This post covers a few important switching concepts that describe how a switch operates including MAC learning and aging, frame switching, frame flooding and the MAC address table.
2.1.a MAC learning
- To switch frames between LAN ports efficiently, the switch maintains an address table called the MAC table.
- When the switch receives a frame, it associates the media access control (MAC) address of the sending network device with the LAN port on which it was received.
- MAC address learning is enabled on all VLANs by default
- The switch dynamically builds the address table by using the MAC source address of the frames received.
- When the switch receives a frame for a MAC destination address not listed in its address table, it floods the frame to all LAN ports of the same VLAN except the port that received the frame.
- When the destination station replies, the switch adds its relevant MAC source address and port ID to the address table.
- The switch then forwards subsequent frames to a single LAN port without flooding all LAN ports.
- You can also enter a MAC address, which is termed a static MAC address, into the table.
- These static MAC entries are retained across a reboot of the switch.
2.1.a MAC aging
- You can configure the amount of time that an entry (the packet source MAC address and port that packet ingresses) remains in the MAC table.
- You can also configure MAC aging time in interface configuration mode or VLAN configuration mode.
- The MAC aging time specifies the time before an entry ages out and is discarded from the MAC address table.
- The range is from 0 to 1000000; the default is 300 seconds.
- Entering the value 0 disables the MAC aging.
- If a VLAN is not specified, the aging specification applies to all VLANs.
2.1.b Frame switching
- LAN switches are characterized by the forwarding method that they support, such as a store-and-forward switch, cut-through switch, or fragment-free switch.
- Store-and-forward switches store the entire frame in internal memory and check the frame for errors before forwarding the frame to its destination.
- Store-and-forward switch operation ensures a high level of error-free network traffic, because bad data frames are discarded rather than forwarded across the network
- With cut-through switching, the LAN switch copies into its memory only the destination MAC address, which is located in the first 6 bytes of the frame following the preamble.
- The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame on to its destination through the designated switch port.
- A cut-through switch reduces delay because the switch begins to forward the frame as soon as it reads the destination MAC address and determines the outgoing switch port
- Fragment-free switching works like cut-through switching with the exception that a switch in fragment-free mode stores the first 64 bytes of the frame before forwarding.
- Fragment-free switching can be viewed as a compromise between store-and-forward switching and cut-through switching.
- The reason fragment-free switching stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes of a frame
2.1.c Frame flooding
- Switches determine which port a frame must be sent out to reach its destination
- If the address is known, the frame is forwarded only on that port
- If the layer 2 MAC address is unknown, the frame is flooded to all ports except the one from which it originated
2.1.d MAC address table
- A MAC address table is made up of the following columns:
- MAC address
- Type (dynamic or static)
- Static entries will persist through a reboot. Dynamic entries will not.