Full duplex

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

Full duplex is a communication mode in which an interface can transmit and receive simultaneously without collisions on the link.

Full duplex lets an interface send and receive data at the same time. This improves performance compared with half duplex, where transmission must take turns. Simplex is the one-way alternative. section 8.1.3 section 8.4

Switched wired Ethernet normally operates in full duplex: each device has its own collision domain, so simultaneous transmissions do not collide on the link. Wireless LANs and hub-based Ethernet use half duplex instead. section 8.1.3 section 8.4

Apply these operational rules:

• Use duplex auto for end-device connections when practical; autonegotiation advertises capabilities and selects the best shared mode, usually prioritizing full duplex. section 8.2
• Configure duplex full manually only when the connected interface is configured compatibly; mismatched settings can leave both interfaces up while severely reducing performance. section 8.3.2
• Check show interfaces status for Duplex values: a-full indicates autonegotiated full duplex, while full indicates manually configured full duplex. section 8.1.3

Cisco interface configuration

SW1(config)# interface g0/1 SW1(config-if)# duplex full SW1(config-if)# interface range f0/1-2 SW1(config-if)# duplex auto The first command manually sets G0/1 to full duplex; the second setting restores autonegotiation for the Fast Ethernet range. Verify the result with do show interfaces status. section 8.1.3

Figure 8.2 Four PCs are connected to a hub, and two attempt to transmit at the same time, resulting in collisions. All four PCs are in the same collision domain. (1) PC2 sends a frame addressed to PC1’s MAC address, and PC4 sends a frame addressed to PC3’s MAC address. (2) The hub attempts to flood both frames at the same time, resulting in collisions. Neither PC1 nor PC3 receive their respective frames intact.

Contrast a hub’s shared collision domain with switched full-duplex links: simultaneous hub transmissions collide, whereas switches isolate connected devices into separate collision domains. section 8.1.3

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

How deep it goes

Defines simplex, half duplex, and full duplex, then adds configuration, autonegotiation, collision domains, and mismatch symptoms.

Examples

Uses switched and hub-based LAN scenarios, Cisco status output, duplex commands, and router-switch autonegotiation examples.

What it assumes

Assumes basic interfaces and Ethernet, while teaching simplex, half duplex, full duplex, collision domains, and autonegotiation from fundamentals.

What you can do after

Operate/configure Cisco interfaces, verify duplex status, and diagnose mismatches using performance symptoms and interface counters.

How it connects

Connects full duplex with half duplex, simplex, Ethernet switches, hubs, autonegotiation, collision domains, and CSMA/CD.

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Related concepts

• digital twinused by · A digital twin is a computational representation of a physical entity, process, or system that models relevant characteristics, state, or behavior.
• WebSocketaddressed by · WebSocket is a network protocol that enables persistent, bidirectional, full-duplex communication between endpoints over a single TCP connection.
• half duplexcontrasts with · Half duplex is a communication mode in which data can travel in both directions, but transmission and reception cannot occur simultaneously.
• duplex mismatchcontrasts with · A duplex mismatch is a link condition in which two connected interfaces use different duplex settings or incompatible autonegotiation behavior.
• collaborationused by · Collaboration is coordinated activity in which multiple participants contribute toward a shared goal or outcome.
• data flowkind of · Data flow is the movement and transformation of data through the components, operations, or processing stages of a system or program.

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.