MAC Address

Media Control Access

Also called BIA (Burned-In Address).

A 6-byte physical address is assigned to the device when it is made.

Is globally unique.

The first 3 bytes are OUI (Organizationally Unique Identifier), which is assigned to the company making the device.

The last 3 bytes are unique to the device itself.

Written as 12 hexadecimal characters.

Unicast Frame

A frame destined for a single target.

Dynamic MAC Address

MAC addresses can be dynamically learned, also called dynamic MAC addresses.

In Cisco switches, these addresses are removed from the MAC address table after 5 minutes of inactivity.

Unknown Unicast Frame

A frame for which a Switch does not have an entry on its MAC address table.

This causes the Switch to Flood

Flooding

Sending a frame for every device (except the one that sent it).

The switch will then await for a device to respond.

Every device that does not recognize the destination address in the frame, ignores the packet.

Known Unicast Frame

Known destination addresses cause the frame to be forwarded to the respective device.

Address Table

These tables are cleared every 5 minutes, this is called aging.

Commands:

• Show table: show mac address-table

• Clear table: clear mac address-table dynamic

• Clear specific address: clear mac address-table dynamic address <mac address>

• Clear mac from a specific interface: clear mac address-table interface <interface>