Half-Duplex and Full-Duplex Ethernet

Learn simplex, half-duplex, and full-duplex Ethernet, collision domains, CSMA/CD, auto-negotiation, duplex mismatches, and Cisco IOS troubleshooting.

Duplex describes whether a network interface can send and receive data, and whether those actions can occur at the same time. Understanding duplex is essential for explaining Ethernet collisions, interpreting interface counters, and troubleshooting slow links.

This lesson focuses on Ethernet behavior at the physical and data-link layers. For background, review the OSI Reference Model and computer networking fundamentals.

Duplex Communication Fundamentals

Communication direction describes how data moves between two endpoints:

One-way communication: Data travels in only one direction.
Alternating two-way communication: Both endpoints can transmit, but they take turns.
Simultaneous two-way communication: Both endpoints can transmit and receive at the same time.

Mode	Transmission Direction	Can Both Ends Send Simultaneously?	Collision Possibility	Typical Example
Simplex	Only one direction	No	Not applicable between the sender and receiver	A basic broadcast-style device or one-way sensor link
Half-duplex	Two-way, but alternating	No	Yes, on a shared medium	Legacy Ethernet connected through a hub
Full-duplex	Two-way, simultaneous	Yes	No on a properly operating point-to-point link	A workstation connected directly to a switch port

Simplex

Simplex communication travels in only one direction. The receiving endpoint does not send traffic back over the same communication arrangement. Simplex is different from both Ethernet half-duplex and full-duplex, because Ethernet interfaces normally support two-way communication.

Half-Duplex

Half-duplex permits two-way communication, but only one side transmits at a time. An interface must wait before transmitting if the shared medium is already being used.

Full-Duplex

Full-duplex permits both connected interfaces to transmit and receive simultaneously. It requires a suitable point-to-point connection and compatible operation at both ends.

Half-Duplex Ethernet

In half-duplex Ethernet, an interface can transmit or receive at a given moment, but it cannot do both at once. This mode is associated primarily with older shared Ethernet designs, including networks built with hubs.

Shared Segments and Collision Domains

A collision domain is a network segment in which simultaneous transmissions can interfere with one another. A hub is a Layer 1 device: it repeats an incoming electrical signal out of its other ports. It does not make forwarding decisions and does not divide the attached devices into separate collision domains.

When several hosts connect to a hub, they share the same physical medium and collision domain. If two hosts transmit at nearly the same time, their signals interfere. The hosts must contend for access and retransmit frames that were damaged by a collision.

CSMA/CD

CSMA/CD means Carrier Sense Multiple Access with Collision Detection. It is the access method associated with shared, half-duplex Ethernet.

Carrier sense: A device listens to determine whether the medium appears idle.
Multiple access: Multiple devices share the same medium and may attempt to transmit.
Transmission: A device sends a frame when the medium is available.
Collision detection: The device monitors the medium while transmitting. If the observed signal indicates interference, it identifies a collision.
Backoff: The device stops transmitting and waits for a time selected by the Ethernet backoff algorithm.
Retransmission: After waiting, the device attempts to send the frame again.

Collisions consume transmission time and cause retransmissions. As more devices contend for the medium, effective throughput falls. The nominal link speed does not represent the useful application throughput available during heavy contention.

Full-Duplex Ethernet

A full-duplex Ethernet link provides separate transmit and receive paths between directly connected interfaces. Each endpoint can send on its transmit path while receiving on its receive path.

Because the paths are independent on a point-to-point link, the endpoints do not contend for one shared medium. Collisions and CSMA/CD do not apply when the link is correctly operating in full-duplex mode.

Full-duplex operation improves performance for bidirectional traffic. For example, a workstation can upload a file while receiving data from a server without alternating access to the link.

