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Difficulty: Very hardDHCP Server, Client, and Relay Agent Operation and Configuration

An network administrator is troubleshooting an issue where client workstations attached to VLAN 20 (subnet 192.168.20.0/24) fail to receive IPv4 dynamic configuration settings from a centralized DHCP server located at 10.1.100.50 in VLAN 100. The administrator inspects the configuration on the Cisco IOS router acting as the default gateway for both subnets:

interface GigabitEthernet0/0/1.20
encapsulation dot1Q 20
ip address 192.168.20.1 255.255.255.0
!
interface GigabitEthernet0/0/1.100
encapsulation dot1Q 100
ip address 10.1.100.1 255.255.255.0
ip helper-address 10.1.100.50

Based on the router configuration output, why are the clients in VLAN 20 failing to obtain IP addresses from the DHCP server?

  1. The ip helper-address command is configured on the router interface facing the DHCP server rather than on the incoming client gateway interface.Answer
  2. B
    The router cannot relay DHCP broadcasts across subnets because routers inherently drop all Layer 2 and Layer 3 broadcast frames regardless of helper address configuration.
  3. C
    The ip helper-address configuration requires the addition of the overload keyword to enable unicast translation for cross-VLAN relay operation.
  4. D
    The subinterface GigabitEthernet0/0/1.20 requires the command ip dhcp relay information trust-all because Option 82 is inserted automatically whenever traffic crosses 802.1Q subinterfaces.

Answer

The ip helper-address command is configured on the router interface facing the DHCP server rather than on the incoming client gateway interface.
DHCP clients broadcast their DHCP Discover messages to 255.255.255.255. To relay these broadcasts to a unicast DHCP server in a different subnet, the 'ip helper-address' command must be configured on the router interface receiving those broadcasts (the client default gateway, GigabitEthernet0/0/1.20). In the provided configuration, the command was incorrectly placed on the subinterface facing the server (GigabitEthernet0/0/1.100), preventing the router from intercepting client requests.

Step-by-Step Solution

1
Analyze client traffic flow and DHCP request behavior.
Clients in VLAN 20 send DHCP Discover packets as Layer 3 broadcasts (255.255.255.255) arriving on interface GigabitEthernet0/0/1.20.
DHCP clients do not initially have an IP address and broadcast their requests to locate a server.
2
Evaluate placement of the ip helper-address statement.
The ip helper-address 10.1.100.50 command is currently under GigabitEthernet0/0/1.100 instead of GigabitEthernet0/0/1.20.
The router listens for incoming broadcasts on the interface where the command is applied. Because it is absent on GigabitEthernet0/0/1.20, incoming broadcasts from VLAN 20 clients are discarded.
3
Formulate the correct configuration change.
Move ip helper-address 10.1.100.50 from subinterface .100 to subinterface .20.
Applying the helper address on GigabitEthernet0/0/1.20 allows the router to intercept client broadcasts, convert them to unicast packets directed to 10.1.100.50, and populate the giaddr field with 192.168.20.1 so the server selects the correct address pool.

Key Concept

DHCP Relay Agent Interface Placement and Operation
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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