Learn how switches separate collision domains, contain broadcasts within VLANs, learn MAC addresses, and respond when forwarding information is unknown or overwhelmed.

What you will be able to do

Describe how a switch uses a MAC address table to choose forwarding ports.
Explain how VLANs define broadcast domains and how broadcast traffic is flooded.
Distinguish known unicast, unknown unicast, broadcast, and unknown multicast forwarding behavior.
Explain how collision-domain segmentation and CAM-table limits affect network behavior.

What a Switch Keeps Track Of

A switch forwards Ethernet frames by using a MAC address table. Each entry connects a MAC address with a switch port and a VLAN.

This table gives the switch a local map of where devices appear. The VLAN matters because the same MAC address information is interpreted within a particular broadcast domain.

When a frame enters, the switch can learn or refresh the source MAC address. It records the incoming port together with the VLAN associated with that frame.

When the Destination Is Known

For a destination MAC address found in the table, the switch has forwarding information for that address. The table points toward a port within the relevant VLAN.

This targeted behavior differs from sending the frame everywhere. The switch can use its learned information instead of treating the destination as unknown.

When the Destination Is Unknown

If the destination MAC address is missing from the table, the switch floods the frame within the same VLAN. It sends the frame through the other ports, not back through the receiving port.

A switch can also flood an unknown unicast when its capacity for learned MAC addresses has been exceeded. For those affected frames, its behavior can resemble a hub.

This is different from ordinary learned forwarding. A known destination receives directed delivery, while an unknown destination triggers wider delivery inside the VLAN.

Broadcast Domains Are VLAN Boundaries

A broadcast domain is the set of devices that receive an Ethernet broadcast. In this lesson, one VLAN defines that domain.

An Ethernet broadcast frame is delivered to every device in the same broadcast domain. Its destination MAC address is FF:FF:FF:FF:FF:FF.

Broadcast forwarding is therefore broad, but it is not unlimited across VLAN boundaries. The VLAN determines which ports belong to the delivery scope.

Broadcast and Unknown Multicast Flooding

Broadcast frames are flooded to all ports in the same VLAN except the incoming port. This prevents the switch from sending the frame back to the port where it arrived.

Unknown multicast frames follow the same flooding pattern described here. They reach the other ports in the VLAN rather than only one learned destination.

Unknown unicast flooding uses the same VLAN-limited, incoming-port-excluded pattern. The address type differs, but the forwarding result is similarly broad.

How Switching Changes Collision Domains

A collision domain is a part of an Ethernet network where transmissions can collide. Using switches to segment these domains can reduce collisions in half-duplex environments.

This idea concerns collisions, not broadcast reach. A switch may separate collision domains while devices in one VLAN still share the same broadcast domain.

Keeping the two concepts separate helps explain why fewer collisions does not automatically mean fewer broadcast recipients.

The Cost of a Large Broadcast Domain

A very large broadcast domain can require a large CAM table and substantial switch memory. The table must support MAC-address information used for forwarding within the network.

A large CAM table can potentially affect MAC-address lookup performance. Scale can therefore influence both resource needs and forwarding behavior.

The practical relationship is simple: more devices and addresses can increase the information the switch must maintain and search.

When the Table Is Deliberately Overloaded

An attacker can send frames with many different source MAC addresses. This can fill the switch’s CAM table with rapidly changing source information.

MAC flooding can force the switch into a flooding state. Once forwarding information is unavailable for affected destinations, frames may be flooded within the VLAN.

In that condition, the switch can behave like a hub for the affected frames. This resembles unknown-unicast flooding, but the trigger is an overloaded table.

A Port-Security Learning Option

Sticky MAC port security allows a switch port to learn MAC addresses dynamically. It is one way to control how addresses become associated with a port.

This feature belongs beside the CAM-table discussion because it concerns MAC learning at the port. It does not change the definition of a broadcast domain or the broadcast destination address.

Recap Before Practice and Prove

A switch maintains MAC information by associating addresses with ports and VLANs. It can learn the source address from an arriving frame, then use the table for later forwarding.