Characteristic	Half-Duplex	Full-Duplex
Transmission	Transmit or receive, but not both at once	Transmit and receive simultaneously
Medium	Shared medium is common	Dedicated point-to-point paths
Collisions	Possible when devices transmit together	Not expected on a correctly operating link
CSMA/CD	Required for shared Ethernet access	Not used
Performance	Reduced by contention, collisions, and backoff	Supports concurrent traffic in both directions
Typical use	Legacy or specialized environments	Modern switched Ethernet

Hubs, Switches, and Collision Domains

Device Type	Collision Domain Behavior	Typical Duplex Mode	CSMA/CD Relevance
Hub	All connected ports share one collision domain	Half-duplex	Required because devices contend for the shared medium
Switch port	Each active port normally forms its own collision domain	Full-duplex	Not used on a correctly operating point-to-point link

A switch is different from a hub at both Layer 1 and Layer 2. A switch still sends and receives electrical or optical signals through physical interfaces, but it also examines Ethernet frames and forwards them based on MAC addresses. A direct switch-to-host connection is normally a dedicated point-to-point link, so it can operate full-duplex.

A full-duplex Ethernet link cannot be created by simply selecting full duplex on one port of a hub segment. The physical connection must provide a point-to-point relationship. A shared hub segment remains a shared medium and normally requires half-duplex operation.

Ethernet Speed and Duplex Negotiation

Auto-negotiation is an Ethernet mechanism that allows two connected interfaces to advertise capabilities and select mutually supported speed and duplex settings. When both peers support it and are configured consistently, they normally select the highest mutually supported mode, with full-duplex preferred over half-duplex at the same speed.

Speed and duplex can also be configured manually. The important rule is that both ends must use compatible settings. A manually configured speed or duplex value on one side must match the operational behavior of the peer.

Why Mixing Fixed and Automatic Settings Is Risky

Suppose one interface is forced to 100 Mbps full-duplex while the other interface is left to auto-negotiate. The auto-negotiating interface may detect the speed through parallel detection, but a fixed peer may provide no duplex negotiation information. In that situation, the auto side commonly selects 100 Mbps half-duplex. The result is a full-duplex-to-half-duplex mismatch.

For this reason, avoid configuring only one side manually. Use auto-negotiation on both ends when supported, or explicitly configure matching speed and duplex values on both ends when a platform, legacy device, or design requirement requires fixed settings.

Cisco IOS Configuration Examples

To restore automatic selection on a supported Cisco IOS interface:

interface gigabitEthernet 0/1
 speed auto
 duplex auto

To configure fixed values, configure the same compatible values on the connected peer:

interface fastEthernet 0/1
 speed 100
 duplex full

Interface command availability varies by platform and interface type. Always verify the resulting operational state after changing a setting.

Duplex Mismatch

A duplex mismatch occurs when two connected interfaces use different duplex modes. The common case is one side operating full-duplex while the other operates half-duplex.

The link may remain physically up and pass small amounts of traffic, which makes this fault easy to overlook. However, transmissions can interfere from the perspective of the half-duplex endpoint, while the full-duplex endpoint continues transmitting without using collision detection. This creates asymmetric symptoms.

The half-duplex side may report collisions and late collisions.
The full-duplex side may report CRC or FCS errors, runts, input errors, or poor receive performance.
Pings may succeed while file transfers, voice, video, or other sustained applications perform badly.
Throughput may be low and retransmissions may make the application appear unreliable.

Observed Symptom	Likely Interface Counter or Condition	Interpretation	Corrective Action
Slow transfers with basic connectivity	Collisions on one side; CRC/FCS or input errors on the other	Possible full-duplex-to-half-duplex mismatch	Compare both ends and align speed and duplex
Late collisions	Late-collision counter increases	Often a duplex mismatch; can also indicate a physical or distance problem	Check duplex first, then cabling and link distance
Corrupted received frames	CRC/FCS and input errors increase	Frames are failing integrity checks; mismatch, cable, transceiver, or interference may be involved	Check the peer's duplex and inspect the physical link
Very small frames	Runt counter increases	May result from collision-related corruption or a physical fault	Check duplex, cabling, connectors, and hardware
Errors begin after a change	Operational duplex differs from the intended configuration	Incomplete or inconsistent configuration change	Review both interface configurations and negotiation results

Interface Verification and Troubleshooting

Begin with facts from both ends of the physical link. Do not infer duplex solely from the configured commands; check the operational speed and duplex and examine whether error counters are increasing.

Cisco IOS Verification Commands

show interfaces gigabitEthernet 0/1
show interfaces status
show interfaces counters errors

show interfaces commonly displays the interface state, line protocol state, operational speed, duplex mode, and detailed counters. show interfaces status provides a compact status view on supported switches. show interfaces counters errors helps summarize error categories on platforms that support the command.

