A network administrator is troubleshooting a DHCP relay issue on a Cisco IOS router. Host … — Cisco CCNA

A network administrator is troubleshooting a DHCP relay issue on a Cisco IOS router. Host devices on VLAN 30 ( $172.16.30.0/24$ ) use subinterface GigabitEthernet0/0.30 ( $172.16.30.1$ ) as their default gateway. The centralized DHCP server is located at $10.10.10.50$ , connected off router interface GigabitEthernet0/1 ( $10.10.10.1$ ).

Packet captures at the DHCP server reveal that incoming relayed DHCP DISCOVER messages contain a Relay Agent IP Address (`giaddr`) of $10.10.10.1$ instead of $172.16.30.1$ . Consequently, the DHCP server fails to assign addresses from the intended $172.16.30.0/24$ pool.

Which router misconfiguration causes the `giaddr` field to be set to $10.10.10.1$ ?

The `ip helper-address 10.10.10.50` command was incorrectly applied to interface GigabitEthernet0/1 instead of subinterface GigabitEthernet0/0.30.Answer
B
The `ip helper-address 10.10.10.50` command was correctly placed on subinterface GigabitEthernet0/0.30, but Option 82 insertion altered the `giaddr` to the egress interface address.
C
The router interface GigabitEthernet0/0.30 failed to intercept broadcast packets because DHCP relay must be globally activated using `ip dhcp relay enable`.
D
The router was configured with `ip helper-address 172.16.30.1` on subinterface GigabitEthernet0/0.30, causing the gateway address to match the outbound interface.

Answer

The `ip helper-address 10.10.10.50` command was incorrectly applied to interface GigabitEthernet0/1 instead of subinterface GigabitEthernet0/0.30.

When a Cisco router acts as a DHCP relay agent, it listens for broadcast UDP port 67 messages on interfaces configured with `ip helper-address <server_ip>`. When a DISCOVER packet is received on such an interface, the router converts the broadcast to a unicast packet sent to the helper IP and populates the `giaddr` (Gateway IP Address) field with the primary IP address of the INCOMING interface. Because `giaddr` was set to

10.10.10.1

, the `ip helper-address` command was erroneously placed on interface GigabitEthernet0/1. Moving the command to GigabitEthernet0/0.30 ensures `giaddr` is populated as

172.16.30.1

, allowing the DHCP server to allocate from the correct

172.16.30.0/24

pool.

Step-by-Step Solution

Analyze how a Cisco IOS DHCP Relay Agent populates the Relay Agent IP Address (`giaddr`) field.

When a router receives a client broadcast DHCP DISCOVER message on an interface enabled with `ip helper-address`, it sets the `giaddr` field to the primary IP address of the INGRESS interface where the request was received.

The DHCP server uses the `giaddr` field to determine which subnet scope/pool to select for IP address assignment.

Evaluate the captured `giaddr` IP address value (

10.10.10.1

10.10.10.1

corresponds to interface GigabitEthernet0/1, which is the egress interface connected to the DHCP server subnet, rather than ingress subinterface GigabitEthernet0/0.30 (

172.16.30.1

This indicates that the `ip helper-address` command was mistakenly configured under interface GigabitEthernet0/1.

Identify the corrective configuration step.

Remove `ip helper-address 10.10.10.50` from interface GigabitEthernet0/1 and place it under subinterface GigabitEthernet0/0.30.

Placing the helper address on GigabitEthernet0/0.30 ensures the router intercepts VLAN 30 client broadcasts and sets `giaddr` to

172.16.30.1

Key Concept

DHCP Relay Agent Ingress Interface Binding and giaddr Population

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Key Concept

Common Mistakes

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