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I am learning for the ICND1 exam through CBTNuggets video courses. In one of their videos they used a command,

switchport port-security maximum

This is what the cisco help says about it,

To set the maximum number of secure MAC addresses on a port

I have tested this command myself on Cisco Packet Tracer, and this command allows to set this value to up to 132.

Under what circumstances would I connect 132 hosts to a single Switch port ? Doesn't one port only support one MAC address at a time ?

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    There can always be another switch (not trunked), a hub, or even a virtual server.
    – Ryan Foley
    Jun 29, 2014 at 15:49

3 Answers 3

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You can have more than one MAC address on a switch port if:

  1. You have a switch connected to it. Could be another managed switch (like a Cisco) or an unmanaged switch (like a consumer Netgear or Linksys switch).
  2. You have a virtual server host attached to it with multiple virtual machines sharing the NIC.
  3. You're using a VoIP phone to carry data for a computer. In this case, you'll have a trunk running to the phone from the switch and a second ethernet cable running from the phone to a computer.
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    (4) some operating systems allow you to define additional "virtual" interfaces for a single network port, each of which gets its own MAC address. (I believe this was originally meant as host operating system support for your (2), but people have found other uses for it.)
    – zwol
    Jun 29, 2014 at 21:35
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    3 is really a special case of 1 (the phone has a switch in it). And for good measure there's 5.) you're still using 10BASE2 or 10BASE5 because nobody told you it's not the 80s anymore.
    – hobbs
    Jun 30, 2014 at 11:54
  • Thanks, hobbs! I'd also forgotten about the option of having a hub connected to a switchport. This is actually a fairly common option for traffic sniffing and troubleshooting.
    – Avery Abbott
    Jun 30, 2014 at 21:22
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You can have more than one MAC on a switchport, for example a big Server with 20 VMs on it, so it needs to acces the switchport with 21 MAC addresses.

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If you have connected the switch-port to a WIFI-AP then you would need to add multiple hosts to a single switch-port as all the hosts would be using a single switch-port.

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    That depends on the access point configuration. If the AP is in routed mode, you would see only one mac address, if it's in bridge mode you would see one for every connectied client.
    – Teun Vink ♦
    Jan 23, 2017 at 10:27

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