5/26/05 External DHCP Servers
OL-7426-03
About External DHCP ServersExternal DHCP Servers
The Operating System is designed to appear as a DHCP Relay to the network and as a DHCP Server to
clients with industry-standard external DHCP Servers that support DHCP Relay. This means that each
Cisco Wireless LAN Controller appears as a DHCP Relay agent to the DHCP Server. This also means that
the Cisco Wireless LAN Controller appears as a DHCP Server at the virtual IP Address to wireless clients.
Because the Cisco Wireless LAN Controller captures the client IP Address obtained from a DHCP Server,
it maintains the same IP Address for that client during same-Cisco Wireless LAN Controller, inter-Cisco
Wireless LAN Controller, and inter-subnet Client Roaming.
Per-WLAN AssignmentPer-WLAN Assignment
All Cisco WLAN Solution WLANs can be configured to use the same or different DHCP Servers, or no
DHCP Server. This allows operators considerable flexibility in configuring their Wireless LANs, as further
described in the Cisco WLAN Solution WLANs section.
Note that Cisco WLAN Solution WLANs that support Management over Wireless must allow the manage-
ment (device servicing) clients to obtain an IP Address from a DHCP Server.
Per-Interface AssignmentPer-Interface Assignment
•The Layer 2 Management Interface can be configured for a primary and secondary DHCP
server.
•The Layer 3 AP-Manager Interface can be configured for a primary and secondary DHCP server.
•Each of the Operator-Defined Interfaces can be configured for a primary and secondary DHCP
server.
•The Virtual Interface does not use DHCP servers.
•The Service-Port Interface can be configured to enable or disable DHCP servers.
Security ConsiderationsSecurity Considerations
For enhanced security, it is recommended that operators require all clients to obtain their IP Addresses
from a DHCP server. To enforce this requirement, all Cisco WLAN Solution WLANs can be configured
with a ‘DHCP Required’ setting and a valid DHCP Server IP Address, which disallows client static IP
Addresses. If a client associating with a WLAN with ‘DHCP Required’ set does not obtain its IP Address
from the designated DHCP Server, it is not allowed access to any network services.
Note that if ‘DHCP Required’ is selected, clients must obtain an IP address via DHCP. Any client with a
static IP address will not be allowed on the network. The Cisco Wireless LAN Controller monitors DHCP
traffic since it acts as a DHCP proxy for the clients.
If slightly less security is tolerable, operators can create Cisco WLAN Solution WLANs with ‘DHCP
Required’ disabled and a valid DHCP Server IP Address. Clients then have the option of using a static IP
Address or obtaining an IP Address from the designated DHCP Server.
Operators are also allowed to create separate Cisco WLAN Solution WLANs with ‘DHCP Required’
disabled and a DHCP Server IP Address of 0.0.0.0. These WLANs drop all DHCP requests and force
clients to use a static IP Address. Note that these WLANs do not support Management over Wireless.