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17-8

Catalyst 3750 Switch Software Configuration Guide

78-16180-02

Chapter 17 Configuring STP

Understanding Spanning-Tree Features

Disabled State

A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree.

An interface in the disabled state is nonoperational.

A disabled interface performs these functions:

• Discards frames received on the interface

• Discards frames switched from another interface for forwarding

• Does not learn addresses

• Does not receive BPDUs

How a Switch or Port Becomes the Root Switch or Root Port

If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest

MAC address becomes the root switch. In Figure 17-3, Switch A is elected as the root switch because

the switch priority of all the switches is set to the default (32768) and Switch A has the lowest MAC

address. However, because of traffic patterns, number of forwarding interfaces, or link types, Switch A

might not be the ideal root switch. By increasing the priority (lowering the numerical value) of the ideal

switch so that it becomes the root switch, you force a spanning-tree recalculation to form a new topology

with the ideal switch as the root.

Figure 17-3 Spanning-Tree Topology

When the spanning-tree topology is calculated based on default parameters, the path between source and

destination end stations in a switched network might not be ideal. For instance, connecting higher-speed

links to an interface that has a higher number than the root port can cause a root-port change. The goal

is to make the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on

Switch B (a 10/100 link) is the root port. Network traffic might be more efficient over the Gigabit

Ethernet link. By changing the spanning-tree port priority on the Gigabit Ethernet port to a higher

priority (lower numerical value) than the root port, the Gigabit Ethernet port becomes the new root port.

86475

RP = Root Port

DP = Designated Port

Cisco WS-C3750-48PS-S User Manual

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