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Table Of Contents
Using the Vendor-Specific Information Field
Sample Configuration for DOCSIS-Compliant Bridging
Configuring the Attached CPE Devices
Reconfiguring DOCSIS-Compliant Bridging
DOCSIS-Bridging Configuration
This chapter describes the default configuration of the Cisco uBR924 cable access router. With this configuration, the Cisco uBR924 router functions in its "plug and play" DOCSIS-bridging mode, performing as a DOCSIS-compliant two-way cable modem. Every DOCSIS-compliant cable modem provides the following minimum set of features:
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Automatically provisions and configures itself using the DOCSIS configuration file that is downloaded from a server at the headend.
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The following sections describe the configuration for "plug and play" DOCSIS bridging:
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The DHCP server configuration and DOCSIS configuration file are required for every DOCSIS-compliant cable modem. The Cisco IOS image and configuration files are optional, depending on the needs of the subscribers. The remaining configurations are optional, depending on the needs of the subscribers.
The information described in this chapter applies to every Cisco uBR924 cable access router that is used in a DOCSIS-compliant network. Additional configuration steps might be needed, however, to support additional features, such as VoIP and IPSec encryption—this additional configuration is described in the other chapters in this guide.
Caution
DHCP Server Configuration
The DOCSIS specification (SP-RFI-IO5-991105 or later revision) requires that a DOCSIS-compliant cable modem connect to a DHCP server at power-on or reset to establish temporary IP connectivity with the cable network. This enables the cable modem to download the additional configuration information needed to establish a permanent connection with the headend and cable network.
The DHCP server can be a CMTS with DHCP server capabilities (such as a Cisco uBR7200 series universal broadband router), or it can be a dedicated server located at the headend. The server can be configured manually for each cable modem or it can be part of an automated provisioning system such as Cisco Network Registrar (CNR).
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The DHCP server provides the information shown in Table 2-1 to each cable modem. (If the modem fails to obtain any of this information, an error messsage appears. Such messages are explained in the book, Cisco Cable CPE Error Messages, viewable online at www.cisco.com/univercd/cc/td/doc/product/cable/cab_modm/ubcmerrs.pdf ).
After making a successful DHCP request, the cable modem contacts the ToD server to get the current date and time. It also begins the TFTP download of the DOCSIS configuration file, which is described in the next section, "DOCSIS Configuration File" section.
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DOCSIS Configuration File
The DOCSIS specification requires that a DOCSIS-compliant cable modem download a DOCSIS configuration file during its power-on or reset sequence. This file must be in the format described in the SP-RFI-IO5-991105 specification (or later revision) and must contain the information shown in Table 2-2.
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Table 2-2 DOCSIS Configuration File Parameters
Downstream Frequency
Specifies the center frequency (in multiples of 62500 Hz) for the downstream channel to be used by the router. (This parameter does not need to be specified in the configuration file because the router will scan the downstream for available frequencies, but typically it is specified to ensure that the router conforms to the provider's channel plan.)
Upstream Channel ID
Specifies channel ID for the upstream channel to be used by the router. (This parameter does not need to be specified in the configuration file because it can be set dynamically by the CMTS during provisioning.)
Network Access Configuration
Determines whether CPE devices attached to the cable modem are allowed access to the cable network. The default is to allow access for CPE devices (which is required for normal operations).
Class of Service ID
Specifies the ID for this class of service (1-16).
Maximum Downstream Rate
Specifies the maximum downstream data rate (in bits/sec) allowed for traffic associated with this class of service. (This is a limit, not a guarantee of service.)
Maximum Upstream Rate
Specifies the maximum upstream data rate (in bits/sec) allowed for traffic associated with this class of service. (This is a limit, not a guarantee of service.)
Upstream Channel Priority
Specifies the priority for upstream traffic (0-7, where 7 is highest priority).
Minimum Upstream Rate
Specifies the minimum upstream data rate (in bits/sec) that is guaranteed for traffic associated with this class of service.
Maximum Upstream Channel Burst
Specifies the maximum size of burst traffic to be allowed on this upstream channel. The size is specified in bytes, 0-65535, where 0 is no limit. If this field is set to a non-zero value, it should be set to at least 1800 so that it is greater than the maximum Ethernet frame size of 1518 plus the associated packet overhead).
Class of Service Privacy Enable
Specifies whether BPI encryption should be enabled on traffic associated with this class of service (1 enables BPI encryption, 0 disables BPI encryption).
Vendor ID
The three-byte Organization Unique Identifier for the vendor, which is also usually the first three bytes of the cable modem's MAC address. This value is usually expressed as a hexadecimal number. This field should be "00000C" for Cisco Systems routers.
