Alcatel-Lucent 6850-48 Guida Di Rete

Using Automatic IP Configuration

Configuring DHCP Relay

page 31-12

OmniSwitch AOS Release 6 Network Configuration Guide

An additional function of the DHCP Relay feature enables a switch to broadcast a BootP or DHCP request 
packet at boot time to obtain an IP address for default VLAN 1. This function is separate from the previ-
ously described functions (such as Global DHCP, per-VLAN DHCP, and related configurable options) in 
that enabling or disabling automatic IP configuration does not exclude or prevent other DHCP Relay func-
tionality. 

Note. Automatic IP address configuration only supports the assignment of a permanent IP address to the 
switch. Make sure that the DHCP server is configured with such an address before using this feature.

Using automatic IP configuration also allows the switch to specify the type of request packet to send; 
BootP (the default) or DHCP. When the BootP/DHCP server receives the request packet from the switch, 
it processes the request and sends an appropriate reply packet. When the switch receives a reply packet 
from the BootP/DHCP server, one or more of the following occurs:

• The router port for VLAN 1 is assigned the IP address provided by the server. 

• If the reply packet contains a subnet mask for the IP address, the mask is applied to the VLAN 1 router 

port address. Otherwise, a default mask is determined based upon the class of the IP address. For exam-
ple, if the IP address is a Class A, B, or C address, then 255.0.0.0, 255.255.0.0, or 255.255.255.0 is 
used for the subnet mask.

• If the reply packet from the server contains a gateway IP address, then a static route entry of 0.0.0.0 is 

created on the switch with the gateway address provided by the server.

Note. If the VLAN 1 router port is already configured with an IP address, the switch does not broadcast a 
request packet at boot time even if automatic IP configuration is enabled.

To verify IP router port configuration for VLAN 1, use the 

commands. For more information about these commands, refer to the OmniSwitch CLI Reference Guide.

By default, this function is disabled on the switch. To enable automatic IP configuration and specify the 
type of request packet, use the 

 command. For example:

-> ip helper boot-up enable DHCP

-> ip helper boot-up enable BOOTP

Once enabled, the next time the switch boots up, DHCP Relay will broadcast a BootP (the default) or 
DHCP request packet to obtain an IP address for default VLAN 1. 

To disable automatic IP configuration for the switch, use the ip helper boot-up command with the disable 
option, as shown below:

-> ip helper boot-up disable