Enterasys Networks X-PeditionTM Dhcp Services, Temporary or Permanent Network Address Allocation

DHCP Services

client used a client ID when it got the lease, it will use the same identifier in the message. Alternately, when a lease is near expiration, the client tries to renew it. If unsuccessful in renewing by a certain period, the client enters a rebinding state and sends a DISCOVER message to restart the process.

DHCP also sets various options/extensions to clients which are outlined in “Assigned Network Configuration Values to Clients: Options” on page 15-3.

DHCP Services

The DHCP services comprising the Bindings Database, leases, network options, and Client Class configuration are described below.

Persistent Storage of Network Parameters for Clients

The first DHCP service is persistent storage of network parameters for network clients, also known as the bindings database. The XSR directs the Server to store a [key:value] entry for each client, where the key is some unique identifier and the value contains configuration parameters for the client.

For example, the key might be the IP-subnet-number/hardware-addresspair. Alternately, the key might be the IP-subnet-number/hostnamepair, allowing the server to assign parameters intelligently to a DHCP client that has been moved to a different subnet or has changed hardware addresses. DHCP defines the key to be IP-subnet-number/hardware-addressunless the client explicitly supplies an identifier using the client identifier option. The XSR stores host IP and client-hardware addresses, intervals, client-identifiers, and client-names in the leases.cfg file.

Temporary or Permanent Network Address Allocation

The second DHCP service is temporary or permanent network (IP) address allocation to clients. Network addresses are dynamically allocated simply by a client requesting an address for an interval with the server guaranteeing not to reallocate that address within the requested time and attempting to return the same network address each time the client requests an address.

Lease

The period over which a network address is allocated to a client is called a lease. A client may extend its lease with follow-up requests and may issue a message to release the address back to the server when the address is no longer needed. Also, a client may request a permanent assignment by asking for an infinite lease. Even if it assigns permanent addresses, a server may distribute lengthy but non-infinite leases to allow detection of a retired client.

In some environments network addresses must be reassigned due to exhaustion of available addresses in which case the allocator reuses addresses whose leases have expired. The server will use any available data in the configuration data repository to choose an address for reuse.

For example, the server may choose the least recently assigned address. As a consistency check, the allocating server will also probe the reused address before allocating the address - e.g., with an ICMP echo request - and the client will also probe the newly received address - e.g., with ARP.

Assigned Network Configuration Values to Clients: Options

With the exception of IP address assignment to clients, the DHCP Server provides a framework for passing configuration data to hosts on a TCP/IP network. Configuration values and other

XSR User’s Guide 15-3