Cisco WS-C4003 Software Guide - Page 99

Chapter 7 Configuring Spanning Tree How Spanning Tree Protocols Work How a Switch or Port Becomes the Root Switch or Root Port If all switches in a network are enabled with default settings, the switch with the lowest MAC address becomes the root switch. In the network shown in Figure 7-1, Switch A, with the lowest MAC address, is the root switch. However, due to traffic patterns, number of forwarding ports, or line types, Switch A might not be the ideal root switch. A switch can be forced to become the root switch by increasing the priority (that is, lowering the priority number) on the preferred switch. This causes the spanning tree to recalculate the topology and make the selected switch the root switch. Figure 7-1 Configuring a Loop-Free Topology DP DP A DP DP D RP DP DP S5688 RP RP DP B C RP = Root Port DP = Designated Port You can also change the priority of a port in order to make it the root port. When the spanning tree topology is based on default parameters, the path between source and destination stations in a switched network might not be ideal. The goal is to make the fastest link the root port, connecting higher-speed links to a port that has a higher number than the current root port can cause a root-port change. For example, assume that a port on Switch B is a fiber-optic link. Also, another port on Switch B (an unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the high-speed fiber-optic link. By changing the Port Priority parameter for the UTP port to a higher priority (lower numerical value) than the fiber-optic port, the UTP port becomes the root port. You could also accomplish this scenario by changing the Port Cost parameter for the UTP port to a lower value than that of the fiber-optic port. How Bridge Protocol Data Units Work BPDUs contain configuration information about the transmitting switch and its ports, including switch and port MAC addresses, switch priority, port priority, and port cost. Each configuration BPDU contains this information: • The unique identifier of the switch that the transmitting switch believes to be the root switch • The cost of the path to the root from the transmitting port • The identifier of the transmitting port The switch sends configuration BPDUs to communicate and compute the spanning tree topology. A MAC frame conveying a BPDU sends the switch group address to the destination address field. All switches connected to the LAN on which the frame is transmitted receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses the information in the frame to calculate a BPDU, and if the topology changes, initiates a BPDU transmission. 78-12647-02 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 7-3