Label the port types (RP=Root Port, DP=Designated Port, NDP=non-Designated Port) and the STP states (F=Forwarding, B=Blocking) in Figure 7-30. The Bridge IDs are labeled. All links are Fast Ethernet. Assume that there is only a single VLAN and that the portvlanpri command has not been used.
Figure A-6 provides the labels requested in Question 1 for Figure 7-30.
Cat-B becomes the Root Bridge because it has the lower BID. Cat-A therefore needs to select a single Root Port. In the previous examples of back-to-back switches, the links did not cross and Port 1/1 became the Root Port because of the lower Port ID (0x8001).
In this case, the crossed links force you to think about the fact that it is the received Port ID that influences the Cat-A, not Cat-A’s local Port ID values. Although Cat-A:Port-1/2 has the higher local value, it is receiving the lower value. As a result, Port-1/2 becomes the Root Port. Understanding this issue is critical to effectively use portvlanpri load balancing.