Registrar for the Cisco CMTS Routers
supplements the Cisco Network Registrar (CNR) documentation by providing
additional cable-specific instructions to provision a hybrid fiber-coaxial
(HFC) network using Cisco universal broadband routers as CMTSs at the headend
of the network.
about the IPv6 provisioning on CNR server, please refer to
IPv6 on Cable.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats,
see the release notes for your platform and software release. To find information about the features documented in this module,
and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this
1 Effective with Cisco IOS-XE Release 3.17.0S, CBR-CCAP-SUP-60G supports 8 cable line cards. The total traffic rate is limited
to 60Gbps, the total number of downstream service flow is limited to 72268, and downstream unicast low-latency flow does not
count against the limits.
on the HFC Network
A TFTP 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.
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.
The DHCP and Domain Name System (DNS) server shown in Figure
above 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 cBR-8 converged
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 theCiscoNetworkRegistrarUserManual.
Cisco Network Registrar Description
CNR is a dynamic IP address management system, running on Windows or
Solaris, that uses the Dynamic Host Configuration Protocol (DHCP) to assign IP
addresses to cable interfaces, PCs, and other devices on the broadband network.
The CNR tool includes script extensions that allow a cable system administrator
to define and view individual DHCP options, define the identity or type of
device on the network, and assign the device to a predefined class or group.
Using the CNR tool, a cable system administrator can specify policies
Integrated DHCP and Domain
Name Server (DNS) services
Time of Day (ToD) and
Trivial File Transfer Protocol (TFTP) server based on the size of the network
DHCP safe failover and
dynamic DNS updates
This is available only in CNR 3.0 or higher.
Using the CNR tool and the extension scripts identified in the
Overview of Scripts
section, a cable system administrator can specify scopes, policies, and options
for the network and each cable interface based on the services and
configuration to support at each subscriber site.
Scopes refer to the administrative grouping of TCP/IP addresses; all
IP addresses within a scope should be on the same subnet.
The cable system administrator defines system default policies for all
standard options and uses scope-specific policies for options related to
particular subnets, such as cable interfaces. This allows DHCP to send the
information with the IP address.
Seven entry points exist for scripts:
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
Overview of DHCP Using CNR
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 universal broadband routers, and provide a guide on how to configure CNR to support this provisioning model.
Converged Broadband Routers and Cable Modems Work
broadband routers and 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) to 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 cBR-8 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
cBR-8 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
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 cBR-8 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 below lists the required DHCP options and fields.
Table 2. Required DHCP
Fields and Options
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 cBR-8 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.
of the TFTP server where configuration files are stored.
hosts running the time server specified in the RFC 868 standard.
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.
of the security server. This should be set if security is required. See RFC
1533 for details.
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
cBR-8 router at the headend must be configured as a forwarder (iphelper is
Serve IP addresses on two
networks; a net-10 network (non-Internet routable) and a net-24 network
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.
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.
The Cisco cBR-8 router upstream
ports must be configured with the primary network address on the net-24
network; such as 126.96.36.199.
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.
cable modem tag with the scope containing nonroutable addresses.
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.
Figure below shows
the trial CNR configuration in an HFC network.
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:
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.
Cable Modem DHCP Response Fields
Each cable interface on the broadband network requires the following
fields in the DHCP response:
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 network number and indicate how your IP network is
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
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
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 set the gateway address field of the DHCP
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 cable
interfaces 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)
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.
Y=0xYYYYYYY is the SNMP index to the CMTS interface.
Suboption 2, MAC address of
the cable interface:
Type 2 (1 byte)
Len 6 (1 byte)
Value xxxx.xxxx.xxxx (6 bytes)
Overview of Scripts
This section lists the scripts applicable to cable interface
Two-way Cable Modem Scripts
To support two-way configurations at a subscriber site, use these
Telco Return Cable Modem Scripts
To support telco return and two-way cable interface configurations on
the same cable interface card or chassis, use these scripts:
Placement of Scripts
For CNR running on Windows NT, place the appropriate scripts in the following directory:
For CNR running on Solaris, place the appropriate scripts in the following directory:
Activating Scripts in Cisco Network Registrar
To activate the scripts after you have placed them in the appropriate
Open up a text editor.
Open one of the scripts at the nrcmd> command prompt.
Create the extension points and attach them to the system.
The easiest way to do this is to simply cut and paste the
command lines from the scripts to the nrcmd> command line.
After you have created and attached the extension points, do a
The scripts are active.
Configuring the Cisco CMTS Routers to Use Scripts
Each cable interface must be set up as a BOOTP forwarder and have the
relay option enabled. The primary and secondary IP addresses for each cable
interface must be in sync with the CNR tool.
To properly communicate with scripts in the system, use the following
commands on the Cisco CMTS router:
To enable option 82, use
To disable the validation
of DHPC relay agent information in forwarded BOOTREPLY messages, use the
You can also use the cable dhcp-giaddr command in cable interface
configuration mode to modify the GIADDR field of DHCPDISCOVER and DHCPREQUEST
packets to provide a relay IP address before packets are forwarded to the DHCP
server. Use this command to set a “policy” option such that primary addresses
are used for CMs and secondary addresses are used for hosts behind the CMs.
