19. Dhcp Client; Dhcp Overview - Nokia ESB26 User Manual

19. DHCP Client

DHCP Overview

DHCP (Dynamic Host Configuration Protocol) 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 is described in RFC 2131: Dynamic Host Configuration Protocol and in RFC 2132:
DHCP Options and BOOTP Vendor Extensions.
DHCP provides configuration parameters to Internet hosts. DHCP consists of two
components: a protocol for delivering host-specific configuration parameters from a DHCP
server to a host and 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 chapter, 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 assignments outside of the
DHCP mechanisms.
The Nokia ESB26 switch uses two bytes in the client-identifier field of the DHCP message to
identify the location of the chassis and the slot that contains the switch. The location is
specified by a twelve-bit number and the slot number is specified by a four-bit number. The
two remaining bytes in the client-identifier field have a fixed value of zero. You can view the
chassis location and slot ID by using the show positioning command described in Displaying
the Slot and Location.
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