CCNA 2.1: Describe and verify switching concepts (MAC learning and aging, Frame switching, Frame flooding, MAC address table)


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.

Study Notes:

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:
    • VLAN
    • MAC address
    • Type (dynamic or static)
    • Ports
  • Static entries will persist through a reboot.  Dynamic entries will not.
  1. Alan Inman
    Alan Inman
    August 21, 2018 at 10:28 am

    Joe, thank you so much for taking the time to provide this great information! What I’ve found different (and most helpful) on your page is how direct you are with answers, “This is what it is. This is what it does.” This allows me to make the most of the limited time I have. I’m not sure how much your hard work is paying off right now, but please keep at it! I believe you’re on to great things with your method.

    • Joe Barger (CCNP/CCDP)
      Joe Barger (CCNP/CCDP) • Post Author •
      August 21, 2018 at 8:19 pm

      Glad to hear this content is helpful. Best of luck on your way to the CCNA. Thanks for reviewing the site.

  2. Isaac Mantey
    Isaac Mantey
    October 10, 2018 at 1:56 pm

    Thank you soo much providing this. God bless you.

  3. Ayan Satalluri
    Ayan Satalluri
    February 1, 2019 at 9:33 am

    Excellent work and I’m sure it’ll make you proud – one fine day when a great Engineer quotes it’s all from e-configs! Thanks Joe. Hopefully CCNA Security is next for you to breakdown and post.

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