Interpreting Counters in Context

Collisions: Expected only in a shared half-duplex environment; unexpected growth on a dedicated full-duplex link suggests a problem.
Late collisions: Collisions detected outside the normal Ethernet collision window. A duplex mismatch is a common cause, but cabling, excessive distance, or other physical problems are also possible.
CRC/FCS errors: Received frames failed an integrity check. Causes include physical corruption, faulty cables or transceivers, interference, and duplex-related problems.
Runts: Frames smaller than the minimum valid Ethernet frame size. Collision-related faults and physical problems can produce them.
Input errors: A broad receive-side category that may include CRC errors, runts, and other receive faults.
Output errors: A broad transmit-side category that may indicate interface, hardware, congestion, or link problems depending on the platform.
Dropped packets: Packets discarded because of congestion, buffer limitations, policy, or other conditions. Drops alone do not prove a duplex mismatch.

Systematic Remediation Process

Identify the exact physical link and inspect the local interface state.
Record operational speed, operational duplex, configured settings, and relevant counters.
Verify the neighbor device's interface and compare both ends. Neighbor-device discovery can help identify the connected device, but direct inspection of that device's interface is still preferred.
Determine whether both sides use auto-negotiation or both sides use matching fixed settings.
Correct the configuration. Prefer auto-negotiation on both ends when supported; otherwise configure identical compatible speed and duplex values.
Clear counters when operationally appropriate, or record a baseline if clearing is not permitted.
Generate representative traffic and monitor whether errors, collisions, or drops increase.
If errors continue after duplex settings match, inspect cable quality, connectors, patch panels, transceivers, electromagnetic conditions, link distance, and interface hardware.

For example, if users report slow file transfers but pings work, inspect both endpoints for speed, duplex, collisions, late collisions, CRC/FCS errors, and runts. If the switch is forced to full-duplex and the endpoint is operating half-duplex, restore auto-negotiation on both sides or configure matching fixed values. Then baseline the counters and test again.

Practical Scenarios

Legacy Hub Segment

Several PCs connect to an Ethernet hub. Every PC shares the same medium and collision domain. Only one station can successfully transmit at a time, so the segment operates half-duplex and uses CSMA/CD. Increasing the number of active hosts increases contention and can reduce effective throughput.

Switch-to-Host Connection

A workstation connects directly to a switch access port. The two interfaces negotiate a common speed and full-duplex mode. Upload and download traffic can occur simultaneously, and Ethernet collisions should not occur on the dedicated link.

Duplex Mismatch Incident

A switch interface is forced to full-duplex while the connected endpoint operates half-duplex. Pings may succeed, but file transfers are slow or unreliable. The half-duplex endpoint accumulates collisions or late collisions, while the full-duplex side may record CRC/FCS errors, runts, or other receive problems.

Inconsistent Settings Corrected

An administrator checks both ends and finds incompatible settings. The administrator either restores auto-negotiation on both interfaces or configures identical speed and duplex values on both ends. After counters are cleared or baselined, traffic is tested and the interface is monitored to confirm that errors stop increasing.

Modern Ethernet Considerations

Modern switched Ethernet commonly uses auto-negotiation and full-duplex operation. Many higher-speed Ethernet standards operate only in full-duplex mode, so legacy collision-based behavior is mainly a historical or specialized troubleshooting concept rather than normal operation on current switch networks.

Best practice is to use auto-negotiation on both ends unless a platform limitation, legacy endpoint, or explicit design requirement calls for fixed settings. When fixed settings are necessary, document and verify the configuration on both connected interfaces.

CCNA Exam Notes

A hub creates one shared collision domain; a switch normally gives each active port its own collision domain.
Half-duplex Ethernet uses CSMA/CD because multiple devices may contend for a shared medium.
Full-duplex point-to-point Ethernet does not use CSMA/CD and should not have collisions.
Full-duplex allows simultaneous traffic in both directions; it does not double the rate of one direction.
A duplex mismatch can leave the link up while severely degrading application performance.
Late collisions strongly suggest a duplex mismatch, but physical-layer faults must also be considered.
When one side is fixed and the other side auto-negotiates, the auto side may select half-duplex because it cannot learn the fixed peer's duplex through negotiation.
Check operational values and counters on both ends, not just the local configuration.