A network engineer is configuring DHCP relay services on a Layer 3 core switch to service … — CompTIA Network+

A network engineer is configuring DHCP relay services on a Layer 3 core switch to service a newly created IoT VLAN (VLAN 50, $172.20.50.0/24$ ). Centralized DHCP services reside on server $192.168.10.10/24$ . The engineer creates Switch Virtual Interface (SVI) `Vlan50` with IP address $172.20.50.1/24$ , but accidentally applies the `ip helper-address 192.168.10.10` command to the Layer 3 point-to-point uplink interface ( $172.20.100.2/30$ ) connected to the datacenter router instead of SVI `Vlan50`. Packet captures confirm client devices on VLAN 50 are sending DHCP DISCOVER broadcast frames, yet no relay traffic reaches the DHCP server. Which of the following best explains why the DHCP relay process fails?

The DHCP relay agent intercepts broadcasts only on the interface where the helper address is configured; applying it to the uplink interface prevents the switch from intercepting and relaying DISCOVER broadcasts arriving on the client SVI.Answer
B
The relay agent successfully forwards DISCOVER packets out the uplink interface using the uplink IP address ( $172.20.100.2$ ) as the gateway IP ( $giaddr$ ), causing the server to fail because no scope exists for the uplink subnet.
C
The relay helper service encapsulates DISCOVER traffic into TCP port 67 packets by default, causing the central firewall to drop the traffic because DHCP servers respond exclusively to UDP port 68.
D
The relay agent converts incoming unicast DISCOVER requests into Layer 2 broadcasts across VLAN 50, but because the uplink interface resides in a separate broadcast domain, Layer 3 switches drop the frames.

Answer

The DHCP relay agent intercepts broadcasts only on the interface where the helper address is configured; applying it to the uplink interface prevents the switch from intercepting and relaying DISCOVER broadcasts arriving on the client SVI.

A DHCP relay agent (such as Cisco's `ip helper-address`) must be enabled on the specific ingress interface (e.g., SVI or subinterface) facing the client subnets. When client devices broadcast DHCP DISCOVER messages (`255.255.255.255`), the Layer 3 device inspects the ingress interface. If no relay service is enabled on that specific interface, the broadcast is dropped at the Layer 3 boundary. Applying the helper command to an outbound uplink interface only instructs the switch to listen for broadcasts arriving on that uplink, failing to capture or relay any client traffic originating on VLAN 50.

Step-by-Step Solution

Analyze client frame transmission on VLAN 50

Workstations broadcast DHCP DISCOVER frames (

255.255.255.255

) at Layer 2/3 onto SVI `Vlan50` (

172.20.50.1

Unconfigured hosts do not possess an IP address and must broadcast locally to locate a DHCP server.

Evaluate Layer 3 boundary interface configuration

SVI `Vlan50` lacks an active `ip helper-address` statement. The Layer 3 engine drops the broadcast packet at the boundary.

Routers and Layer 3 switches discard Layer 2/3 broadcasts by default unless an active relay agent daemon is configured on that specific ingress interface.

Assess uplink interface configuration

The `ip helper-address` statement placed on the point-to-point uplink interface (

172.20.100.2

) listens only for broadcasts arriving on that specific uplink link.

DHCP relay statements inspect incoming traffic per interface. A helper address on an outbound uplink does not retroactively capture traffic entering through other SVIs.

Key Concept

DHCP Relay Agent Interface Placement and Broadcast Handling

Estimated Time:2m 0s

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