Cisco Systems ME3400G2CSA How a Switch or Port Becomes the Root Switch or Root Port

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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide

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Chapter14 Configuring STP Understanding Spanning-Tree Features

A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwa rding sta te

from the learning state.

An interface in the forwarding state performs these functions:

•Receives and forwards frames received on the interface

•Forwards frames switched from another interface

•Learns addresses

•Receives BPDUs

A Layer 2 interface in the disabled state does not particip ate 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 forwar ding

•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 Figure14-2, 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.

Figure14-2 Spanning-Tree Topology

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RP = Root Port

DP = Designated Port