Cisco WS-X6K-SUP1A-2GE-RF Software Guide - Page 115

Chapter 8 Configuring Spanning Tree Understanding How Spanning Tree Protocols Work In a switched network, the root switch is the logical center of the spanning tree topology. A spanning tree protocol uses BPDUs to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment. Understanding How a Switch Becomes the Root Switch If all switches are enabled with default settings, the switch with the lowest MAC address in the network becomes the root switch. In Figure 8-1, Switch A is the root switch because it has the lowest MAC address. 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 numerical priority number) on the preferred switch. This action causes the spanning tree to recalculate the topology and make the selected switch the root switch. Figure 8-1 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 change the priority of a port 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. Connecting higher-speed links to a port that has a higher number than the current root port can cause a root-port change. The goal is to make the fastest link the root port. 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 fiber-optic port to a higher priority (lower numerical value) than the UTP port, the fiber-optic port becomes the root port. You could also accomplish this scenario by changing the Port Cost parameter for the fiber-optic port to a lower value than that of the UTP port. Understanding 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 78-13315-02 Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 8-3