Cisco uBR7200 Series Universal Broadband Routers

Cisco Network Registrar for the uBR7200 Series

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Table of Contents

Cisco Network Registrar for the CiscouBR7200 Series Universal Broadband Routers


Servers Required on the HFC Network

How CiscouBR7200 Series Routers and Cable Modems Work

DHCP Fields and Options for Cable Modems

Installing Cisco Network Registrar and Server Software

Cisco Network Registrar Sample Configuration

For More Information

Obtaining Documentation and Submitting a Service Request

Cisco Network Registrar for the Cisco uBR7200 Series Universal Broadband Routers

Revised: March 3rd, 2009, 78-6125-01

This document describes the Cisco Network Registrar (CNR) software product and how to install CNR and use it to provision a hybrid fiber coaxial (HFC) network that uses Cisco uBR7200 series universal broadband routers at the headend. This document contains the following sections:


Cisco Network Registrar (CNR) is a dynamic IP address management system that uses the Dynamic Host Configuration Protocol (DHCP) and assigns IP addresses to PCs and other devices on a network based on a predefined set of policies, such as class of service. CNR assigns available IP addresses from address pools based on the identity or type of the requesting device and the policies in effect. For example, CNR can distinguish between registered devices, unregistered devices, and registered devices that have been assigned to a particular class of service.

CNR also provides extensions that can be customized (via programming or a script) so that you can view individual DHCP options, determine the identity or type of a device based on the content of the options, and assign a device to a predefined class or group. Using these extensions, you can determine the difference between PCs and cable modems and assign them IP addresses from different address pools.

In typical data-over-cable environments, service providers are interested in simplifying provisioning to limit the amount of information that must be collected about subscribers’ customer premise equipment (CPEs). To support current provisioning models, a field technician must be sent to a subscriber’s home or business to install and setup a cable modem. During this site visit, the technician might register the

serial number and MAC address of the cable modem in the customer account database. Because a field technician must go to a subscriber’s site to replace a cable modem, you can easily track modem information.

Manually registering and tracking information about a cable subscriber’s PC is more difficult. A subscriber might purchase a new PC or exchange the network interface card (NIC) without notifying you of the change. Automatic provisioning with CNR reduces the amount of customer service involvement needed to track customer equipment. To use the provisioning model described in this document, you must still track serial numbers and MAC addresses for cable modems, but you do not need to track information about the PC or NIC cards installed at a subscriber site.

The remainder of this document describes how to configure CNR to support this model. The following sections describe the equipment and servers required for the cable headend, provide an overview of the interaction between DOCSIS-compatible cable modems and the Cisco uBR7200 series universal broadband routers, and provide a guide on how to configure CNR to support this provisioning model.

The Cisco uBR7200 series consists of the 6-slot Cisco uBR7246 (4 cable modem cards and 2 port adapters) and the 3-slot Cisco uBR7223 (2 cable modem cards and 1 port adapter). The Cisco uBR7200 series routers are known as cable modem termination systems (CMTSs).

Servers Required on the HFC Network

A TFTP (Trivial File Trasnfer Protocol) server, DHCP server, and time-of-day (TOD) server are required to support two-way data cable modems on an HFC network. A cable modem will not boot if these servers are not available. The log server and security servers are not required to configure and operate a cable modem. If the log server or security servers are not present, a cable modem will generate warning messages, but it will continue to boot and function properly.

Figure 1 Servers Required on a Two-Way HFC Network


In this provisioning model, TOD and TFTP servers are standard Internet implementations of the RFC 868 and RFC 1350 specifications. Most computers running a UNIX-based operating system supply TOD and TFTP servers as a standard software feature. Typically, the TOD server is embedded in the UNIX inetd and it requires no additional configuration. The TFTP server is usually disabled in the standard software but can be enabled by the user. Microsoft NT server software includes a TFTP server that can be enabled with the services control panel. Microsoft NT does not include a TOD server. A public domain version of the TOD server for Microsoft NT can be downloaded from several sites. For more information, see the “Installing Cisco Network Registrar and Server Software” section.

