Black Box LR11xx Series Router Configurations Guide

Figure 2 BOOTP Requests

Broadcast BOOTREQUEST

 

Unicast BOOTREQUEST

DHCP Client

DHCP Relay Agent

DHCP Server

1.1.2.2 BOOTP Replies

BOOTP replies are messages from the server to the client. Reply messages include DHCP OFFER, DHCP ACK, DHCP NAK, etc. The relay agent looks up the MAC address and either sends the packet to the client or broadcasts it on the LAN.

Figure 3 BOOTP Replies

Unicast/Broadcast

BOOTREPLY

Unicast

BOOTREPLY

DHCP Client

DHCP Relay Agent

DHCP Server

1.1.3 Using DHCP Relay with NAT

When NAT is enabled, the DHCP server may discard packets because the giaddr does not match the source of the packet. Additionally, it may not know how to route the packet back to the client. See Figure 4. The solution is that the gateway address (giaddr) field needs to have IP address 192.168.20.1 (in this example). The DHCP server configuration should be able to give 10.1.1.x addresses for packets from 192.168.20.1. However, there may be a limitation that the DHCP server does not allow configuration using IP addresses from a different subnet, although this is mentioned in the RFC.

Figure 4 A Typical Scenario

DHCP Client

10.1.1.x Network

DHCP Client

DHCP Client

Network Address Translation

PRIVATE

PUBLIC

192.168.20.1

10.1.1.1

 

 

Router

DHCP Relay Agent

DHCP Server

1.1.4 Command Line Interface

The following are examples of command strings relevant to DHCP relay:

1.1.4.1 Enabling DHCP Relay

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Black Box LR1112A-T1/E1 manual Using Dhcp Relay with NAT, Command Line Interface, Bootp Replies, Enabling Dhcp Relay