Learn how Ethernet switches use MAC addresses, VLAN context, and forwarding decisions to move Layer 2 frames efficiently.

What you will be able to do

Explain what a MAC address identifies and distinguish unicast, multicast, universally administered, and locally administered addresses.
Describe how a switch learns source MAC addresses and maintains dynamic table entries.
Predict whether a switch forwards, floods, or filters a frame based on its destination MAC address and incoming port.
Contrast dynamic and static MAC address table entries, including the effect of aging on dynamic entries.

1. MAC addresses identify interfaces

A MAC address is an identifier used at the Ethernet layer. It identifies a network interface, giving a switch a local delivery target for a frame.

A unicast MAC address points to one network interface. A multicast MAC address represents a group, so devices listening to that group can receive frames addressed to it.

Universally administered addresses are the most common MAC address type. A locally administered address may be configured manually or assigned by an administrator instead of using the device’s burned-in address.

2. The MAC address table

An Ethernet switch uses a MAC address table to connect MAC addresses with switch ports and VLANs. This table supports Layer 2 forwarding decisions.

Think of each entry as a forwarding clue: a particular MAC address belongs in a particular VLAN and can be reached through a particular port. The switch consults that clue when a frame arrives.

The table is not a list of every possible device. It contains learned entries and can also contain entries configured by an administrator.

3. Learning from the source

When a frame enters a switch, the switch first examines the frame’s source MAC address. It records the incoming port and VLAN associated with that source.

For example, suppose a frame from interface A arrives on port 4 in VLAN 20. The switch can associate interface A’s MAC address with port 4 and VLAN 20.

This process is automatic for dynamic entries. If the address is new, the switch adds it. If the same address now appears through another port, the switch updates the entry.

4. Dynamic entries change over time

A dynamic MAC entry remains useful while traffic continues to arrive from its associated MAC address. If no such traffic appears during the entry’s aging period, the switch removes the entry.

Aging prevents an old location from remaining in the table forever. The address can be learned again when a later frame arrives from that source.

An aging period that is too short can create repeated learning. That repetition increases the switch’s overhead because entries must be rebuilt more often.

5. Checking the destination

After examining the source, the switch examines the frame’s destination MAC address. It searches the MAC address table for that destination within the relevant VLAN.

Suppose a frame enters port 2, and the table places its destination on port 7 in the same VLAN. The switch sends the frame through port 7 rather than sending it through every other port.

This targeted action differs from learning. Source information updates the table, while destination information determines where the current frame should go.

6. Unknown destinations are flooded

If the destination MAC address is not in the table, the switch cannot select one destination port. It floods the frame within the same VLAN.

Flooding means sending the frame through all ports in that VLAN except the port where the frame arrived. The incoming port is excluded because the frame has already entered through it.

For example, a frame arriving on port 3 with an unfamiliar destination may leave through the other ports belonging to that VLAN. Later traffic can provide the source information needed for future table decisions.

7. When the destination is on the incoming port

Sometimes the table associates the destination MAC address with the same port that received the frame. In that case, the switch filters the frame by dropping it.

Filtering avoids sending the frame back through the port where the destination is already located. It is different from forwarding to one other port and different from flooding across the VLAN.

A simple decision sequence is: learn from the source, look up the destination, then forward, flood, or filter according to the table result.

8. Comparing entry types

A dynamic MAC entry is learned automatically when frames arrive from a source MAC address. Its location can be updated when the switch observes that address through another port, and it can disappear after aging.

A static MAC entry is configured manually. It permanently associates a MAC address with a specific port and VLAN.

The key contrast is how the association is created and maintained. Dynamic entries respond to observed traffic and aging, while static entries are deliberately assigned to a chosen port and VLAN.

9. Recap before Practice and Prove

A MAC address identifies an Ethernet network interface or, for multicast, a listening group. Unicast addresses target one interface, while address administration may be universal or locally configured.

A switch learns dynamic table entries from source MAC addresses. Each learned association includes the incoming port and VLAN, and a new location can replace an earlier one.

The destination lookup controls the immediate action. A known destination on another port receives the frame there, an unknown destination causes same-VLAN flooding, and a destination on the incoming port causes filtering.

Dynamic entries age out when traffic is absent, while static entries are manually tied to a port and VLAN. In Practice and Prove, use these rules to trace table learning and predict each forwarding result.

Ethernet Frames and MAC Addresses