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Ethernet is based on CSMA/CD. When using VLANs, the number of stations can become very high, making CSMA/CD inefficient. Are there any mechanisms that VLANs use to overcome this?

  • VLANs create separate smaller collision domains. Also, almost all modern networking is full-duplex, so CSMA/CD does not apply; a switch can receive a frame on every interface simultaneously, with sufficient buffer space. (true hubs haven't been manufactured for decades.) – Ricky Beam Jan 14 at 2:28
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    @RickyBeam VLANs do not affect collision domains. You may be thinking of broadcast domains. – user253751 Jan 14 at 12:35
  • Hubs don't do VLANs, and if they did, it would create partitions allowing more than one port (one per partition) to transmit at the same time. (actually had a 10b2 hub like that -- port groups, not vlans) If P1=V10, P2=V20, then as long as no port is in both vlans (and the vlans are quiet), P1 and P2 can transmit at the same time. – Ricky Beam Jan 14 at 15:40
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VLANs segregate a shared link-layer (L2) topology into smaller broadcast domains.

CSMA/CD is the physical-layer (L1) access method for half-duplex Ethernet, normally only used with repeaters, hubs, or ancient Ethernet variants. Practically, it isn't even supported with current gigabit-and-faster variants any more.

Since VLANs can only (reasonably) be used with switches and switches predominantly use full-duplex links without CSMA/CD, both technologies don't really come into touch with each other.

Of course, you could configure a VLAN trunk to use half duplex on purpose, but why would you do so? Additionally, CSMA/CD would work just as well with or without VLAN tags.

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