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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

Users on a newly provisioned branch office subnet (172.16.40.0/24172.16.40.0/24) report an inability to reach intranet services or external websites. Running `ipconfig /all` on an affected client workstation yields the following output:

IPv4 Address. . . . . . . . . . . : 169.254.14.88
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . :

The central DHCP server is located on the headquarters data center VLAN (192.168.10.0/24192.168.10.0/24). Network operations confirms that the DHCP scope for 172.16.40.0/24172.16.40.0/24 is online and contains 150 available IP addresses. Which of the following is the most likely root cause of this issue?

  1. The router interface serving as the default gateway for 172.16.40.0/24172.16.40.0/24 lacks a configured DHCP relay agent (IP helper address).Answer
  2. B
    The client workstations were manually misconfigured with static APIPA addresses that bypass the default gateway.
  3. C
    The DHCP scope for 172.16.40.0/24172.16.40.0/24 has exhausted all usable host IP addresses due to incorrect network address calculations.
  4. D
    The workstations successfully leased an IP address but were provisioned with a Default Gateway residing on a different subnet.

Answer

The router interface acting as the default gateway for the branch subnet is missing a DHCP relay agent (IP helper address) to forward broadcast DHCP requests across subnets.
DHCP requests begin as Layer 2 broadcast packets. Routers drop Layer 2 broadcasts by default. When the DHCP server resides on a different network segment from the clients, the router interface facing the client subnet must be configured with a DHCP relay agent (such as `ip helper-address` in Cisco IOS) to convert broadcast requests into unicast traffic directed to the remote DHCP server. Without this, clients fallback to APIPA (169.254.x.x169.254.x.x).

Step-by-Step Solution

1
Analyze the client workstation IP address output.
The client has an address of 169.254.14.88169.254.14.88 with no Default Gateway or DHCP Server listed.
Addresses in the 169.254.0.0/16169.254.0.0/16 range indicate that APIPA assigned the IP after the client sent DHCP DISCOVER requests without receiving a DHCP OFFER response.
2
Evaluate the network architecture and DHCP server status.
The DHCP server is on a separate subnet (192.168.10.0/24192.168.10.0/24) and has active, unallocated leases available.
Since the server has capacity, the failure is occurring in transport between the client subnet (172.16.40.0/24172.16.40.0/24) and the server subnet.
3
Identify Layer 3 boundary behavior for broadcast traffic.
DHCP DISCOVER messages are Layer 2 broadcasts (255.255.255.255255.255.255.255) and do not cross routers by default.
To forward these broadcast requests as unicast packets to a DHCP server on another subnet, the local router interface must be configured with a DHCP relay agent (`ip helper-address`).

Key Concept

DHCP Relay / IP Helper Agent Configuration
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Topics

Troubleshooting IP Addressing and DHCP Services

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