Cisco WS-C2980G-A Software Guide - Page 99

Chapter 7 Configuring Spanning Tree Understanding How STPs Work The following three things determine the topology of an active switched network: • The unique switch identifier (MAC address of the switch) that is associated with each switch • The path cost to the root associated with each switch port • The port identifier (MAC address of the port) associated with each switch port 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 and the root port and designated port for each switched segment. Understanding 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. You can force a switch to become the root switch by increasing the priority (that is, lowering the 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 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 the source and the 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. 78-15486-01 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 7-3