Understanding How A Switch Or Port Becomes The Root Switch Or Root Port - Cisco WS-C2948G-GE-TX Configuration Manual

Chapter 7 Configuring Spanning Tree
The following three things determine the topology of an active switched network:
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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
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 (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
DP
RP = Root Port
DP = Designated Port
You can also 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 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.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
The unique switch identifier (MAC address of the switch) that is associated with each switch
The path cost to the root that is associated with each switch port
The port identifier (MAC address of the port) that is associated with each switch port
Figure
Configuring a Loop-Free Topology
DP
DP
A
RP
DP
RP DP
RP
B C
7-1, Switch A, with the lowest MAC address, is the root switch.
D
DP DP
Understanding How STPs Work
7-3