Alteon OS Application Guide

DHCP Relay Agent

DHCP is described in RFC 2131, and the DHCP relay agent supported on the GbESM is described in RFC 1542. DHCP uses UDP as its transport protocol. The client sends messages to the server on port 67 and the server sends messages to the client on port 68.

DHCP defines the methods through which clients can be assigned an IP address for a finite lease period and allowing reassignment of the IP address to another client later. Additionally, DHCP provides the mechanism for a client to gather other IP configuration parameters it needs to operate in the TCP/IP network.

In the DHCP environment, the switch acts as a relay agent. The DHCP relay feature

(/cfg/l3/bootp) enables the switch to forward a client request for an IP address to two BOOTP servers with IP addresses that have been configured on the switch.

When a switch receives a UDP broadcast on port 67 from a DHCP client requesting an IP address, the switch acts as a proxy for the client, replacing the client source IP (SIP) and desti- nation IP (DIP) addresses. The request is then forwarded as a UDP Unicast MAC layer mes- sage to two BOOTP servers whose IP addresses are configured on the switch. The servers respond as a a UDP Unicast message back to the switch, with the default gateway and IP address for the client. The destination IP address in the server response represents the interface address on the switch that received the client request. This interface address tells the switch on which VLAN to send the server response to the client.

DHCP Relay Agent Configuration

To enable the GbESM to be the BOOTP forwarder, you need to configure the DHCP/BOOTP server IP addresses on the switch. Generally, you should configure the command on the switch IP interface closest to the client so that the DHCP server knows from which IP subnet the newly allocated IP address should come.

The following figure shows a basic DHCP network example:

GbESM

DHCP

Server

BladeCenterl

Figure 1-3DHCP Relay Agent Configuration

42C4911, January 2007

Chapter 1: Accessing the Switch „ 31

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Nortel Networks 42C4911 manual Dhcp Relay Agent Configuration
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42C4911 specifications

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