Forwarding State; Disabled State; How A Switch Or Port Becomes The Root Switch Or Root Port - Cisco NCS 4200 Series Configuration Manual

Lan switching

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Configuring PVST+ and RPVST+

Forwarding State

A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state 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 participate in frame forwarding or in the spanning tree. An

interface in the disabled state is non operational.

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

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 figure below , 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

LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)

Forwarding State

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