If a network topology has a VLAN which spans multiple switches, what happens if a frame is sent to a MAC address that is connected to the source device's "home" switch? By that I mean if a device wants to send a frame to another device connected directly to the same switch.

Will the frame be sent to all switches associated with the VLAN even across trunks? Or will the first switch know that the destination MAC address is local to the switch so doesn't need to be sent on any trunks?


Do unicast frames get sent to all switches associated with the target VLAN?

No. Switches forward unicast frames out of the port associated with their destination address only (might be another switch or the destination itself). The port associations are learned and updated by inspecting each frame's source address. If you use VLANs, frames do not leave their VLAN (packets encapsulated within the frames might, via a gateway/router).

Only if a switch has no port association for a frame's destination address (unlikely case) then it floods that frames out of all ports except the one it was received from, essentially emulating a repeater hub.

  • So the VLAN tag in a frame is used to prevent frames from leaving their VLAN? (The switches will only allow frames to be forwarded through ports that match the VLAN tag.) – CJ7 Mar 8 at 7:45
  • Essentially, yes. Note that VLAN membership can also be implied when a single "native" VLAN isn't tagged on a port. – Zac67 Mar 8 at 9:00

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