how ingress configuration BPDUs are processed.

The handling of an ingress BPDU depends on whether its priority vector is:

Better than the one currently known to the receiving bridge's port

In this case, the BPDU triggers a configuration update that includes the new root port, the designated ports, and the new state for all ports.

Figure 15-22. Processing ingress configuration BPDUs

The same as the one already known to the receiving bridge's port

This is what would ...