A network administrator is troubleshooting an issue where workstations on a newly provisio… — CompTIA Network+

A network administrator is troubleshooting an issue where workstations on a newly provisioned subnet, VLAN 55 ( $172.18.16.0/21$ ), fail to acquire dynamic IP configurations and instead assign themselves addresses in the range $169.254.1.1$ to $169.254.254.254$ .

The central DHCP server ( $10.100.5.10$ ) resides on VLAN 10. The Layer 3 switch routing interface for VLAN 55 is configured as follows:

text
interface Vlan55
ip address 172.18.16.1 255.255.248.0
ip helper-address 10.100.5.10

A packet capture performed at the DHCP server shows that unicast `DHCPDISCOVER` packets forwarded by the Layer 3 switch arrive at the server with a Gateway IP Address (GIADDR) of $172.18.16.1$ , but the DHCP server returns no `DHCPOFFER`. All other subnets using this DHCP server are receiving leases normally.

Which of the following is the MOST likely root cause of this failure?

A
The `ip helper-address` command must be configured on the DHCP server interface on VLAN 10 rather than on the client VLAN interface.
B
The client workstations have a misconfigured default gateway that prevents APIPA addresses from routing across VLAN boundaries.
The DHCP server does not have an active scope defined that matches the network segment of the GIADDR (172.18.16.0/21).Answer
D
The /21 subnet mask defines 2,046 usable host addresses, which exceeds the maximum address pool capacity allowed per DHCP scope.

Answer

The DHCP server lacks an active scope matching the GIADDR network segment (172.18.16.0/21).

When a DHCP relay agent forwards client broadcasts to a central DHCP server, it inserts its local interface IP address into the GIADDR field of the DHCP packet header. The central DHCP server relies on GIADDR to match incoming requests to an active DHCP scope. If the server does not have an active scope corresponding to the relay agent's subnet (172.18.16.0/21), it cannot allocate an address and drops the request without responding.

Step-by-Step Solution

Analyze client symptoms and network packet capture evidence.

Workstations self-assign APIPA addresses (169.254.x.x), indicating DHCP requests are unfulfilled. The packet capture confirms DHCPDISCOVER packets arrive at the central server with GIADDR = 172.18.16.1.

Because the relay agent correctly forwards unicast DHCPDISCOVER packets to 10.100.5.10, physical and Layer 3 reachability between the relay agent and DHCP server is functional.

Evaluate how a DHCP server processes relayed requests based on the GIADDR field.

The server inspects GIADDR (172.18.16.1) to identify which subnet pool to allocate an IP address from.

DHCP servers use the GIADDR IP address to perform a lookup in their scope database for a matching subnet.

Identify why no DHCPOFFER is returned while other subnets function normally.

If no scope exists or if the scope for 172.18.16.0/21 is deactivated/deconfigured, the DHCP server cannot select an available address and silently drops the request.

A missing or inactive scope corresponding to the GIADDR prevents address allocation.

Key Concept

DHCP Relay GIADDR Scope Selection

Estimated Time:2m 0s

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