The DHCP and Domain Name System (DNS) server shown in Figure 1 must be the DHCP/DNS server available in Cisco Network Registrar version 2.0 or later. CNR is the only DHCP server that implements policy-based assignment of IP addresses. The headend must be a Cisco uBR7200 series universal broadband router. The remote access server is only required on HFC networks that are limited to one-way (downstream only) communication. In a one-way HFC network, upstream data from a PC through the headend to the Internet is carried over a dialup connection. This dialup connection for upstream data is referred to as telco return. For simplification, the model will not include a log or security server. Cable modems can be set up to use the logging and security servers by including the appropriate DHCP options in the cable modem policy as described in the Cisco Network Registrar User Manual . (See the “For More Information” section.)

How Cisco uBR7200 Series Routers and Cable Modems Work

Cisco uBR7200 series routers and Cisco uBR904 cable modems are based on the Data Over Cable Service Interface Specification (DOCSIS) standards. These standards were created by a consortium of cable service providers called Multimedia Cable Network Systems, Ltd. (MCNS) so that cable headend and cable modem equipment produced by different vendors will interoperate. The key DOCSIS standards provide the basis for a cable modem to communicate with any headend equipment and headend equipment to communicate with any cable modem.

Cable modems are assigned to operate on specific cable channels so activity can be balanced across several channels. Each Cisco uBR7200 series router installed at the headend serves a specific channel. Part of network planning is to decide which channel each cable modem can use.

A cable modem cannot connect to the network until the following events occur:

  • The cable modem initializes and ranges through available frequencies until it finds the first frequency that it can use to communicate to the headend. The cable modem might be another vendor’s DOCSIS-compatible device and the headend might have a Cisco uBR7200 series router installed. At this point on the initial connection, the cable modem cannot determine if it is communicating on the correct channel.
  • The cable modem goes through the DHCP server process and receives a configuration file from the server.
  • One of the parameters in the configuration file tells the cable modem which channel it can use.
  • If the assigned channel is not available on the Cisco uBR7200 series router to which the cable modem is currently connected, it resets itself and comes up on the assigned channel.
  • During this second DHCP process, the modem will be connected to the correct CMTS. This time, the configuration file will be loaded. For a DOCSIS-compatible cable modem to access the network, it might go through the DHCP server two times on two different networks; therefore, one-lease-per-client IP addressing is critical.

DHCP Fields and Options for Cable Modems

DHCP options and packet fields are required to enable cable modems to boot and operate properly. Table 1 lists the required DHCP options and fields.


Table 1 Required DHCP Fields and Options

Required Field/Option
Field/Option In Cisco Network Registrar


IP address. As a DHCP packet passes through the relay agent to the DHCP server, the relay agent supplies a unique IP address to the packet and stores it in this field. The relay agent is a uBR7200 series router with the iphelper attribute defined.


Subnet mask for the IP address stored in the giaddr field. This value is also stored in the DHCP packet by the relay agent.



Name of the cable modem configuration file that will be read from a TFTP server.



IP address of the TFTP server where configuration files are stored.



List of hosts running the time server specified in the RFC 868 standard.


Time offset of a cable modem internal clock from Universal Time Coordinated (UTC). This value is used by cable modems to calculate the local time that is stored in time-stamping error logs.


IP address of the security server. This should be set if security is required. See RFC 1533 for details.

Installing Cisco Network Registrar and Server Software

If you order a Cisco uBR7200 series router with Cisco Network Registrar, you will receive a CD-ROM with the Cisco Network Registrar software. Install the software from this CD-ROM. If you order Cisco Network Registrar as a spare, you must download the software from

Cisco Network Registrar software for the Cisco uBR7200 series routers is available on at:

Cisco does not sell TOD server software. You can download or purchase a TOD server software of your choice.

Cisco Network Registrar Sample Configuration

You can use the following information to set up Cisco Network Registrar in a trial configuration. The configuration describes DHCP-related setup only; it does not cover setting up DNS or configuring dynamic DNS (DDNS). You should be familiar with important CNR concepts including scopes, primary and secondary scopes, scope selection tags, client classes, and CNR policies. See the Using Network Registrar publication for detailed information on these concepts.

In the trial configuration, you can configure CNR to perform the following operations:

  • Receive DHCP requests from a cable modem and a PC on an HFC network via a port supporting multiple network numbers. The Cisco uBR7200 series router at the headend must be configured as a forwarder (iphelper is configured).
  • Serve IP addresses on two networks; a net-10 network (non-Internet routable) and a net-24 network (Internet routable).
  • Tell the difference between a cable modem and a PC based on the MAC address of the device and provide net-24 addresses to the PC and net-10 addresses to the cable modem.
  • Refuse to serve IP addresses to MAC addresses that it does not recognize.

