- Enterasys Security Router User's Guide

DHCP Services
15-4 Configuring DHCP XSR User’s Guide
control data are carried in tagged data items which are stored in the options field of the DHCP
message. The data items themselves, also called options, are enabled on the XSR by the
options
command specifying IP address, hex or ASCII string values. Supported options are defined in the
“Dynamic Host Configuration Protocol Commands” chapter of the XSR CLI Reference Guide.
RFC-1122 specifies default values for most IP/TCP configuration parameters.
Provisioning Differentiated Network Values by Client Class
One DHCP option - supported on the XSR by the client-class command - groups clients into
classes with differentiated configuration. A DHCP Server selects appropriate parameters for the
clients belonging to this class. For example, a Client Class can configure all enterprise users in
Accounting with a different lease time than users in Marketing. RFC-3004 defines the User Class
(Client Class) option for DHCP.
BOOTP Legacy Support
The XSR provides backward compatibility with BOOTP clients. When configured with a manual
binding, it supplies a specified IP address to the client as well as a TFTP server IP address and file
name to download (with the
next-server command).
Refer to “BOOTP Client Support Example” on page 15-12 for more information.
Nested Scopes: IP Pool Subsets
As mentioned earlier, one of the main functions of the DHCP Server is to allocate IP addresses to
clients. In that process, the DHCP Server works with three scopes or resource sets responsible for
aggregated DHCP attributes - Pools or subnets, Client Classes, and Hosts. Scopes can be assigned
other attributes as well as IP addresses, and can nest these attributes hierarchically much like files
are organized in a directory tree. How these scopes interrelate can be loosely illustrated as shown
in Figure 15-1.
Figure 15-1 DHCP Nested Scopes
The Pool scope in Figure 15-1 defines and manipulates IP addresses and parameters. The Client
Class scope manages sets of clients requesting DHCP Server services. The Host scope controls
DHCP user parameters.
Pool (subnet)
Client Class
Host
Values are inherited
from outer scopes
A nested scope may
override an outer
scope attribute
Attributes persist
at the Host level
Values are inherited
from outer scopes
192.168.57.0
lcurtis-xp
Elite
Pool (subnet)
192.168.57.0