Duplex

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Duplex is an interface transmission setting that specifies whether an Ethernet interface operates in full-duplex or half-duplex mode.

Duplex is an Ethernet interface setting that determines whether the interface can send and receive data simultaneously. Half-duplex interfaces send or receive, but not both at once; full-duplex interfaces do both at the same time. section 8.1.3

Duplex behavior depends on the connected medium and device:

• Switched wired Ethernet normally operates in full-duplex mode, allowing simultaneous transmission and reception. section 8.1.3
• Wireless LANs and legacy hub-based Ethernet operate in half-duplex mode because devices share the transmission medium. section 8.1.3 section 8.4
• Autonegotiation lets connected interfaces advertise capabilities and select the best shared speed and duplex mode. section 8.2
• Leave autonegotiation enabled for end devices such as PCs; manual settings may be used on infrastructure links but introduce human-error risk. section 8.2

Cisco IOS configuration and verification

Configure a full-duplex link manually with `interface g0/1` followed by `duplex full`. Configure end-user ports to negotiate with `interface range f0/1-2` followed by `duplex auto`. Verify the result with `show interfaces status`; manually configured full duplex appears as `full`, while negotiated full duplex appears as `a-full`, where `a-` indicates autonegotiation. section 8.1 section 8.1.3

A duplex mismatch occurs when connected interfaces use different duplex settings or when only one side uses autonegotiation. Both interfaces can remain up/up, but performance falls sharply. The half-duplex side may show increasing collisions and late collisions, while the full-duplex side may show Runts or CRC errors. section 8.3.2

Figure 8.1 Interface configurations on R1 and SW1: (1) configuring R1’s G0/1 interface with an IP address, description, and manual speed and duplex settings; (2) configuring SW1’s G0/1 interface with a description, and manual speed and duplex settings; (3) configuring SW1’s connections to end-user devices (PCs) using autonegotiation; (4) disabling SW1’s unused interfaces.

Compare router and switch interface configurations, including manually configured duplex, autonegotiated end-user ports, and disabled unused interfaces. section 8.1

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Explains half- and full-duplex behavior, autonegotiation, CLI configuration, verification output, and mismatch symptoms.

Examples

Uses switched Ethernet, Wi-Fi, hub collisions, Cisco IOS commands, status output, and router-switch autonegotiation scenarios.

What it assumes

Defines interfaces, ports, switches, hubs, and shared media before explaining duplex operation.

What you can do after

Diagnose duplex mismatches with interface status, collision, late-collision, Runt, and CRC information.

How it connects

Connects duplex with Ethernet switches, hubs, wireless LANs, autonegotiation, collisions, CSMA/CD, speed, and interface errors.

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Related concepts

• autonegotiationconfigured by · Autonegotiation is a link establishment technique in which connected interfaces automatically agree on compatible communication parameters such as speed and duplex.
• network interfacepart of · A network interface is a physical or logical connection point through which a system sends and receives network traffic.
• etherchannelused by · EtherChannel is a link-aggregation technology that combines multiple physical Ethernet links into one logical connection.
• show interfaces statusmeasured by · show interfaces status is a Cisco IOS switch command that displays interface status, descriptions, speed, and duplex information.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.