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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

A network administrator is troubleshooting network connectivity issues for automated guided vehicles (AGVs) deployed on a newly provisioned warehouse floor subnet (VLAN 45: 10.45.0.0/2310.45.0.0/23). The AGVs cannot communicate with the central controller on another subnet. Inspecting the IP configuration on an AGV displays the following terminal output:

IPv4 Address. . . . . . . . . . . : 169.254.12.88
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :

The central DHCP server is located on VLAN 10 (10.10.0.5010.10.0.50) with a valid, unexhausted scope for VLAN 45. Which TWO of the following statements correctly identify the underlying cause and the required resolution?

  1. The host automatically generated an Automatic Private IP Addressing (APIPA) address because DHCP broadcast requests are not reaching the central DHCP server.Answer
  2. An IP helper address (DHCP relay) must be configured on the VLAN 45 default gateway router interface pointing to the DHCP server at 10.10.0.50.Answer
  3. C
    The host cannot reach external subnets because the default gateway was not manually specified as 169.254.0.1 on the client network adapter.
  4. D
    The DHCP server scope allocation failed because a /23 prefix length only provides a maximum of 254 usable host IP addresses.

Answer

The host automatically generated an APIPA address because DHCP broadcast requests were blocked across subnets, and an IP helper address (DHCP relay) must be configured on the VLAN 45 gateway router interface pointing to 10.10.0.50.
The presence of a 169.254.x.x address confirms that the client failed to receive a DHCP lease and assigned itself an Automatic Private IP Address (APIPA). Because the central DHCP server is situated on a different subnet (VLAN 10) than the client (VLAN 45), Layer 2 DHCP Discover broadcasts are blocked by default at the router interface. Configuring an IP helper address (DHCP relay) on the VLAN 45 router interface allows the router to relay client requests as unicast packets directly to the central DHCP server.

Step-by-Step Solution

1
Analyze the client IP configuration snippet.
The IP address 169.254.12.88 belongs to the link-local Automatic Private IP Addressing (APIPA) block.
Operating systems automatically assign an APIPA address when dynamic configuration via DHCP times out or fails.
2
Identify why DHCP communications are failing across subnets.
DHCP Discover packets are sent as Layer 2 broadcasts, which routers do not forward across VLAN boundaries.
The client resides on VLAN 45 while the central DHCP server is located on VLAN 10.
3
Determine the necessary corrective action.
Configure an IP helper address (DHCP relay) on the VLAN 45 router interface pointing to 10.10.0.50.
A DHCP relay agent captures local DHCP broadcast traffic, encapsulates it as unicast IP packets, and forwards it to the designated DHCP server.

Key Concept

Troubleshooting APIPA Assignment and Cross-Subnet DHCP Relay (IP Helper)
Estimated Time:1m 30s
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Topics

Troubleshooting IP Addressing and DHCP Services

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