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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

A network administrator provisions a new VLAN at a remote office site, routing inter-VLAN traffic through a local Layer 3 switch. Clients connected to the new VLAN are unable to access network resources and receive self-assigned IP addresses starting with 169.254.x.x169.254.x.x. The centralized DHCP server located at the main headquarters has a valid, non-exhausted scope created for the new subnet. Which of the following is the most likely cause of this issue?

  1. The Layer 3 switch interface serving the new VLAN lacks a DHCP relay agent configuration.Answer
  2. B
    The workstation operating systems automatically assigned APIPA addresses due to an incorrect default gateway setting on the client NICs.
  3. C
    The DHCP scope was miscalculated by neglecting to subtract the network and broadcast addresses from the usable host pool.
  4. D
    The centralized DHCP server rejected client requests because the server's own interface resides on a different IP subnet than the new gateway.

Answer

The Layer 3 switch interface serving the new VLAN lacks a DHCP relay agent configuration.
DHCP DISCOVER messages are sent as Layer 2 broadcast frames. Because routers and Layer 3 switches do not forward broadcast traffic, a DHCP relay agent (or IP helper address) must be enabled on the client-facing gateway interface to convert broadcasts into unicast traffic directed to the remote DHCP server. Without this relay, clients fail to reach the server and fall back to Automatic Private IP Addressing (APIPA).

Step-by-Step Solution

1
Analyze client symptoms and network topology.
Clients receive 169.254.x.x169.254.x.x (APIPA) addresses, indicating DHCP DISCOVER messages are failing to receive a DHCP OFFER from the server across the Layer 3 boundary.
DHCP broadcasts (Layer 2 broadcast destination address FF:FF:FF:FF:FF:FF) are restricted to their local broadcast domain and cannot traverse Layer 3 devices by default.
2
Evaluate the state of the centralized DHCP server.
The server has a configured, active, and unexhausted scope for the target subnet, ruling out server-side scope exhaustion.
If the scope is available and valid, the delivery mechanism between the client broadcast domain and the unicast DHCP server is broken.
3
Identify the required network service to bridge Layer 2 broadcasts across Layer 3 boundaries.
A DHCP relay agent (such as the `ip helper-address` directive) must be configured on the gateway interface to forward broadcast requests as unicast packets to the remote server.
Without a relay agent, DHCP DISCOVER broadcasts are dropped by the Layer 3 interface.

Key Concept

DHCP Relay / IP Helper Operation across Layer 3 Boundaries
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Topics

Troubleshooting IP Addressing and DHCP Services

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