Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring DHCP Features (Rev. 03)
DHCP Server
© 2008 Foundry Networks, Inc.
Page 5 of 63
DHCP Server
Overview
Configuration Parameters Repository
The first service provided by DHCP is a persistent storage of network parameters for network
clients. The model of DHCP persistent storage is that the DHCP service stores a key-value entry
for each client, where the key is some unique identifier and the value contains the configuration
parameters for the client.
Dynamic Allocation of Network Addresses
The second service provided by DHCP is the allocation of temporary or permanent network (IP)
addresses to clients. The basic mechanism for the dynamic allocation of network addresses is
simple: a client requests the use of an address for a time interval. The allocation mechanism (the
collection of DHCP servers) guarantees not to reallocate that address within the requested time and
attempts to return the same network address each time the client requests an address. The period
over which a network address is allocated to a client is referred to as a
lease
. The client may extend
its lease with subsequent requests. The client may issue a message to release the address back to
the server when the client no longer needs the address. The client may ask for a permanent
assignment by asking for an infinite lease. Even when requesting
permanent
addresses, a server
may choose to give out lengthy but non-infinite leases to allow detection of the fact that the client
has been retired.
In some environments, it will be necessary to reassign network addresses due to exhaustion of
available addresses. In such environments, the allocation mechanism will reuse addresses whose
lease has expired. The server should use whatever information is available in the configuration
information repository to choose an address to reuse. For example, the server may choose the least
recently assigned address. As a consistency check, the allocating server should probe the reused
address before allocating the address, e.g., with an ICMP echo request, and the client should probe
the newly received address, e.g., with ARP.
The DHCP Database Agent
A DHCP database agent is an FTP or TFTP server that stores the DHCP binding database.
How the DHCP Database Agent Process Works
The DHCP database agent is periodically synchronized with the DHCP switch at pre-defined
periods. It is highly recommended to reduce the database synchronization period before rebooting
the device. The DHCP database agent process is initiated when the device is reset and the DHCP
database is erased from the DHCP server device. This process is automatic, but to use the DHCP-
based functionality (such as enabling unicast TFTP requests) the DHCP server must be properly
preconfigured.
Automatic Loading of the DHCP Database
After the switch is up, the DHCP database agent process will initiate attempts to download the
DHCP database configuration file.
The DHCP device automatically attempts to download the DHCP database file using one of the
TFTP or FTP methods.