VMware ESXi and vSphere Cluster Management

Collision Domains Explained: Hubs, Switches, Bridges, and Routers

Learn how Ethernet collisions occur and how hubs, bridges, switches, routers, duplex modes, and VLANs affect collision and broadcast domains.

An Ethernet collision domain is a shared network segment in which simultaneous transmissions can interfere with one another. Devices in that segment contend for the opportunity to transmit.

Collisions affect Ethernet frames, which are Layer 2 protocol data units. Introductory material sometimes calls these “packet collisions,” but frame collision is the more precise term.

How Ethernet Collisions Occur

On a shared, half-duplex Ethernet segment, a device first listens to determine whether the medium is in use. If two or more devices transmit at nearly the same time, their electrical signals overlap.

The overlapping signals corrupt the transmitted frames. The affected frames are discarded, so the sending devices must retransmit them. Retransmissions add delay and reduce usable throughput.

CSMA/CD

CSMA/CD means Carrier Sense Multiple Access with Collision Detection. It was the traditional Ethernet method for shared half-duplex media:

A device senses whether the medium is available.
The device transmits if the medium appears idle.
The device detects a collision if another transmission overlaps its own.
The devices stop, wait for different random intervals, and retry.

CSMA/CD is mainly a legacy concept in modern switched Ethernet because normal point-to-point full-duplex links do not have ordinary collisions.

Hubs and Shared Bandwidth

A hub is a Layer 1 multiport repeater. When a hub receives an electrical signal on one port, it repeats that signal to every other port.

Because every hub port belongs to the same repeated medium, all devices connected to one hub share:

One collision domain.
The available segment bandwidth.
The opportunity to transmit.

On a half-duplex hub segment, only one device can successfully transmit at a time. If two devices transmit together, a collision can occur.

Two interconnected hubs

Connecting a second hub to the first extends the same shared Ethernet segment. It increases the number of attached devices, but it does not create a second collision domain. Every device on the interconnected hub chain remains in one collision domain.

Computer A     Computer B
     \           /
       [Hub 1]====[Hub 2]
                    /   \
             Computer C  Computer D

All four computers and both hubs: one collision domain

How Bridges, Switches, and Routers Create Boundaries

A device creates a collision-domain boundary when it separates one Ethernet segment from another. Count each distinct link or shared segment, not each device twice.

Bridge

A bridge is a Layer 2 device that forwards or filters frames between its ports. Each bridge port connects to a separate collision domain. A frame crossing from one bridge port to another crosses from one collision domain to another.

Switch

A switch is a multiport bridge. Each active switch port creates a separate collision domain. For example, four computers connected to four separate switch ports represent four collision domains.

Although the switch separates collision domains, it normally forwards Layer 2 broadcast frames between ports in the same VLAN. Therefore, a switch port boundary is not automatically a broadcast-domain boundary.

Router

A router is a Layer 3 device. Each Ethernet interface is a collision-domain boundary and normally also a broadcast-domain boundary. A routed link connected to one router interface is a separate Ethernet segment from a link connected to another router interface.

How Common Devices Affect Collision Domains

Device or link type	Collision-domain behavior	Shared bandwidth behavior	Typical duplex mode	Broadcast-domain behavior
Hub	All connected ports are one collision domain.	All attached devices share the segment bandwidth.	Half duplex	Broadcasts are repeated to all ports.
Bridge	Each port connects to a separate collision domain.	Different ports can represent separate segments.	Half or full duplex, depending on the link	Broadcasts can be forwarded between ports.
Layer 2 switch	Each active port is a separate collision domain.	Each point-to-point link has its own link capacity.	Normally full duplex	Broadcasts normally remain within the same VLAN.
Router	Each Ethernet interface separates collision domains.	Each interface connects to a different segment.	Normally full duplex on switched Ethernet	Interfaces separate broadcast domains.
Full-duplex point-to-point Ethernet link	The link is a separate segment, but normal collisions do not occur.	Both ends can send and receive simultaneously.	Full duplex	Depends on the Layer 2 device and VLAN design.

Full-Duplex Ethernet and Modern Networks

A point-to-point switch-to-host or switch-to-switch Ethernet link normally operates in full duplex. Full duplex means both endpoints can transmit and receive simultaneously.

Full-duplex Ethernet does not use CSMA/CD and does not experience normal shared-media collisions. Simultaneous sending and receiving is possible, so the aggregate capacity is greater than that of a half-duplex link with the same nominal speed.

A duplex mismatch occurs when one endpoint uses full duplex while the other uses half duplex. This is different from an ordinary shared-medium collision, but it can cause poor throughput, late collisions, retransmissions, and interface errors.

Normal switch-interface settings

No configuration is required to understand or count collision domains. For practical Cisco IOS verification, these commands can help inspect a link:

show interfaces
show interfaces status
show interfaces counters errors
show mac address-table

A typical modern switch interface uses auto-negotiation:

interface GigabitEthernet1/0/1
 speed auto
 duplex auto

Both ends should use compatible speed and duplex settings. Forcing half duplex on a modern switched link is generally not recommended.

Method for Identifying and Counting Collision Domains

Locate every shared hub segment first.
Treat an entire interconnected chain of hubs as one collision domain.
Identify every bridge, switch, and router interface that separates one Ethernet segment from another.
Count each distinct link or shared segment once.
Do not count both ends of the same physical link as two domains. A switch-to-router cable is one collision domain, even though both the switch port and router interface are boundaries.
For full-duplex links, count the link as a separate segment when analyzing topology, but note that normal collisions do not occur on it.

