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Difficulty: MediumIP Addressing Services

Workstations in a newly configured branch office network (VLAN 20) are unable to obtain IPv4 addresses dynamically. The network administrator verifies that the centralized DHCP server is online in the datacenter (VLAN 100) and that IP routing between the subnets is functioning properly. Packet captures at the datacenter reveal that broadcast DHCP DISCOVER messages originated by branch clients never arrive at the server. Which of the following configuration steps must be performed on the branch router to resolve this issue?

  1. Configure an IP helper address pointing to the DHCP server IP on the client VLAN 20 gateway interface.Answer
  2. B
    Configure an IP helper address on the datacenter router interface facing the centralized DHCP server.
  3. C
    Create a DNS CNAME record mapping the client gateway IP address to the DHCP server hostname.
  4. D
    Enable a firewall rule allowing outbound UDP port 53 traffic from client workstations to the DHCP server.

Answer

An IP helper address must be configured on the client VLAN 20 gateway interface of the branch router, pointing directly to the centralized DHCP server's IP address.
The correct action is to configure an IP helper address on the client VLAN 20 gateway interface of the branch router. By default, Layer 3 devices drop Layer 2 broadcast frames, preventing DHCP DISCOVER messages from reaching servers outside the local broadcast domain. The IP helper command enables a DHCP relay agent on the client-facing gateway interface, transforming broadcast traffic into unicast packets routed across the network to the central DHCP server.

Step-by-Step Solution

1
Analyze client traffic behavior across Layer 3 boundaries
Identify that Layer 2 broadcast messages (such as DHCP DISCOVER packets) are dropped by routers by default.
Routers establish broadcast domain boundaries and do not forward L2 broadcast packets across subnets without explicit relay configuration.
2
Determine the required IP service mechanism
Recognize the requirement for a DHCP Relay Agent (IP helper address).
A DHCP relay agent intercepts local client broadcasts and encapsulates them into unicast IP packets directed to the designated DHCP server.
3
Identify the correct interface placement for the relay agent
Apply the IP helper configuration on the inbound interface receiving the client broadcast traffic (VLAN 20 gateway interface).
The relay agent must listen on the specific local interface where client broadcast requests originate to successfully relay them to another subnet.

Key Concept

DHCP Relay Agent and IP Helper Address Placement
Estimated Time:1m 15s
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Topics

IP Addressing Services

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