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Example: DHCP Relay Agent Configuration with Multiple Clients and Servers

This example shows an extended DHCP relay agent configuration for a network that includes multiple DHCP clients and DHCP servers. Additional details follow the example.

[edit forwarding-options]dhcp-relay {server-group {sp-1 {;;}sp-2 {;;;}}active-server-group sp-1;overrides layer2-unicast-replies;group clients_a {relay-option-82 circuit-id;interface fe-1/0/1.1;interface fe-1/0/1.2;interface fe-1/0/1.3;}group clients_b {relay-option-82 {circuit-id {prefix routing-instance-name;}}interface fe-1/0/1.4;interface fe-1/0/1.5;interface fe-1/0/1.6;}group eth_dslam_relay {active-server-group sp-2;overrides {trust-option-82;layer2-unicast-replies;}interface fe-1/0/1.7;interface fe-1/0/1.8;interface fe-1/0/1.9;}}

This example creates two server-groups: sp-1, which includes DHCP server addresses and, and sp-2, which includes DHCP server addresses,, and The active server group to which the DHCP relay agent configuration applies is sp-1. A global override is set that causes the DHCP relay agent to use Layer 2 unicast transmission to send DHCP reply packets from the DHCP server to DHCP clients during the discovery process.

The example also creates three groups of subscribers and their associated Fast Ethernet interfaces: clients_a, clients_b, and eth_dslam_relay. These groups are configured to meet different needs, as follows:

  • The clients_a and clients_b groups consist of basic subscribers. The service provider for these groups inserts option 82 information in the DHCP packets that are destined for the DHCP server.
  • The subscribers in eth_dslam_relay are connected to an Ethernet digital subscriber line access multiplexer (DSLAM) that functions as a Layer 2 DHCP relay agent. The active server group for eth_dslam_relay is sp-2. Overrides are set for the eth_dslam_relay group that enable the DHCP relay agent to trust option 82 information and to use Layer 2 unicast transmission to send DHCP reply packets to DHCP clients during discovery.

Published: 2010-04-15

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