Worked topology: seven computers and seven collision domains

The following topology contains exactly seven computers, two interconnected hubs, a bridge, a switch, and a router. The router has three Ethernet interfaces for this example.

 C1       C2
  \       /
   [Hub 1]====[Hub 2]---C3
                  |
                B port 1
                [Bridge]
                B port 2
                  |
                  | D2
                  |
             S port 1
                [Switch]---S port 2---C4
                  |  \ 
                  |   \ S port 3---C5
                  |
             S port 4
                  |
              R interface 1
                [Router]
              /           \
 R interface 2             R interface 3
      |                         |
      C6                        C7

D1: Hub 1, Hub 2, C1, C2, C3, and the bridge's first port
D2: Bridge's second port to switch port 1
D3: Switch port 2 to C4
D4: Switch port 3 to C5
D5: Switch port 4 to router interface 1
D6: Router interface 2 to C6
D7: Router interface 3 to C7

Total: 7 collision domains

Why the count is seven: C1, C2, and C3 share the interconnected hub chain, so they belong to D1 rather than three separate domains. The bridge separates D1 from D2. Each switch-host link is its own domain, the switch-router link is one domain, and each router interface connects to its own separate segment.

Collision-domain counting rules

Topology feature	How to count it	Reason
Devices connected to one hub	Count the entire hub segment as one domain.	The hub repeats signals to every connected port.
Two or more interconnected hubs	Count the complete hub chain as one domain.	Interconnected hubs extend the same shared medium.
Individual bridge port	Count the segment attached to that port once.	Bridge ports separate the attached segments.
Individual switch port	Count the link attached to that port once.	Each active switch port is a distinct collision domain.
Individual router Ethernet interface	Count the connected Ethernet segment once.	Router interfaces separate Ethernet and broadcast segments.
Host-to-switch full-duplex link	Count it as one separate link, with no normal collisions.	It is point-to-point rather than a shared half-duplex medium.

Collision Domains Versus Broadcast Domains

A broadcast domain is the Layer 2 scope within which a broadcast frame is forwarded. The two concepts describe different kinds of boundaries.

        C1       C2       C3
         |        |        |
       Switch ports in VLAN 10
                  |
             Router interface
                  |
             Other network

Collision boundaries: normally one per host-switch link.
Broadcast boundary: the router interface.
VLAN 10: one Layer 2 broadcast domain.

Characteristic	Collision domain	Broadcast domain
Primary concern	Whether simultaneous transmissions can interfere.	Where Layer 2 broadcast frames are forwarded.
Typical layer	Ethernet physical and data-link behavior	Data-link layer
What creates a boundary	Separate bridge, switch, or router segments; shared-media changes.	Usually a router interface or VLAN boundary.
Effect of a hub	Places all attached devices in one domain.	Repeats broadcasts across all attached ports.
Effect of a switch	Creates one collision domain per active port.	Normally forwards broadcasts within the same VLAN.
Effect of a router	Each interface connects to a separate collision domain.	Separates broadcast domains.
Effect of a VLAN	Does not by itself describe collision behavior; switch ports remain separate collision domains.	Each VLAN is generally its own Layer 2 broadcast domain.

For example, three hosts connected to three switch ports in one VLAN normally occupy three collision domains but one broadcast domain. A router uplink separates the hosts' broadcast domain from the network on the router's other interface.

Troubleshooting Examples

Slow performance on a hub

Symptom: Users connected to a hub experience inconsistent performance when several devices are active.

Likely cause: The users share one half-duplex collision domain and contend for the medium.

Verification: Identify the hub or other shared segment and check whether performance worsens as concurrent activity increases.

Resolution: Replace the hub with a switch and use separate full-duplex point-to-point links where supported.

Late collisions on a switched link

Symptom: A switched link has poor throughput, late collisions, or duplex-related interface errors.

Likely cause: A duplex mismatch, such as one endpoint forced to half duplex while the other uses full duplex or auto-negotiation.

Verification: Inspect speed and duplex on both endpoints and review counters for collisions, late collisions, errors, and drops.

Resolution: Make both endpoints compatible, preferably by using supported auto-negotiation on both sides.

Counting every hub-connected computer separately

Symptom: A learner counts each computer attached to a hub as a separate collision domain.

Cause: Confusing physical endpoints or ports with shared Ethernet segments.

Resolution: Trace the signal path. If a hub repeats signals across the same segment, count the entire interconnected hub portion as one collision domain.

Assuming every switch port is a broadcast domain

Symptom: A learner assumes that a switch creates a separate broadcast domain for every port.

Cause: Confusing collision-domain separation with broadcast-domain separation.

Resolution: Check the VLAN assignments and look for a router or Layer 3 boundary. Switch ports separate collision domains, while VLANs and router interfaces determine typical broadcast-domain boundaries.

Exam-Ready Summary

A collision domain is a shared Ethernet segment where simultaneous half-duplex transmissions can interfere.
Ethernet collisions corrupt frames, which are discarded and retransmitted.
A hub places all attached devices, including devices across an interconnected hub chain, in one collision domain.
A bridge separates collision domains between its ports.
A switch creates one collision domain per active port.
A router interface is a collision-domain boundary and normally a broadcast-domain boundary.
Full-duplex point-to-point Ethernet does not use CSMA/CD and does not have normal Ethernet collisions.
Count each physical link or shared segment once; never count both endpoints of one link as two domains.
Switches usually split collision domains without splitting broadcasts within the same VLAN.
VLANs normally define separate Layer 2 broadcast domains.