A network engineer is troubleshooting IP configuration failures on a newly provisioned sub… — CompTIA Network+

A network engineer is troubleshooting IP configuration failures on a newly provisioned subinterface for host workstations on VLAN 40 ( $10.40.16.0/22$ ). Client devices on this VLAN are unable to communicate with remote subnets and consistently auto-assign Automatic Private IP Addressing (APIPA) addresses in the range of $169.254.x.x$ . Packet captures performed on the router interface `Gi0/0.40` reveal that incoming `DHCPDISCOVER` broadcast frames from clients are arriving at the router interface, but no corresponding unicast packets are forwarded to the centralized DHCP server located at $10.10.2.100/24$ .

The subinterface configuration on the router is displayed below:

text
interface GigabitEthernet0/0.40
encapsulation dot1Q 40
ip address 10.40.16.1 255.255.252.0

Which of the following identifies the root cause of this issue and the correct command required on interface `Gi0/0.40` to resolve host connectivity?

The subinterface lacks a relay configuration to bridge Layer 2 broadcasts across subnets; configure `ip helper-address 10.10.2.100` on interface `Gi0/0.40`.Answer
B
The DHCP scope is depleted on the server; increase the pool size on the server by modifying the subnet mask to `255.255.240.0`.
C
The default gateway IP address resides on an invalid subnet boundary; change the interface IP configuration to `10.40.16.1 255.255.255.0`.
D
The router interface is blocking DHCP packets due to a missing default route; execute `ip route 0.0.0.0 0.0.0.0 10.10.2.100` on the subinterface.

Answer

The subinterface lacks a relay configuration to bridge Layer 2 broadcasts across subnets; configure `ip helper-address 10.10.2.100` on interface `Gi0/0.40`.

Because client hosts and the DHCP server reside on different IP subnets (VLAN 40 vs. the server subnet 10.10.2.0/24), the router interface facing the clients must act as a DHCP Relay Agent. By default, routers block Layer 2 broadcast traffic such as `DHCPDISCOVER`. Configuring `ip helper-address 10.10.2.100` on subinterface `Gi0/0.40` allows the router to forward DHCP requests as unicast traffic across the Layer 3 boundary to the target server address.

Step-by-Step Solution

Analyze symptom and packet capture evidence

Workstations auto-assign APIPA (

169.254.x.x

) addresses, and incoming Layer 2 `DHCPDISCOVER` broadcasts reach `Gi0/0.40`, but no unicast frames leave the router toward the DHCP server at

10.10.2.100

DHCP broadcast messages operate at Layer 2 and are stopped at the router boundary by default.

Evaluate subinterface configuration

`Gi0/0.40` has 802.1Q tagging and an IP address assigned, but lacks an `ip helper-address` directive.

Without an IP helper address configured, the router cannot relay Layer 2 DHCP broadcasts across Layer 3 subnets to a remote DHCP server.

Determine remediation step

Add `ip helper-address 10.10.2.100` under `interface GigabitEthernet0/0.40`.

This instructs the router to encapsulate incoming `DHCPDISCOVER` broadcasts into unicast UDP packets addressed directly to

10.10.2.100

with the gateway IP address (`giaddr`) set to

10.40.16.1

Key Concept

DHCP Relay Agent and IP Helper Address Placement

Estimated Time:2m 0s

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