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Difficulty: HardTroubleshooting IP Addressing and DHCP Services

A network engineer is troubleshooting a newly connected point-of-sale terminal on VLAN 45 (10.45.10.0/2410.45.10.0/24) that cannot communicate with the accounting database. Executing `ip addr show eth0` on the terminal produces the following output:

text
eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default
inet 169.254.112.85/16 brd 169.254.255.255 scope global eth0
valid_lft forever preferred_lft forever

Inspection of the default gateway router configuration for VLAN 45 displays the following settings:

text
interface GigabitEthernet0/0.45
encapsulation dot1Q 45
ip address 10.45.10.1 255.255.255.0
ip helper-address 10.100.20.14

A recent IP audit confirms that the active central enterprise DHCP server resides at address 10.100.20.1510.100.20.15. What is the primary root cause of the terminal failing to acquire a valid network IP address?

  1. The `ip helper-address` directive on the gateway router subinterface is misconfigured with an incorrect DHCP server IP address.Answer
  2. B
    The host self-assigned an APIPA address because the local default gateway IP address on VLAN 45 is down or unreachable.
  3. C
    The DHCP Relay Agent service must be installed directly on the client terminal operating system to unpack relay packets.
  4. D
    The /24 subnet mask on the router subinterface limits the pool size, causing an IP address collision with the subnet broadcast address.

Answer

The `ip helper-address` statement on the gateway subinterface specifies 10.100.20.1410.100.20.14, which does not match the active central DHCP server IP address (10.100.20.1510.100.20.15).
When client devices display an address within the 169.254.0.0/16169.254.0.0/16 block (APIPA), it signifies that the host attempted a dynamic lease request but received no DHCP OFFER. Because the DHCP server sits on a different subnet (10.100.20.0/2410.100.20.0/24), the router's subinterface uses `ip helper-address` to forward broadcast requests via unicast. The configuration lists 10.100.20.1410.100.20.14 instead of the active server address 10.100.20.1510.100.20.15, causing all forwarded DHCP requests to be dropped by the network.

Step-by-Step Solution

1
Analyze client IP output snippet
The terminal displays IP address 169.254.112.85/16169.254.112.85/16, identifying an Automatic Private IP Addressing (APIPA) self-assigned address.
When a DHCP client fails to receive a DHCP DISCOVER response/OFFER, it defaults to APIPA.
2
Examine gateway relay settings
Interface `GigabitEthernet0/0.45` routes requests using `ip helper-address 10.100.20.14`.
DHCP broadcast messages generated on VLAN 45 must be converted into unicast packets destined for the DHCP server IP address specified in the helper directive.
3
Compare helper address against active server inventory
The configured helper IP (10.100.20.1410.100.20.14) is incorrect because the central DHCP server is hosted at 10.100.20.1510.100.20.15.
Forwarded unicast DHCP DISCOVER frames are sent to an inactive or non-DHCP endpoint, resulting in lost requests and client lease timeout.

Key Concept

DHCP Relay (IP Helper) Address Configuration and APIPA Diagnosis
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Topics

Troubleshooting IP Addressing and DHCP Services

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