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Difficulty: HardDHCP Server, Client, and Relay Agent Operation and Configuration

Clients connected to VLAN 10 successfully receive dynamic IP addresses from a remote server located at 192.168.100.50, but clients on VLAN 20 fail to obtain IP addresses and assign themselves 169.254.x.x addresses.

An engineer inspects the running configuration of router R1:

text
interface GigabitEthernet0/0.10
encapsulation dot1Q 10
ip address 10.10.10.1 255.255.255.0
ip helper-address 192.168.100.50
!
interface GigabitEthernet0/0.20
encapsulation dot1Q 20
ip address 10.10.20.1 255.255.255.0
!
interface GigabitEthernet0/1
description Connection to DHCP Server Subnet
ip address 192.168.100.1 255.255.255.0
ip helper-address 192.168.100.50

Which configuration change on router R1 will restore DHCP functionality for clients on VLAN 20?

  1. Remove the helper address command from interface GigabitEthernet0/1 and add ip helper-address 192.168.100.50 under subinterface GigabitEthernet0/0.20.Answer
  2. B
    Add ip helper-address 10.10.20.1 under interface GigabitEthernet0/1 to forward server response packets back to the subinterface.
  3. C
    Configure ip helper-address 10.10.20.255 under interface GigabitEthernet0/0 to flood DHCP broadcasts across all subinterfaces.
  4. D
    Add ip helper-address 192.168.100.255 under subinterface GigabitEthernet0/0.20 to broadcast requests across the destination subnet.

Answer

Remove the helper address configuration from interface GigabitEthernet0/1 and configure ip helper-address 192.168.100.50 under subinterface GigabitEthernet0/0.20.
To relay DHCP broadcast requests from clients to a server on a different subnet, the Cisco IOS `ip helper-address <server-ip>` command must be configured on the client-facing layer 3 interface (subinterface GigabitEthernet0/0.20). In the provided scenario, subinterface GigabitEthernet0/0.20 is missing the command, while interface GigabitEthernet0/1 mistakenly has it configured. Moving the command to GigabitEthernet0/0.20 allows R1 to intercept VLAN 20 broadcast DISCOVER messages, insert 10.10.20.1 as the gateway address (GIADDR), and unicast the request to 192.168.100.50.

Step-by-Step Solution

1
Analyze client-to-relay path requirements
DHCP DISCOVER packets generated by clients are local Layer 2 broadcasts targeted to UDP port 67.
Routers drop Layer 2/Layer 3 broadcast packets by default and do not forward them between interfaces or subinterfaces.
2
Identify correct placement of the ip helper-address command
The `ip helper-address` command must be applied to the incoming (ingress) interface where client broadcasts originate (GigabitEthernet0/0.20).
When applied on the client interface, the router intercepts UDP broadcast traffic, populates the GIADDR field with the interface's IP address (10.10.20.1), and unicasts the packet to the specified DHCP server.
3
Evaluate the current flawed configuration on R1
Subinterface GigabitEthernet0/0.20 lacks `ip helper-address`, while the egress interface GigabitEthernet0/1 has an unnecessary `ip helper-address` configured.
Configuring `ip helper-address` on the egress interface towards the server does nothing for clients on VLAN 20 because client broadcasts enter on subinterface GigabitEthernet0/0.20 and are dropped before reaching interface GigabitEthernet0/1.

Key Concept

DHCP Relay Agent Configuration and Placement
Estimated Time:1m 30s
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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