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Difficulty: MediumDHCP Architecture and Relay Services

A network engineer is configuring a redundant centralized DHCP infrastructure with primary and secondary DHCP servers located on a dedicated management subnet (10.50.100.0/2410.50.100.0/24). Multiple client subnets, including VLAN 12 (172.16.12.0/24172.16.12.0/24) and VLAN 24 (172.16.24.0/24172.16.24.0/24), must be able to receive lease offers from both servers. Which of the following configuration steps must be implemented on the Layer 3 interface for each client subnet to ensure redundant DHCP relay service? (Select TWO.)

  1. Configure an `ip helper-address` statement specifying the unicast IPv4 address of the primary DHCP server.Answer
  2. Configure an additional `ip helper-address` statement specifying the unicast IPv4 address of the secondary DHCP server.Answer
  3. C
    Configure ACL rules on the client Layer 3 interfaces to permit inbound TCP port 67 and outbound TCP port 68 traffic.
  4. D
    Configure a single `ip helper-address` entry using the directed subnet broadcast address 10.50.100.25510.50.100.255.

Answer

The correct steps are to configure an ip helper-address specifying the primary DHCP server's unicast IP address and a second ip helper-address specifying the secondary DHCP server's unicast IP address on each client subnet's Layer 3 boundary interface.
To support redundant DHCP servers across a Layer 3 boundary, the relay agent (router or Layer 3 switch interface) must be configured with an explicit unicast `ip helper-address` for each server. When multiple `ip helper-address` commands are configured on the same interface, the device replicates incoming client DHCP broadcasts into individual unicast packets sent to each specified DHCP server IP address.

Step-by-Step Solution

1
Identify how Layer 3 relay agents handle DHCP broadcast requests from clients.
The router intercepts Layer 2/3 local broadcasts (UDP port 67) on the client SVI/subinterface.
DHCP Discover and Request packets are broadcasts and cannot cross Layer 3 boundaries without relay assistance.
2
Apply unicast helper address entries for both primary and secondary servers on the client-facing Layer 3 interface.
Multiple `ip helper-address <IP>` commands populate the relay destination table for that interface.
When multiple helper addresses are configured on an interface, the router duplicates the client's broadcast packet and forwards unicast copies to each configured server address.
3
Verify transport layer protocol requirements for DHCP relay forwarding.
DHCP traffic utilizes UDP ports 67 (server listening port) and 68 (client listening port).
TCP is not used during the DORA process or relay agent forwarding.

Key Concept

DHCP Relay Agent Multi-Server Redundancy
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Topics

DHCP Architecture and Relay Services

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