Configuring the System Default Policy
Add these options to the system default policy for:
Cable modems to support on
PCs to support behind each
cable interface on your network
Define these settings following the CNR tool documentation:
TFTP server (IP address)
for those cable interfaces using BOOTP
Time-server (IP address)
Time-offset (Hex value,
1440 for Eastern Standard Time)
Packet-siaddr (IP address of CNR)
Router (set to 0.0.0.0)
Boot-file (name of .cm file for those cable interfaces using BOOTP)
Packet-file-name (.cm file name)
Define these settings following the CNR tool documentation:
Name servers (IP address of DNS servers)
Creating Selection Tag Scopes
When you create your scope selection tags:
Cut and paste the scope selection tag create commands from the
scripts into the nrcmd> command line.
These names have to be exactly as they appear in the scripts.
Then attach the selection tags to the appropriate scripts:
Telco Return for
the Cisco cBR-8 Router
Before you begin
If you are
using the prepacketencode and postclientlookup .tcl scripts for telco return,
the telco return scope does not have a selection tag associated to the scope.
Put the tag
Telcocablemodem on the primary cable interface scope to pull addresses from
that pool instead.
same procedure as above, but use a telco return policy which has a different
.cm file with telco-specific commands in it.
Following is an example for creating
scopes for your network. This example assumes two Cisco cBR-8 converged
broadband routers in two locations, with one cable interface card on one Cisco
cBR-8 configured for telco return.
cm-toledo1_2-0 10.2.0.0 255.255.0.0 assignable 10.2.0.10-10.2.254.254 tagCablemodem tagTelcomodem Default GW=10.2.0.1 (assigned by scripts)
cm-toledo1_3-0 10.3.0.0 255.255.0.0 assignable 10.3.0.10-10.3.254.254 tagCablemodem tagTelcomodem Default GW=10.3.0.1 (assigned by scripts)
pc-toledo1_2-0 188.8.131.52 255.255.255.248 assignable 184.108.40.206-220.127.116.11 tagComputer Default GW=18.104.22.168 (assigned by scripts)
pc-toledo1_3-0 22.214.171.124 255.255.255.248 assignable 126.96.36.199-188.8.131.52 tagComputer Default GW=184.108.40.206 (assigned by scripts)
telco_return_2-0 192.168.1.0 255.255.255.0 (No assignable addresses, tag was put on cable modem primary scope to force telco-return cable modem to pull address from primary scope)
cm-arlington1_2-0 10.4.0.0 255.255.0.0 assignable 10.4.0.10-10.4.254.254 tagCablemodem Default GW=10.4.0.1 (assigned by scripts)
cm-arlington1_3-0 10.5.0.0 255.255.0.0 assignable 10.5.0.10-10.5.254.254 tagCablemodem Default GW=10.5.0.1 (assigned by scripts)
pc-arlington1_2-0 220.127.116.11 255.255.255.248 assignable 18.104.22.168-22.214.171.124 tagComputer Default GW=126.96.36.199 (assigned by scripts)
pc-toledo1_3-0 188.8.131.52 255.255.255.248 assignable 184.108.40.206-220.127.116.11 tagComputer Default GW=18.104.22.168 (assigned by scripts)
last valid address in the .248 subnet range is the broadcast address; do not
Creating Policies for Class of Service or for Upgrading Cable Modem
Cisco IOS Images
To support Class of Service (CoS), define:
tags—Identifiers that describe types of scope configurations
This is needed for Option82.
Client classes—Class with
which a group of clients is associated
Scope selection tags are excluded from or included in
clients and the defined class to which they belong
To assign the CoS or use Option82, make a client entry with a MAC
address and point to the appropriate policy. To use client-based MAC
provisioning, add a client entry “default - exclude,” then put in MAC addresses
for all devices (for example, cable interfaces and PCs) in the client tab and
select the policy to use, including the appropriate tag.
CNR Steps to
The CNR configuration is done
differently if subinterfaces are configured. Here is an example. If you have
configured two ISP subinterfaces and one management subinterface on a Cisco
cBR-8 router, make sure that the management subinterface is the first
subinterface that is configured. If cable interface three—c3/0/0—is being used,
create c3/0/0.1, c3/0/0.2 and c3/0/0.3 as three subinterfaces and c3/0/0.1 as
the first subinterface configured as the management subinterface.
The Cisco cBR-8 router requires
management subinterfaces to route DHCP packets from CMs when they first
initialize because the Cisco cBR-8 router does not know the subinterfaces they
belong to until it has seen the IP addresses assigned to them by gleaning DHCP
reply message from CNR.
In CNR, complete
the following steps for such a configuration:
scope selection tags such as: isp1-cm-tag and isp2-cm-tag
three scopes; for example, mgmt-scope, isp1-cm-scope, and isp2-cm-scope such
that isp1-cm-scope and isp2-cm-scope each define mgmt-scope to be the primary
configure two scopes for PCs for each of the ISPs; isp1-pc-scope and
isp2-pc-scope. For scope isp1-cm-scope, configure isp1-cm-tag to be the scope
selection tag. For scope isp2-cm-scope, configure isp2-cm-tag to be the scope
client classes; for example, isp1-client-class and isp2-client-class
entries with their MAC addresses for CMs that belong to ISP1 and assign them to
isp1-client-class. Also assign the scope selection tag isp1-cm-tag
entries for CMs that belong to ISP2 and assign them to isp2-client-class. Also
assign the scope selection tag isp2-cm-tag
class processing from the scope-selection-tag window
address ranges cannot be configured on these subinterfaces because software
gleans the DHCP reply to figure out the subinterface it really belongs to.
Although CNR can be configured with overlapping address range scopes, it cannot
be used to allocate addresses from these scopes.
sections provide references related to Cisco Network Registrar for use with the
Cisco CMTS routers.
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