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Collision Domains Explained

Learn how Ethernet collisions occur, how CSMA/CD works, and how hubs, switches, bridges, routers, duplex settings, and VLANs affect collision and broadcast domains.

A collision domain is a network segment in which devices can contend for the same shared Ethernet transmission medium. If two devices transmit at the same time on a shared, half-duplex Ethernet segment, their signals can interfere and create an Ethernet collision.

Collision domains are a Layer 1 and Layer 2 segmentation concept. They are not the same as broadcast domains, which describe the Layer 2 area that receives a broadcast frame. A network can have one broadcast domain but many collision domains.

What Is a Collision Domain?

In older Ethernet networks, several devices could use the same physical medium. Every device connected to that medium had to take turns transmitting. The portion of the network sharing that medium was one collision domain.

For example, four PCs connected to a hub share one collision domain. The hub repeats each signal out of its other ports, so all four PCs use the same shared medium. If two PCs begin transmitting together, their electrical signals overlap and the frames are corrupted.

A collision domain can contain one or many devices. The important question is whether those devices can compete for the same half-duplex Ethernet medium.

How Ethernet Collisions Occur

Shared-media Ethernet uses a common transmission path. Before sending, a host listens to determine whether the medium appears idle. However, two hosts may listen at nearly the same time, both detect an idle medium, and both begin transmitting. Their signals then meet on the shared segment.

The resulting collision damages both frames. The transmitting devices stop their current transmissions, signal that a collision occurred, wait for different random periods, and try again. As more devices compete for a shared medium, contention increases. More contention generally means more waiting, retransmissions, and reduced throughput.

Collisions are normal behavior on legacy shared Ethernet. They should be rare or absent on a properly operating modern switched Ethernet link running full duplex.

CSMA/CD

CSMA/CD means Carrier Sense Multiple Access with Collision Detection. It is the media-access method used by half-duplex Ethernet.

1. Carrier sense: A device listens to the medium to check whether another transmission is in progress.
2. Multiple access: Multiple devices share and may attempt to use the same medium.
3. Transmit: If the medium appears idle, the device sends its frame.
4. Collision detection: The device monitors the medium while transmitting and detects a signal conflict.
5. Jam signaling: Devices involved in the collision send a jam signal so other stations recognize that the transmission failed.
6. Randomized backoff: Each device waits for a randomly selected interval before attempting retransmission.

CSMA/CD applies to half-duplex Ethernet, not to normal full-duplex switched links. Full-duplex links have separate transmit and receive paths, so both endpoints can send simultaneously without competing for one shared path.

Half-Duplex and Full-Duplex Ethernet

A duplex mismatch occurs when one endpoint uses full duplex and the other uses half duplex. The full-duplex side may transmit whenever it wants, while the half-duplex side still follows collision rules. The result can include late collisions, CRC errors, runts, poor throughput, and intermittent connectivity.

How Network Devices Affect Collision Domains

Hub, Repeater, Switch, Bridge, and Router Examples

Hub with four PCs

Four PCs attached to one hub form one collision domain. A transmission from one PC is repeated toward the other ports. If two PCs transmit together, a collision can occur.

Switch with four PCs

Four PCs attached to four switch ports form four collision domains: one PC-to-switch link per domain. If the links operate full duplex, normal Ethernet collisions do not occur.

Two hubs connected to one switch

Each hub and all devices attached to it form one shared collision domain. The switch port connected to the first hub represents one collision-domain segment, and the switch port connected to the second hub represents another. The switch separates the two domains, even though it may forward frames between them.

Router connected to two switched LANs

Each router interface connects to its own Layer 2 segment and collision domain. The router also separates the two broadcast domains, so a broadcast on one LAN is not forwarded into the other LAN.

Collision Domain Versus Broadcast Domain

A broadcast domain is the set of devices that receive a Layer 2 broadcast frame. A switch ordinarily forwards broadcasts to other ports in the same VLAN, so a switched VLAN is typically one broadcast domain even though each switch port is a separate collision domain.

Routers divide broadcast domains because their interfaces terminate Layer 2 networks. VLANs also divide broadcast domains logically: ports assigned to different VLANs do not ordinarily share the same broadcast domain.

