Disabled State; How A Switch Or Port Becomes The Root Switch Or Root Port - Cisco WS-C3560-48PS-S Software Configuration Manual

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Configuring STP

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:

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

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 15-2 Spanning-Tree Topology

RP = Root Port

DP = Designated Port

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.

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Does not learn addresses

Does not receive BPDUs

15-2, Switch A is elected as the root switch because

Catalyst 3560 Switch Software Configuration Guide

Understanding Spanning-Tree Features

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