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Difficulty: EasyDHCP Architecture and Relay Services

Hosts on a newly provisioned user subnet fail to dynamically acquire IP addresses from an established DHCP server located on a separate administrative network segment. Why is a DHCP relay service required on the local gateway router to resolve this issue?

  1. DHCP discovery messages are sent as local broadcasts, which routers do not forward across subnets by default.Answer
  2. B
    Devices on different subnets share the same Layer 2 broadcast domain, requiring a relay agent to filter duplicate requests.
  3. C
    DHCP relies on connection-oriented TCP traffic, which requires a gateway relay to maintain stateful session tables across routers.
  4. D
    DHCP requests are transmitted over port 80, which requires network address translation on the router to reach external servers.

Answer

DHCP discovery messages are sent as local broadcasts, which routers do not forward across subnets by default.
DHCP clients initially send broadcast messages to discover available servers. Because routers isolate subnets by dropping broadcast traffic by default, a DHCP relay agent (or IP helper address) must be configured on the router interface facing the client. The relay converts the client's broadcast discovery into a unicast packet directed straight to the central DHCP server's IP address.

Step-by-Step Solution

1
Analyze how clients initiate DHCP requests
DHCP clients broadcast a DHCPDISCOVER message using destination IP address 255.255.255.255255.255.255.255.
The client does not yet have an IP address or knowledge of the local gateway and network topology.
2
Evaluate router behavior regarding local broadcast traffic
Routers drop Layer 3 broadcast frames by default to isolate broadcast domains.
Preventing broadcast propagation across subnets reduces network congestion and prevents broadcast storms.
3
Identify the role of the DHCP relay agent (IP Helper)
The relay agent intercepts the broadcast on the client-facing interface and forwards it as a unicast packet to the central DHCP server.
Unicast packets are routable across network boundaries, allowing the remote DHCP server to assign an IP address from the appropriate scope.

Key Concept

DHCP Relay Services and Broadcast Boundaries
Estimated Time:45s
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Topics

DHCP Architecture and Relay Services

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