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We have core, distribution and access layer switches. the switches in access layer connect several workstations on VLAN 20 only. we have DHCP server on VLAN 1 but is on another switch with all servers. the workstations reach these servers on regular basis.

now the access switches (that connect workstations on VLAN 20), should their uplink (to distribution switches) be:

  • TRUNK mode, native VLAN 20, allow VLANs 20 and 1; or
  • ACCESS VLAN 20

and why please thank

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Unless you have access interfaces on VLAN 1 (not good to use VLAN 1 or a native VLAN) connected to the layer-2 access switch, you do not need to trunk VLAN 1 to the access switch. The gateway on VLAN 20 will route traffic between VLAN 20 and VLAN 1.

To let the hosts on VLAN 20 get DHCP from a server on VLAN 1, you use the ip helper-address interface command on the VLAN 20 gateway layer-3 interface:

ip helper-address <DHCP server address>
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