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Difficulty: EasyDHCP Architecture and Relay Services

A network administrator is setting up a new client subnet on VLAN 10. The centralized enterprise DHCP server resides on a separate subnet across a router on VLAN 20. When client workstations on VLAN 10 boot up, they fail to obtain an IP address because DHCP Discover broadcast messages are stopped by the router. Which command or feature should be configured on the router's VLAN 10 interface to forward these requests to the DHCP server?

  1. DHCP Relay Agent (ip helper-address) specifying the IP address of the DHCP serverAnswer
  2. B
    Proxy ARP enabled on the router interface to forward Layer 2 broadcasts across VLANs
  3. C
    Reconfiguring the DHCP server and clients to communicate using TCP port 67
  4. D
    Static NAT mapping the router interface IP address directly to UDP port 68

Answer

Configure a DHCP Relay Agent (or ip helper-address) on the router interface facing the client subnet, configured with the unicast IP address of the remote DHCP server.
Configuring a DHCP relay agent (or ip helper-address) on the interface connected to the client subnet intercept broadcasts on UDP port 67 and forwards them as unicast traffic directly to the DHCP server's IP address on the remote subnet.

Step-by-Step Solution

1
Identify the cause of the DHCP failure
Routers drop broadcast traffic by default, preventing DHCP Discover broadcast messages originating on VLAN 10 from reaching the server on VLAN 20.
DHCP discovery relies on Layer 2 and Layer 3 broadcasts (255.255.255.255) which cannot cross network boundaries without assistance.
2
Determine the necessary mechanism to bridge subnets for DHCP traffic
A DHCP Relay Agent (configured via 'ip helper-address' in Cisco IOS) must be enabled on the client-facing router interface.
The relay agent intercepts client DHCP broadcasts on the local interface, encapsulates them into unicast IP packets directed to the designated DHCP server IP, and relays the unicast replies back to the client.

Key Concept

DHCP Relay Services and IP Helper Address
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Topics

DHCP Architecture and Relay Services

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