Examkin

Question

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

A network engineer is configuring a Cisco IOS router (R1) to act as a DHCP relay agent servicing clients across multiple subnets. Central DHCP services reside at IP address 172.16.100.5. The subinterface configuration on R1 is as follows:

text
interface GigabitEthernet0/0.10
encapsulation dot1Q 10
ip address 10.1.10.1 255.255.255.0
ip helper-address 172.16.100.5
!
interface GigabitEthernet0/0.20
encapsulation dot1Q 20
ip address 10.1.20.1 255.255.255.0
!
interface GigabitEthernet0/1
description Egress link towards DHCP Server
ip address 172.16.100.1 255.255.255.0

Clients connected to VLAN 10 successfully receive IP addresses, while clients on VLAN 20 fail to obtain lease parameters. Which TWO statements accurately explain the operation of the DHCP relay agent or the required configuration changes?

  1. The command ip helper-address 172.16.100.5 must be configured under interface GigabitEthernet0/0.20 so that DHCP broadcasts from VLAN 20 clients are relayed to the server.Answer
  2. When forwarding a DHCP Discover packet from VLAN 10, the router inserts its local subinterface IP address (10.1.10.1) into the GIADDR field of the DHCP header.Answer
  3. C
    The command ip helper-address 172.16.100.5 should be removed from subinterface GigabitEthernet0/0.10 and applied under GigabitEthernet0/1, as helper addresses must be positioned on the egress interface pointing toward the server.
  4. D
    The router converts client DHCP Discover broadcast frames into Layer 2 Ethernet broadcast frames before forwarding them out GigabitEthernet0/1 to reach all devices in the server broadcast domain.

Answer

The helper address command must be added to subinterface GigabitEthernet0/0.20, and the router populates the GIADDR field with its local client-facing interface IP address when forwarding relay requests.
The correct options accurately reflect DHCP relay principles: first, helper addresses must be enabled on every client-facing gateway interface (such as GigabitEthernet0/0.20) where client broadcasts originate; second, the relay agent populates the GIADDR field with its local gateway IP address so the central DHCP server knows which scope to assign.

Step-by-Step Solution

1
Analyze the subinterface configuration for VLAN 20.
GigabitEthernet0/0.20 is missing the `ip helper-address 172.16.100.5` command, causing client DHCP Discover broadcasts on VLAN 20 to be dropped by default.
DHCP relay helper addresses operate on a per-interface basis and must be configured on every incoming client gateway interface.
2
Examine the DHCP packet transformation performed by the relay agent.
When relaying client broadcast requests to the server, the router inserts its own ingress interface IP address into the GIADDR (Gateway IP Address) field of the DHCP payload.
The central DHCP server uses the GIADDR field to determine the appropriate IP pool and subnet range from which to lease an address.

Key Concept

DHCP Relay Agent Configuration and GIADDR Operation
Rate this question

Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

More questions from this topic