Dhcp Server Standards; How Dhcp Works - Enterasys Security Router X-PeditionTM User Manual

How DHCP Works

• Provisioning of differentiated network values by Client Class.
• Persistent and user-controllable conflict avoidance to prevent duplicate IP address including
configurable ping checking.
• Visibility of DHCP network activity and leases through operator reports statistics and logs.
• Nested scopes.

DHCP Server Standards

The XSR supports the following:
• DHCP Server as defined in RFC-2131, BOOTP Server and BOOTP Relay as defined in RFC-
951/RFC-1542, and BOOTP Client as defined in RFC-1534: Interoperation Between DHCP and
BOOTP (static BOOTP only)
• DHCP Server also supports RFC-2132: DHCP Options and BOOTP Vendor Extensions and
RFC-3004: User Class Option for DHCP.
• DHCP Server and DHCP/BOOTP Relay services run on FastEthernet ports only.
How DHCP Works
The DHCP client-server model defines a set of messages exchanged between two systems. A
simplified description of client-server communications follows:
1. A client issues a broadcast message (DISCOVER) to locate available DHCP Servers on its local
subnet. This message may include suggested values for the network address and duration of a
lease. Also, BOOTP relay agents may pass the message on to DHCP Servers not on the same
physical subnet.
2. A response (OFFER) is sent from a DHCP Server to the client with an offer of configuration
parameters including an available network IP address and settable options. Before the server
actually allocates the new address, it will check that the address is free by pinging it.
3. A client sends a message (REQUEST) to servers for one of the following purposes:
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4. The selected server sends a message (ACK) to a client with configuration parameters - a
binding - including a committed network address, client-identifier or hardware-address and
commits its lease to the binding database. Or, the server sends a message (NACK) indicating
the client's idea of a network address is incorrect or the client's lease has expired.
5. The client performs a final check (ARPs the allocated network address) on the parameters and
at this point is configured.
6. The client may relinquish its lease on a network address by sending a message (RELEASE) to
the server identifying the lease with its client identifier (hardware/network addresses). If the
15-2 Configuring DHCP
Note: If either DHCP/BOOTP Relay (using the ip helper-address command) or DHCP Server
is enabled on one FastEthernet port, you cannot also configure the other service on the second
FastEthernet port. The XSR permits either one or the other service to operate, not both.
Requesting offered parameters from one server and implicitly declining offers from all
others,
Confirming the correctness of a previously allocated address after, for example, a system
reboot,
Extending the lease on a particular network address.
XSR User's Guide