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Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide
OL-2239-03
Chapter7 Overview of the Cisco Network Registrar for the Cisco uBR7200 Series
Cisco Network Registrar Description
• pre-client-lookup
• 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
Cable Modem DHCP Response Fields
Each cableinterface on the broadband network requires the following fields in the DHCP response:
• CM’s IP address
• CM’s subnet mask
Note For cable operators with less experience in networking, you can fill in a guess based on the
network number and indicate how your IP network is divided.
• Name of the DOCSIS configuration file on the TFTP server intended for the cableinterface
• Time offset of the cableinterface from the Universal Coordinated Time (UTC), which the
cableinterface uses to calculate the local time when time-stamping error logs
• Time server address from which the cableinterface obtains the current time
DOCSIS DHCP Fields
DOCSIS DHCP option requirements include:
• 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 cableinterface downloads from the TFTP server
Note If the DHCP server is on a different network that uses a relay agent, then the relay agent must
set the gateway address field of the DHCP response.
• IP address of the security server should be set if security is required
DHCP Relay Option (DOCSIS Option 82)
DOCSIS Option82 modifies DHCPDISCOVER packets to distinguish cableinterfaces from the CPE
devices or “clients” behind them. The DOCSIS Option82 is comprised of the following two suboptions:
• 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 cableinterface.
x=1 Cable Modem REQ
x=0 CPE device
(Behind the cableinterface with the cableinterface MAC address shown in suboption 2.)
The rest of the bits make up the SNMP index to the CMTS interface.
Y=0xYYYYYYY is the SNMP index to the CMTS interface.