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Difficulty: HardIP Addressing Services

A network administrator is deploying a multi-VLAN enterprise network architecture. Workstations connected to VLAN 40 (172.16.40.0/24172.16.40.0/24) fail to receive dynamic IP address configurations and revert to Automatic Private IP Addressing (169.254.x.x169.254.x.x). The centralized DHCP server is located on VLAN 10 (172.16.10.50172.16.10.50). Analysis indicates that client broadcast DHCPDISCOVER messages are arriving at the default gateway interface but are dropped at the Layer 3 boundary. Which of the following commands must the administrator configure on the VLAN 40 router interface to resolve this issue?

  1. ip helper-address 172.16.10.50Answer
  2. B
    ip dhcp pool 172.16.40.0/24
  3. C
    dns-server 172.16.10.50
  4. D
    ip forward-protocol tcp 67

Answer

ip helper-address 172.16.10.50
Applying the `ip helper-address 172.16.10.50` command configures the router interface as a DHCP Relay Agent. It intercepts local broadcast DHCPDISCOVER traffic from VLAN 40 clients and forwards it as unicast UDP traffic to the target DHCP server IP address on VLAN 10.

Step-by-Step Solution

1
Identify the cause of client IP configuration failure across subnets
DHCPDISCOVER broadcast packets cannot traverse Layer 3 boundaries without explicit relay intervention.
Routers drop Layer 2/3 broadcast frames by default to prevent broadcast storms.
2
Determine the correct IP addressing service mechanism required
A DHCP Relay Agent command (`ip helper-address`) must be applied to the client subnet's ingress router interface.
The relay agent intercepts local broadcast DHCPDISCOVER messages and encapsulates them into unicast UDP packets destined for the centralized DHCP server address.

Key Concept

DHCP Relay Agent and IP Helper Configuration
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Topics

IP Addressing Services

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