How to configure DHCP server on L2 switch with helper-address ?

[sh-door]

Hello 

I am configuring a DHCP pool to use a Cisco 2960 switch as a DHCP server.

If the device IP of the 2960 switch and the IP range used for the DHCP pool are different, routing functionality is required.

Since the 2960 is an L2 switch, I understand that an IP helper address must be used to forward DHCP requests to an L3 device.

Is this correct?

There is a Nexus device installed above the 2960 switch.

In this case, where should the helper address be configured?

1. SVI of the Nexus device
2. SVI of the DHCP pool subnet on the 2960 switch
3. SVI of the device subnet on the 2960 switch
4. Another device

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Additionally, when configuring the DHCP server, why are the connected device ports set as access ports?

Would it not work if they were set to trunk mode?

I have reviewed related materials, but I do not fully understand the reasoning.

[M02@rt37 · Accepted solution]

Hello @sh-door 

Since the Nexus switch is the L3 device, the ip helper-address should be configured on the SVI of that Nexus that corresponds to the vlan where the DHCP clients exist.

C2960 is an L2 switch, it can not perform routing. 

Also, access ports are used for end devices because typical client devices (PCs, phones, printers) do not send VLAN-tagged frames. An access port ensures that the connected device is placed in the correct vlan without requiring any vlan awareness from the device. If the port were set to trunk mode, the DHCP request might not reach the DHCP server correctly because untagged frames could be dropped or placed into an unintended VLAN. Trunk mode is only necesary when a device needs access to multiple VLANs, such as another switch, an AP, or a server running multiple services on different VLANs...