B-9
User Guide for Cisco Network Registrar, 7.1
OL-16002-01
Appendices, Glossary, and Index
Appendix B DHCP Options
Option Descriptions
Server Identifier
54
4 octets
Used in DHCPOFFER and DHCPREQUEST messages, and can
optionally be included in the DHCPACK and DHCPNAK
messages. DHCP servers include this option in the DHCPOFFER
in order to allow the client to distinguish between lease offers.
DHCP clients use the contents of the server identifier field as the
destination address for any DHCP messages unicast to the DHCP
server. DHCP clients also indicate which of several lease offers is
being accepted by including this option in a DHCPREQUEST
message. The identifier is the IP address of the selected server.
Parameter
Request List
55
1 octet
minimum
Used by a DHCP client to request values for specified
configuration parameters. The list of requested parameters is
specified as n octets, where each octet is a valid DHCP option code
as defined in this document. The client can list the options in order
of preference. The DHCP server does not have to return the options
in the requested order, but must try to insert the options in the order
that the client requested.
Message
56
1 octet
minimum
Used by a DHCP server to provide an error message to a DHCP
client in a DHCPNAK message in the event of a failure. A client
can use this option in a DHCPDECLINE message to indicate why
the client declined the offered parameters. The message consists of
n
octets of NVT ASCII text, which the client can display on an
available output device.
Maximum
DHCP Message
Size
57
2 octets
Maximum-length DHCP message that a server is willing to accept.
The length is specified as an unsigned 16-bit integer. A client can
use the maximum DHCP message size option in
DHCPDISCOVER or DHCPREQUEST messages, but should not
use the option in DHCPDECLINE messages. Value: 576 minimum
Renewal (T1)
Time Value
58
4 octets
Time interval from address assignment until the client transitions
to RENEWING state.
Value: seconds, as 32-bit unsigned integer
Rebinding (T2)
Time Value
59
4 octets
Time interval from address assignment until the client transitions
to REBINDING state.
Value: seconds, as 32-bit unsigned integer
Vendor Class
Identifier
60
1 octet
minimum
Used by DHCP clients to optionally identify the vendor type and
configuration of a DHCP client. The information is a string of
n
octets, interpreted by servers. Vendors can choose to define
specific vendor class identifiers to convey particular configuration
or other identification information about a client. For example, the
identifier can encode the client hardware configuration. Servers
not equipped to interpret the class-specific information sent by a
client must ignore it (although it can be reported). Servers that
respond should only use option 43 to return the vendor-specific
information to the client.
Table B-7
DHCPv4 Extensions (continued)
Option Name
No. Length
Description
