Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring DHCP Features (Rev. 03)
About DHCP Features
© 2008 Foundry Networks, Inc.
Page 3 of 63
About DHCP Features
Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration
information to hosts on a TCP/IP network. DHCP, based on the Bootstrap Protocol (BOOTP), adds
the capability of automatic allocation of reusable network addresses and additional configuration
options. DHCP captures the behavior of BOOTP relay agents and DHCP participants can
interoperate with BOOTP participants.
DHCP provides configuration parameters to Internet hosts. DHCP consists of two components: (1)
a protocol for delivering host-specific configuration parameters from a DHCP server to a host and
(2) a mechanism for allocating network addresses to hosts.
DHCP is built on a client-server model, where designated DHCP server hosts allocate network
addresses and deliver configuration parameters to dynamically configured hosts. Throughout the
remainder of this document, the term
server
refers to a host providing initialization parameters
through DHCP, and the term
client
refers to a host requesting initialization parameters from a
DHCP server.
DHCP supports three mechanisms for IP address allocation:
•
Automatic allocation
- DHCP assigns a permanent IP address to a client.
•
Dynamic allocation -
DHCP assigns an IP address to a client for a limited period of time (or
until the client explicitly relinquishes the address). Dynamic allocation allows automatic reuse
of an address that is no longer needed by the client to which it was assigned. Thus, dynamic
allocation is particularly useful for assigning an address to a client that will be connected to the
network only temporarily or for sharing a limited pool of IP addresses among a group of
clients that do not need permanent IP addresses. Dynamic allocation may also be a good choice
for assigning an IP address to a new client being permanently connected to a network where IP
addresses are scarce and it is important to reclaim them when old clients are retired
•
Manual allocation -
a client’s IP address is assigned by the network administrator, and DHCP
is used simply to convey the assigned address to the client. A particular network will use one
or more of these mechanisms, depending on the policies of the network administrator. Manual
allocation allows DHCP to be used to eliminate the error-prone process of manually
configuring hosts with IP addresses in environments where (for whatever reasons) it is
desirable to manage IP address assignment outside of the DHCP mechanisms.
As shown in
Figure 1
, the parameter negotiation starts with a DHCPDISCOVER broadcast
message from the client seeking a DHCP server. The DHCP Server responds with a DHCPOFFER
unicast message offering configuration parameters (such as an IP address, a MAC address, a
domain name, and a lease for the IP address) to the client. The client returns a DHCPREQUEST
unicast message requesting the offered IP address from the DHCP Server. The DHCP Server
responds with a DHCPACK unicast message confirming that the IP address has been allocated to
the client.