804 Appendix E:DHCP server

NN47250-500 (Version 03.01)

How the WSS software DHCP server works

When WSS Software receives a DHCP Discover packet, the DHCP server allocates an address from the configured
range according to RFC 2131 and ARPs the address to ensure that it is not already in use. If the address is in use, the
server allocates the next address in the range, and ARPs again. The process continues until WSS Software finds an
address that is not in use. WSS Software then offers the address to the Distributed AP or client that sent the DHCP
Discover. If there are no unused addresses left in the range, WSS Software ignores the DHCP Discover and generates a
log message.
If the client does not respond to the DHCP Offer from the WSS Software DHCP server within 2 minutes, the offer
becomes invalid and WSS Software returns the address to the pool.
The siaddr value in the DHCP exchanges is the IP address of the VLAN. The yiaddr value is an unused address within
the range the server is allowed to use.
In addition to an IP address, the Offer message from the WSS Software DHCP server also contains the following
options:
• Option 54—Server Identifier, which has the same value as siaddr.
• Option 51—Address Lease, which is 12 hours and cannot be configured.
• Option 1—Subnet Mask of the VLAN’s IP interface.
• Option 15—Domain Name. If this option is not set with the set interface dhcp-server command’s dns-domain
option, the WSS Software DHCP server uses the value set by the set ip dns domain command.
• Option 3—Default Router. If this option is not set with the set interface dhcp-server command’s default-router
option, the WSS Software DHCP server can use the value set by the set ip route command. A default route
configured by set ip route can be used if the route is in the DHCP client’s subnet. Otherwise, the WSS Software
DHCP server does not specify a router address.
• Option 6—Domain Name Servers. If these options are not set with the set interface dhcp-server command’s
primary-dns and secondary-dns options, the WSS Software DHCP server uses the values set by the set ip dns
server command.

Configuring the DHCP server

You can configure the DHCP server on an individual VLAN basis. To configure the server, use the following command:

set interface vlan-id ip dhcp-server [ena ble | disable] [start ip-addr1 stop ip-addr2]

[dns-domain domain-name] [primary-dns ip-addr [secondary-dns ip-addr]]

[default-router ip-addr]

The vlan-id can be the VLAN name or number.
The start ip-addr1 and stop ip-addr2 options specify the beginning and ending addresses of the address range (also
called the address pool). By default, all addresses except the host address of the VLAN, the network broadcast address,
and the subnet broadcast address are included in the range. If you specify the range, the start address must be lower than
the stop address, and all addresses must be in the same subnet. The IP interface of the VLAN must be within the same
subnet but is not required to be within the range.
(For information about the other options, see the Nortel WLAN Security Switch 2300 Series Command Line Reference.)