What is DHCP Relay?

A DHCP relay agent forwards DHCP messages between clients and a DHCP server that sits on a different subnet, since discovery broadcasts do not cross routed network boundaries on their own. Positioned on the client-facing interface, it unicasts each request to one or more configured servers and relays the replies back to the client.

Key takeaways

• Forwards DHCP broadcasts to a server across a routed subnet boundary.
• Configured as a relay agent profile bound to a VOS interface.
• Supports multiple forwarding addresses for redundant DHCP servers.
• Option 82 sub-options add circuit and remote identifiers to requests.
• Pass-through actions decide whether relay data is dropped, kept, or replaced.

On this page: Why it matters · How it works · Types · DHCP Relay & Versa · FAQ

Why is DHCP relay important?

DHCP discovery relies on Layer 2 broadcasts, which routers do not forward between subnets. Without a relay, every VLAN or subnet would need its own DHCP server – multiplying the servers a network must deploy and keep synchronized as it grows.

DHCP relay solves this with a lightweight forwarding function on the client-side gateway – the router or SD-WAN device already on the client’s subnet. Instead of a broadcast, it unicasts the request to one or more centralized servers and relays the reply back. This matters most in branch and campus architectures with many segmented VLANs, where centralizing DHCP administration on a few servers beats synchronizing state across every site, and standardizes lease policy, options, and logging in one place.

What is DHCP relay?

When a relay agent receives a client’s broadcast, it inserts its own interface address – the gateway IP address (giaddr) – and forwards the message as a unicast packet to the DHCP server. The server uses that address to pick which subnet to allocate from, then replies back through the relay to the client. If VRRP is enabled, the shared virtual IP address serves as the gateway address instead, so gateway failover does not disrupt DHCP.

A relay agent can be scoped with match conditions – interfaces or networks, client identifiers, hardware addresses – so only qualifying requests are relayed. Where a network runs both a local server and a relay agent, match conditions on each determine which requests it handles.

Types of DHCP relay

Relay behavior varies along a few axes. Pass-through actions control relay information already present in a client’s message: Drop removes it, Keep retains it unchanged, and Replace overwrites it with the current agent’s information – relevant when a message already passed through another relay upstream.

Relay agent information (Option 82) lets a relay attach identifying sub-options to a request – a circuit ID for the link it arrived on, a remote ID, and a vendor-specific circuit ID – giving the server visibility into a request’s origin even though the relay is now the visible source. Link selection goes further, overriding the gateway IP address so the server allocates from a specific subnet.

Relay agents also differ in redundancy: a profile can forward to a single server or to multiple forwarding addresses, so a backup takes over if the primary is unreachable. IPv4 relay has been supported the longest; IPv6 relay, including IPv6 static mapping, was added in Release 23.1.1.

DHCP Relay & Versa

On a VOS device, DHCP relay is configured in two parts: a DHCP relay agent profile – gateway IP address, pass-through action, Option 82 sub-options, link selection, forwarding addresses – and the relay agent itself, which applies match conditions and references the profile from its Relay Settings tab.

On Titan branch devices, the same capability is simpler through LAN connections: a DHCP Relay checkbox on the LAN profile, with one or more comma-separated relay IP addresses, alongside the DHCP Server checkbox on the same interface – one or the other, not both. Multitenant gateway devices expose the identical options per tenant during activation.

Frequently asked questions

What is the difference between a DHCP relay agent and a DHCP server?

A DHCP server allocates addresses from its own pool and answers requests directly. A relay agent allocates nothing; it forwards messages between a client and a server on a different subnet, necessary because discovery broadcasts do not cross routed network boundaries on their own.

What is the gateway IP address (giaddr) in DHCP relay?

The gateway IP address is the relay agent’s own interface address, inserted into a forwarded message so the server knows which subnet to allocate from. If VRRP is enabled, the shared virtual IP address is used instead, so gateway failover does not disrupt address assignment.

What is DHCP Option 82?

Option 82 is relay agent information added to a forwarded request, including a circuit ID identifying the link it arrived on and a remote ID. It gives the DHCP server visibility into a request’s origin even though the relay, not the client, is now the visible sender.

Can a DHCP relay agent forward to more than one DHCP server?

Yes. A relay profile’s forwarding information can list multiple server addresses, each with its own routing instance, so a backup server keeps handing out addresses if the primary becomes unreachable – building redundancy into address assignment without changing anything on the client.

Does DHCP relay support IPv6?

Yes. VOS devices support DHCPv6 relay alongside DHCPv4 relay, including IPv6 static mapping profiles, added in Release 23.1.1. The relay agent forwards DHCPv6 messages the same way it does for DHCPv4, using a separate V6 tab in the relay profile and relay agent configuration.

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