Enterasys Networks X-PeditionTM manual How Dhcp Works, Dhcp Server Standards

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.

Note: If either DHCP/BOOTP Relay (using the ip helper-addresscommand) 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.

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:

–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.

4.The selected server sends a message (ACK) to a client with configuration parameters - a binding - including a committed network address, client-identifieror hardware-addressand 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 XSR User’s Guide