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Difficulty: HardTroubleshooting IP Addressing and DHCP Services

A network administrator provisions a new subinterface on a branch router to serve VLAN 50 (`192.168.50.0/24`). Workstations connected to VLAN 50 fail to obtain network configuration parameters and continuously assign themselves addresses in the `169.254.x.x` range. Meanwhile, devices on VLAN 10 connected to the same switch successfully acquire leases from the centralized DHCP server at `10.10.10.50`. The technician inspects the router subinterface configuration:

text
interface GigabitEthernet0/0/1.50
encapsulation dot1Q 50
ip address 192.168.50.1 255.255.255.0

Which of the following configuration changes on the router interface will resolve the connectivity issue for VLAN 50 hosts?

  1. Add the `ip helper-address 10.10.10.50` command under the `GigabitEthernet0/0/1.50` interface.Answer
  2. B
    Configure static default gateways on the VLAN 50 client devices pointing to 169.254.0.1.
  3. C
    Change the subinterface IP address to `10.10.10.50 255.255.255.0` to match the DHCP server's IP address.
  4. D
    Change the subinterface subnet mask to `255.255.255.128` to reduce the usable host count and clear lease scope exhaustion.

Answer

Add the ip helper-address 10.10.10.50 command under the GigabitEthernet0/0/1.50 subinterface configuration.
DHCP client discovery messages are Layer 2 broadcasts that cannot cross router boundaries by default. Adding the `ip helper-address 10.10.10.50` command to the subinterface enables the router to act as a DHCP relay agent, forwarding local DHCP requests to the centralized server on another subnet.

Step-by-Step Solution

1
Analyze client IP symptoms
Clients receive 169.254.x.x (APIPA) addresses, confirming that DHCP broadcast requests are not receiving replies.
When a DHCP client fails to obtain an IP lease via DORA, standard operating systems self-assign an Automatic Private IP Addressing address.
2
Evaluate network topology and broadcast boundaries
The DHCP server resides on subnet 10.10.10.0/24, whereas clients are on subnet 192.168.50.0/24 across a Layer 3 router interface.
Routers drop Layer 2 broadcast frames (such as DHCPDISCOVER) by default, preventing DHCP requests from crossing subnets.
3
Identify the missing router configuration element
The router subinterface lacks an IP helper address directive to forward broadcast DHCP requests to the central DHCP server.
Configuring `ip helper-address 10.10.10.50` converts incoming client DHCP broadcasts into unicast UDP messages directed to the specified server.

Key Concept

DHCP Relay Agent and IP Helper Configuration
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Topics

Troubleshooting IP Addressing and DHCP Services

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