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Switches SW1 and SW2 both have ports in VLAN 10 and VLAN 20. The two switches are to be connected with a single cable so that devices in the same VLAN can communicate across the switches. Which configuration of the connecting ports is appropriate?

  1. AOn both switches, make the connecting port an access port in VLAN 10
  2. BRemove VLAN 20, merge it into VLAN 10, and make the connecting port an access port in VLAN 10
  3. COn both switches, make the connecting port a trunk port and send frames with IEEE 802.1Q tags that identify their VLAN
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Answer: C. On both switches, make the connecting port a trunk port and send frames with IEEE 802.1Q tags that identify their VLAN

An access port belongs to a single VLAN and is used to connect end devices. To carry frames from several VLANs over one link, the port is made a trunk port and each frame is sent with an IEEE 802.1Q tag holding its VLAN ID. The receiving switch reads the tag to tell which VLAN the frame belongs to and forwards it only to ports in that VLAN. One native VLAN can be chosen whose frames are sent untagged, and it must match on both ends.

  • A. Only VLAN 10 frames would pass, so devices in VLAN 20 could not communicate across the switches.
  • B. Communication would work, but the purpose of separating the VLANs (splitting broadcast domains and isolating traffic for security) would be lost.
  • C. Correct. With tags, a trunk port carries several VLANs over one link.
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