How Switches Learn and Forward

The switch decision cycle

A switch connects devices inside a Layer 2 network and decides where each Ethernet frame should leave. It does not begin by asking for the sender's IP route or application. It works with the frame arriving on a physical or logical switch interface.

For every ordinary frame, remember this order:

1. Receive the frame on an ingress interface.
2. Learn the source MAC address on that ingress interface.
3. Look up or classify the destination MAC address.
4. Forward, filter or flood through eligible interfaces—never back through ingress.

Think of a receptionist recording the door through which a person entered, then consulting a directory for the recipient. The analogy stops at one important limit: a switch learns from observed source frames, not from a trusted identity system, and an entry can age or move.

Learn the source address

When a frame from 02:00:00:00:00:0A arrives on Gi0/1, the switch records or refreshes 02:00:00:00:00:0A → Gi0/1. That evidence answers one narrow question: where was this source most recently observed in this Layer 2 scope?

The switch does not learn the destination from this frame. If the same source later arrives on Gi0/3, source learning refreshes the entry to Gi0/3. This is how normal host movement is learned; the table is dynamic rather than a permanent cabling inventory.

Look up the destination

After source learning, the switch examines the destination MAC:

• A current unicast entry on another interface identifies one egress interface.
• A current unicast entry on the ingress interface means no other interface is required.
• A missing unicast entry remains unicast, but its location is unknown.
• FF:FF:FF:FF:FF:FF is explicitly broadcast and is not narrowed by a unicast table entry.

The source and destination fields have different jobs: the source teaches the switch; the destination guides the forwarding decision.

Forward, filter or flood

Flooding describes a switch action, not automatic acceptance by every endpoint. A network interface can receive a copy and discard it because the destination does not apply: receipt is not acceptance. The switch also stays within its current Layer 2 scope; routing and VLAN boundaries are separate decisions.

Interactive switch learning

Watch the frame enter a labelled interface, open its Ethernet fields, and follow the MAC table through source learning, destination lookup, egress selection and transmission. The player starts automatically unless reduced motion is enabled; all stages remain available through manual controls.

Watch the switch make one decision

Change the evidence, then replay the same receive → learn → look up → forward sequence.

Choose the forwarding evidence

Known destinationUnknown destinationBroadcastHost moved

The destination entry identifies Gi0/2, so one copy leaves that interface.

• Host A: Host A eth0↔Switch: Switch Gi0/1
• Switch: Switch Gi0/2↔Host B: Host B eth0
• Switch: Switch Gi0/3↔Host C: Host C eth0

Active: Host A, Switch; link Host A to Switch

PreviousPlayNextRestartPlayback speed0.5×1×1.5×2×

Step 1 of 5

Frame arrives on Gi0/1

Host A transmits one Ethernet frame. The switch receives it on Gi0/1 before making any forwarding decision.

Packet inspector

Inside the packet

Choose packet inspection depth

Plain-language viewTechnical inspection

Ethernet frame

Source MAC

02:00:00:00:00:0A

Destination MAC

02:00:00:00:00:0B

Technical packet details

Ingress interface

Gi0/1

MAC table before

02:00:00:00:00:0B → Gi0/2