The next item to specify is the cost of each link between branch devices (end stations are not counted). Since we do not want to forward traffic onto low-speed links if we can avoid it, we assign a link cost based upon port speed. The recommended cost figures are shown in Table 1. A 100 Mbps port would cost 19 while a 10 Mbps port would cost 100. Two 100 Mbps cascaded links (two links requiring two bridges to reach the root) would cost 38. This would represent the path cost. Therefore, all designated ports on Bridge C in Figure 2 would advertise a path cost of 38 — assuming traffic is at 100 Mbps. STP uses path cost to the root for arbitrating the ideal route. All other routes are blocked. If path costs for each port on each bridge are manually assigned by the operator, configuration could be tedious. Once these costs are entered, we can let the STP determine the best topology that will not introduce loops.

Table 1 — Link Cost Recommendations

Bridges must communicate with one another to execute the STP, and they accomplish this by sending configuration messages in the form of Bridge Protocol Data Units (BPDUs). The BPDU is sent as a multicast message (01-80-C2-00-00-00) within a reserved range of MAC addresses which are consumed by each bridge and not forwarded. Each bridge must periodically advertise its understanding of the topology and the path cost to the root for each of its ports. The BPDU format is shown in Figure 3. The more important fields will be discussed in the following example.

Figure 3 — There are two types of Bridge Protocol Data Unit types. Configuration messages are normally sent; however, a designated bridge can initiate a Topology Change message.

Refer to Figure 2 where we have four interconnected bridges connected to end stations. We will assume a stable network with a duly elected root bridge (R) and several designated bridges (A, B, C). All ports on the root bridge are designated ports since they emanate to end stations from the root. All other ports are designated ports or candidates for designated ports except three ports that point towards the root. They are Port 1 on Bridge A, Port 1 on Bridge B and Port 1 on Bridge C. These are root ports. Assume that all ports on all bridges are rated for 100 Mbps except Bridge B which is only rated for 10 Mbps. Examining Table 1 we find that the corresponding link cost for 100 Mbps is 19 and for 10 Mbps it is 100. This information will be needed when constructing the BPDUs. There are four links of interest (S, T, U, and V).

The root bridge begins the process of sending a periodic configuration message based upon the Hello Time which is typically two seconds. In this message, the Root Identifier and Bridge Identifier would be the same since this bridge thinks it is the root by having the lowest-value bridge identifier. The Root Path Cost will be zero because this is the root bridge.

Since all ports on the root bridge are designated ports, a configuration BPDU will go out each port along with the corresponding port identifier. This process repeats every Hello Time.

First-tier bridges (those directly connected to the root bridge — in our case, Bridges A and B) will receive the BPDUs on Links S and T and will analyze the data. Each bridge will verify that the Root Identifier is indeed lower than its own bridge identifier. If that is true, each bridge will assemble its own set of BPDUs for transmission out its designated ports. The Root Identifier Field will not change. Each designated port will increase the total cost of getting back to the root by that port's individual Root Path Cost. Since both of these are tier-1 bridges, the Root Path Cost would be the link cost for Bridge A (19) and for Bridge B (100). The bridge and port identifiers would represent data from each bridge. The Hello Time will remain that specified by the root. BPDUs are then sent out each bridge's designated ports. Second-tier bridges receive the BPDUs (in our case, Bridge C) and the path cost is upped again. In our example, the Root Path Cost from Bridge C would be 38. Bridge C will send out its set of BPDUs — ending the process since no more tier bridges remain. End stations do not participate in the process and ignore the messages. Because the propagation of messages from the root will take time, the STP standard establishes an arbitrary limit to the number of cascaded bridges at seven. This limit does not apply to conventional switches that, in any event, should not be present in an STP network.