How A Switch Or Port Becomes The Root Switch Or Root Port; How Bridge Protocol Data Units Work - Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual

Chapter 7 Configuring Spanning Tree

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
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
DP
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 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.
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Configuring a Loop-Free Topology
DP
DP
A
RP
DP
RP DP
RP
B C
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
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
Figure 7-1, Switch A, with the lowest MAC address,
D
DP DP
How Spanning Tree Protocols Work
7-3