Do not combine the two counting tasks. First count collision domains by examining shared media and Layer 2 port boundaries. Then count broadcast domains by examining router interfaces and VLAN boundaries.

Counting Collision Domains in Network Diagrams

1. Identify shared-media devices such as hubs, repeaters, and legacy coaxial segments.
2. Place every hub and all devices attached to it in one collision domain.
3. For a bridge or switch, count each active port connection as a separate collision domain.
4. For a router, treat each Ethernet interface connection as its own Layer 2 segment and collision domain.
5. Check whether a switch port connects to a hub. That port and the hub's attached hosts represent one shared collision domain.
6. Count broadcast domains separately using router boundaries and VLAN membership.

Performance and Troubleshooting

In a shared collision domain, every additional device increases contention. Devices spend more time waiting, collisions cause retransmissions, and the available bandwidth is shared rather than dedicated. Hubs and shared coaxial Ethernet therefore scale poorly compared with switched Ethernet.

Common signs of an unexpected half-duplex link or duplex mismatch include increasing collisions, late collisions, CRC errors, runts, input errors, severe slowness, intermittent application failures, and a link that remains up despite poor performance.

Useful Cisco IOS commands

Use interface commands to check speed, duplex, and error counters:

show interfaces <interface-id>
show interfaces <interface-id> status
show interfaces counters errors

In the detailed output, review the negotiated or configured speed and duplex. Also examine collision, late-collision, runt, CRC, and input-error counters. Collision counters should not continually increase on a correctly operating full-duplex link.

For a controlled troubleshooting case, configure compatible autonegotiation on the Cisco interface:

configure terminal
interface <interface-id>
speed auto
duplex auto

Prefer compatible autonegotiation at both ends when supported. If manual settings are required, configure matching speed and duplex values on both endpoints. Forcing full duplex on only one side is not a valid fix for a mismatch.

Example: collisions on an expected full-duplex link

1. Check duplex and speed at both endpoints.
2. Review collision, late-collision, CRC, runt, and input-error counters.
3. Determine whether a hub or other shared-media device is connected anywhere in the path.
4. Correct matching or autonegotiation settings.
5. Replace hub-based connectivity with dedicated switched Ethernet where possible.

Wireless Collision Considerations

A wireless LAN uses a shared radio medium, so clients still contend for access and performance declines as activity and interference increase. Wireless Ethernet does not use Ethernet CSMA/CD in the same way because a station generally cannot transmit and listen for collisions reliably on the same radio channel. Wireless networks use contention-avoidance techniques such as CSMA/CA instead.

For CCNA purposes, distinguish a wireless shared medium from a wired half-duplex Ethernet collision domain. Both involve contention, but their access-control methods and collision behavior differ.

Modern Networking Context

Hubs and shared coaxial Ethernet are largely legacy technologies. Modern designs normally use switches, dedicated links, and full-duplex operation. As a result, normal CSMA/CD collisions are not expected in a properly configured contemporary switched network.

The collision-domain concept remains important for CCNA exams, legacy environments, diagram analysis, and troubleshooting. It also reinforces a broader design principle: switches reduce contention by giving each port its own physical Ethernet segment, while routers and VLANs control Layer 2 broadcast scope.

Key Takeaways

• A collision domain is a portion of a network where devices can contend for one shared half-duplex Ethernet medium.
• Two simultaneous transmissions on that medium can interfere and cause a collision.
• CSMA/CD detects collisions and uses jam signaling, randomized backoff, and retransmission.
• A hub or repeater extends one shared collision domain; it does not separate it.
• A bridge and a switch separate collision domains; each active switch port is normally one collision domain.
• Each router interface terminates a Layer 2 segment and separates collision and broadcast domains.
• Full-duplex switched Ethernet eliminates normal collisions, while a duplex mismatch can cause late collisions and errors.
• Switches divide collision domains per port, but same-VLAN broadcasts ordinarily remain in one broadcast domain.
• Count collision domains and broadcast domains as separate diagram-analysis tasks.

For foundational context, review the OSI Reference Model, Computer Network Expained, and VLAN interfaces.