How does DHCP relay operate across VLANs in a segmented enterprise network?

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How does DHCP relay operate across VLANs in a segmented enterprise network?

Asked on Oct 19, 2025

Answer

DHCP relay is used in segmented enterprise networks to facilitate the communication between DHCP clients and servers across different VLANs. This is essential when the DHCP server is not on the same VLAN as the client, allowing the relay agent to forward DHCP requests and responses between them.

Example Concept: DHCP relay operates by using a relay agent, typically configured on a router or Layer 3 switch, which listens for DHCP requests from clients on one VLAN and forwards them to a DHCP server on another VLAN. The relay agent uses the "giaddr" field in the DHCP packet to insert its own IP address, which helps the DHCP server identify the subnet of the requesting client. Once the server responds, the relay agent forwards the response back to the client, ensuring proper IP address allocation across VLANs.

Additional Comment:

Ensure the relay agent is correctly configured with the IP address of the DHCP server.
Verify that the VLANs are properly routed and that the necessary ACLs allow DHCP traffic.
Check that the "ip helper-address" command (or equivalent) is used on the interface connected to the client VLAN.
Monitor DHCP traffic to troubleshoot any issues with IP address allocation or relay operation.

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