A network engineer provisions a new subinterface `GigabitEthernet0/0.40` on a branch route… — CompTIA Network+

A network engineer provisions a new subinterface `GigabitEthernet0/0.40` on a branch router to serve an isolated IoT subnet ( $172.16.40.0/24$ ). Devices connected to VLAN 40 are unable to reach external resources and auto-assign link-local IPv4 addresses in the $169.254.x.x$ range. A terminal capture on the router shows the following configuration:

text
interface GigabitEthernet0/0.10
description Corporate_Users
encapsulation dot1Q 10
ip address 172.16.10.1 255.255.255.0
ip helper-address 10.10.10.50
!
interface GigabitEthernet0/0.40
description IoT_Devices
encapsulation dot1Q 40
ip address 172.16.40.1 255.255.255.0

Existing client workstations on `GigabitEthernet0/0.10` receive leases dynamically from the central DHCP server at $10.10.10.50$ without issue. Which configuration change on the branch router will resolve the IP addressing failure for VLAN 40?

Add the command `ip helper-address 10.10.10.50` under the `GigabitEthernet0/0.40` subinterface interface configuration mode.Answer
B
Reconfigure `GigabitEthernet0/0.40` to use `encapsulation dot1Q 10` so it can share the existing DHCP relay agent setup.
C
Configure the central DHCP server to enable APIPA relaying for unconfigured subinterfaces.
D
Increase the usable host capacity on `GigabitEthernet0/0.10` to include the $172.16.40.0/24$ network range.

Answer

Add the command `ip helper-address 10.10.10.50` under the `GigabitEthernet0/0.40` subinterface interface configuration mode.

DHCP broadcasts (UDP ports 67 and 68) are confined to their local Layer 2 broadcast domain and do not pass through Layer 3 boundaries. Subinterface `GigabitEthernet0/0.40` acts as the default gateway for VLAN 40. Without the `ip helper-address` command on this specific subinterface, the router drops the client DHCPDISCOVER broadcast frames. Adding `ip helper-address 10.10.10.50` configures the router to act as a DHCP relay agent for VLAN 40, converting incoming client broadcasts into unicast UDP packets sent directly to the central DHCP server at

10.10.10.50

Step-by-Step Solution

Analyze client symptoms and subinterface configuration.

IoT devices on VLAN 40 receive 169.254.x.x addresses, indicating APIPA assignment caused by a failure to complete the DHCP DORA process.

DHCP client broadcast requests (DHCPDISCOVER) are stopped by default at Layer 3 router interfaces unless a relay agent is configured.

Compare working subinterface `GigabitEthernet0/0.10` with non-working subinterface `GigabitEthernet0/0.40`.

`GigabitEthernet0/0.10` has `ip helper-address 10.10.10.50` configured, while `GigabitEthernet0/0.40` is missing any relay statement.

DHCP relay agent directives must be explicitly applied to each routed logical interface/subinterface that receives client broadcasts.

Identify the required remediation step.

Applying `ip helper-address 10.10.10.50` to subinterface `GigabitEthernet0/0.40` allows the router to convert client UDP broadcasts into unicast packets routed to the centralized server.

This establishes the necessary Layer 3 relay mechanism for VLAN 40.

Key Concept

DHCP Relay Agent and IP Helper Placement on VLAN Subinterfaces

Estimated Time:2m 0s

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