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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

A network administrator is troubleshooting connectivity for a workstation on a newly configured VLAN 25 (172.16.25.0/24172.16.25.0/24). The central DHCP server is located on VLAN 10 (172.16.10.100172.16.10.100). Running `ipconfig /all` on the workstation yields the following output snippet:

IPv4 Address. . . . . . . . . . . : 169.254.88.12
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . :

Which of the following is the most likely cause of this issue?

  1. The router interface serving VLAN 25 lacks a configured IP helper address pointing to the central DHCP server.Answer
  2. B
    The client workstation was configured with an incorrect default gateway subnet mask, causing DHCP request drops.
  3. C
    The network interface assigned an APIPA address because the client's static IP configuration conflicts with the default gateway.
  4. D
    The DHCP scope for VLAN 25 exhausted its address space because unusable network and broadcast addresses were included as valid host allocations.

Answer

The router interface serving VLAN 25 lacks a configured IP helper address pointing to the central DHCP server.
DHCP clients broadcast DHCPDISCOVER messages on their local subnet. Because routers do not forward Layer 3 broadcast traffic by default, clients on subnets separate from the central DHCP server cannot obtain an IP address unless an IP helper address (DHCP relay) is configured on the router gateway interface. Without a relay, the client times out and assigns itself an APIPA address (169.254.x.x169.254.x.x).

Step-by-Step Solution

1
Analyze the `ipconfig /all` output snippet provided for the workstation.
The workstation has self-assigned an APIPA address (169.254.88.12169.254.88.12) with no Default Gateway or DHCP Server listed.
When a client configured for dynamic IP addressing fails to receive a response to its DHCPDISCOVER broadcast, operating systems default to Automatic Private IP Addressing (APIPA).
2
Evaluate the network topology details.
The client resides on VLAN 25 (172.16.25.0/24172.16.25.0/24), whereas the DHCP server resides on a different subnet, VLAN 10 (172.16.10.100172.16.10.100).
Layer 3 routers block broadcast traffic by default, preventing DHCPDISCOVER broadcasts on VLAN 25 from reaching VLAN 10.
3
Identify the required network configuration for cross-subnet DHCP.
An IP helper address (DHCP relay agent) must be configured on the router's VLAN 25 interface.
The DHCP relay agent intercepts local DHCP broadcasts and forwards them as unicast packets across the router to the central DHCP server address.

Key Concept

DHCP Relay (IP Helper) Configuration across VLAN boundaries
Estimated Time:1m 15s
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Topics

Troubleshooting IP Addressing and DHCP Services

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