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I was wondering whether once the DHCP offer made by the DHCP server arrives at the DHCP relay agent, the relay agent is able to (assuming the network has one VLAN per subnet):

  • examine the DHCP payload for the offer IP and determine what subnet the address will be on and hence
  • translate that to the one VLAN in that subnet using some internal table
  • tag the frame with a VLAN using .1q before entering the switch such that the FF:FF:FF:FF:FF:FF egresses to only ports of that VLAN

Or does this defy the rules of 802.1q tagging; would the switch remove this tag if it were placed before entering the switch at the access port.

  • Are you wondering if this can be done, or if there is an actual existing DHCP relay agent which does this? – dirkt Oct 5 '18 at 5:37
  • Well, both, I suppose. Also assuming that the client doesn't support pre IP assignment unicast. – Lewis Kelsey Oct 5 '18 at 5:48
  • Assuming Linux, VLANs are just additional network interfaces, and not different from other interfaces. So the kernel will take care of VLAN tags. Nothing prevents a DHCP relay agent from relaying to multiple networks/subnets using an internal table, but I am not familiar enough with existing agents to know if there's one that already does that. – dirkt Oct 5 '18 at 5:52

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