C H A P T E R 7 Overview Of The Cisco Network Registrar For The Cisco Ubr7200 Series; Cable Modem Dhcp Response Fields; Docsis Dhcp Fields; Dhcp Relay Option (Docsis Option 82) - Cisco 7246VXR - uBR Router Software Configuration Manual

Cisco Network Registrar Description
•
•
•
•
•

Cable Modem DHCP Response Fields

Each cable interface on the broadband network requires the following fields in the DHCP response:
•
•
•
•
•

DOCSIS DHCP Fields

DOCSIS DHCP option requirements include:
•
•
•

DHCP Relay Option (DOCSIS Option 82)

DOCSIS Option82 modifies DHCPDISCOVER packets to distinguish cable interfaces from the CPE
devices or "clients" behind them. The DOCSIS Option82 is comprised of the following two suboptions:
•
Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide
7-2
post-client-lookup—Examines and takes action on results of the client-class process, places data items in
the environment dictionary to use at the pre-packet-encode extension point, includes DHCP relay option
check-lease-acceptable
pre-packet-encode
post-sent-packet
pre-dns-add-forward
CM's IP address
CM's subnet mask
For cable operators with less experience in networking, you can fill in a guess based on the
Note
network number and indicate how your IP network is divided.
Name of the DOCSIS configuration file on the TFTP server intended for the cable interface
Time offset of the cable interface from the Universal Coordinated Time (UTC), which the
cable interface uses to calculate the local time when time-stamping error logs
Time server address from which the cable interface obtains the current time
IP address of the next server to use in the TFTP bootstrap process; this is returned in the siaddr field
DOCSIS configuration file that the cable interface downloads from the TFTP server
If the DHCP server is on a different network that uses a relay agent, then the relay agent must
Note
set the gateway address field of the DHCP response.
IP address of the security server should be set if security is required
Suboption 1, Circuit ID:
Type 1 (1 byte)
Len 4 (1 byte)
Value (8 bytes)
<bit 31,30,....................0)
<xYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY>
where the MSB indicates if the attached device is a cable interface.
x=1 Cable Modem REQ
x=0 CPE device
(Behind the cable interface with the cable interface MAC address shown in suboption 2.)
The rest of the bits make up the SNMP index to the CMTS interface.
is the SNMP index to the CMTS interface.
Y=0xYYYYYYY
Chapter 7 Overview of the Cisco Network Registrar for the Cisco uBR7200 Series
OL-2239-03