To perform these options, you must implement the following CNR configuration items:

  • Create two scope selection tags; one for PCs, one for cable modems.
  • Create two client-classes; one for PCs , one for cable modems.
  • Create a lease policy appropriate for the cable modem devices.
  • Create a lease policy appropriate for the PC devices.
  • Create a scope containing Class A net-24 (routable) addresses.
  • Create a scope containing Class A net-10 (nonroutable) addresses.
  • Identify the scope containing the net-24 addresses as the primary scope and configure the other scope containing the net-10 addresses as secondary to the net-24 scope.

Note The Cisco uBR7200 series router upstream ports must be configured with the primary network address on the net-24 network; such as

  • Assign the policies to the appropriate scope.
  • Add the MAC address of the cable modem and the PC to the client-entry list.
  • Associate the PC tag with the scope containing routable addresses.
  • Associate the cable modem tag with the scope containing nonroutable addresses.
  • Associate the cable modem tag with the cable modem client-class.
  • Associate the PC tag with the PC client-class.
  • Assign the PC MAC to the PC class.
  • Assign the cable modem MAC to the cable modem class.
  • Enable client-class processing.

Figure 2 shows the trial CNR configuration in an HFC network.

Figure 2 Trial Configuration in an HFC Network


These configuration items and their associations can be created using either the CNR management graphical user interface (GUI) or command-line interface (CLI). The following sample script configures DHCP for a sample server:

File: cabledemo.rc
Command line: nrcmd -C <cluster> -N <user name> -P <password> -b < cabledemo.rc
scope-selection-tag tag-CM create
scope-selection-tag tag-PC create
client-class create class-CM
client-class class-CM set selection-criteria=tag-CM
client-class create class-PC
client-class class-PC set selection-criteria=tag-PC
policy cmts-cisco create
policy cmts-cisco setleasetime 1800
policy cmts-cisco setoption domain-name-servers
policy cmts-cisco setoption routers
policy cmts-cisco setoption time-offset 604800
policy cmts-cisco setoption time-servers
policy cmts-cisco set packet-siaddr=
policy cmts-cisco setoption log-servers
policy cmts-cisco setoption mcns-security-server
policy cmts-cisco set packet-file-name=golden.cfg
policy cmts-cisco set dhcp-reply-options=packet-file-name,packet-siaddr,mcns-security-server
policy pPC create
policy pPC set server-lease-time 1800
policy pPC setleasetime 1800
policy pPC setoption domain-name-servers
policy pPC setoption routers
scope S24.1.1.0 create
scope S24.1.1.0 addrange
scope S24.1.1.0 set policy=pPC
scope S24.1.1.0 set selection-tags=tag-PC
scope S10.1.1.0 create
scope S10.1.1.0 addrange
scope S10.1.1.0 set policy=cmts-cisco
scope S10.1.1.0 set selection-tags=tag-CM
scope S10.1.1.0 set primary-scope=S24.1.1.0
client 01:02:03:04:05:06 create client-class-name=class-PC
client ab:cd:ef:01:02:03 create client-class-name=class-CM
client default create action=exclude
dhcp enable client-class
dhcp enable one-lease-per-client
dhcp reload

In addition to the DHCP server setup, you might want to enable packet-tracing. When packet-tracing is enabled, the server parses both requests and replies, and then adds them to the logs. If you do enable tracing, performance will be adversely affected, and the logs will roll over quickly.

Use the following nrcmd command to set packet tracing.

DHCP set log-settings=incoming-packet-detail,outgoing-packet-detail

For More Information

For more information on Cisco Network Registrar, MCNS specifications, and DHCP specifications, see the following web sites and documents:

sp-rfi-i04-980724.pdf— RF Interface Specification

sp-cmtri-i01-970804.pdf— Telco Return Interface Document

sp-cmci-i02-980317.pdf— Cable Modem to Customer Premise Equipment Interface

RFC 2131— Dynamic Host Configuration Protocol

RFC 2132— DHCP Options and BOOTP Vendor Extensions

Obtaining Documentation and Submitting a Service Request

For information on obtaining documentation, using the Cisco Bug Search Tool (BST), submitting a service request, and gathering additional information, see What’s New in Cisco Product Documentation at: .

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