Vendor-Specific Options
Contains any arbitrary values that are defined by the manufacturer of the cable modem. The Cisco uBR924 cable access router uses this field to identify the Cisco IOS configuration file that should be downloaded (if any). Arbitrary Cisco IOS commands can also be specified in this field.
SNMP Write-Access Control and SNMP MIB Objects
Allows the service provider to set arbitrary SNMP attributes on the cable modem. For the Cisco uBR924 router, these two fields are typically used to enable SNMP management of the router because SNMP management is disabled by default.
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Authorize Wait Timeout
Specifies the retransmission interval, in seconds, of Authorization Request messages from the Authorize Wait state. Valid values are 2-30 seconds.
Reauthorize Wait Timeout
Specifies the retransmission interval, in seconds, of Reauthorization Request messages from the Authorize Wait state. Valid values are 2-30 seconds.
Authorization Grace Timeout
Specifies the grace period for re-authorization, in seconds. Valid values are 1-1800 seconds.
Operational Wait Timeout
Specifies the retransmission interval, in seconds, of Key Requests from the Operational Wait state. Valid values are 1-10 seconds.
Rekey Wait Timeout
Specifies the retransmission interval, in seconds, of Key Requests from the Rekey Wait state. Valid values are 1-10 seconds.
TEK Grace Time
Specifies the grace period for re-keying, in seconds. Valid values are 1-1800 seconds.
Authorize Reject Wait Timeout
Specifies how long, in seconds, a cable modem waits in the Authorize Reject Wait state after receiving an Authorization Reject. Valid values are 60-1800 seconds.
Maximum Number of CPEs
Determines the maximum number of CPE devices that can use the cable modem to connect to the cable network. The default value is 1. In bridging mode, the Cisco uBR924 router supports a maximum number of either 3 or 254 CPE devices, depending on the Cisco IOS software release being used.
CPE Ethernet MAC Address
Configures the cable modem with the MAC addresses for one or more CPE devices that are allowed to connect to the cable network. Entering values in this field is optional because the cable modem can learn the MAC addresses of CPE devices dynamically, up to the maximum allowable number. However, DOCSIS cable modems give priority to the CPE devices whose MAC addresses are in the configuration file.
TFTP Software Server IP Address
Specifies the IP address for the TFTP server that will provide software images. This server does not necessarily have to be the same TFTP server that provided the DOCSIS configuration file.
Software Image Filename
Specifies the fully qualified path name for the software image that the cable modem should be running. If necessary, the cable modem uses TFTP to download this image from the software server.
Concatenation Support
Specifies whether the cable modem supports DOCSIS 1.1 concatenation of upstream packet requests.
Use RFC2104 HMAC-MD5
Specifies the algorithm used to compute the CMTS Message Integrity Check (MIC). If yes, the HMAC-MD5 algorithm specified in RFC 2104 is used; otherwise, the algorithm specified by RFC 1321 is used. (The algorithm used must match the one used on the CMTS.)
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CMTS Authentication
Specifies an authentication string to be used between the provisioning server (which creates the configuration files) and the CMTS. It allows the CMTS to authenticate the CM provisioning with a central authentication service, such as a RADIUS server. This field is typically used only for one-way cable modems that use telco-return.
1 The DOCSIS configuration file also contains fields for one-way cable modems that use telco-return, but these fields do not apply to the Cisco uBR924 router, which is a two-way cable modem.
Cisco IOS Software Image
The DOCSIS configuration file contains the filename for the software image that the Cisco uBR924 router must be running. If this filename does not match the software image that is currently installed on the router, the router must use the TFTP protocol to download the new image from the server specified in the DOCSIS configuration file.
After the new software image has been downloaded, the Cisco uBR924 router resets itself and repeats the entire power-on and provisioning process. This includes downloading the DOCSIS configuration file again. However, because the software image is stored in non-volatile Flash memory, the router does not have to download it again—the software download occurs only when the service provider specifies a new software image filename in the DOCSIS configuration file.
If the Cisco uBR924 router cannot download the new image, it retries the download, up to a maximum of 16 attempts. If the router still cannot download the image, it falls back to its previous software image and attempts to go online with that image.
The service provider can also force the Cisco uBR924 router to download new software by putting a new image filename in the DOCSIS configuration file and resetting the router. This should be done only after warning the customer that the modem will be offline for a period of several minutes.
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Cisco IOS Configuration File
The DOCSIS configuration file uses the type 43 Vendor-Specific Options field to specify that the Cisco uBR924 router should download a Cisco IOS configuration file. The router's console port is automatically disabled as part of this process to prevent users at the remote site from reconfiguring the router.
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Using the Vendor-Specific Information Field
Table 2-3 shows the values that would be entered in the Vendor-Specific Information Field (VSIF) to download a Cisco IOS configuration file and automatically disable the console port.
Table 2-4 shows the values that would be entered in the Vendor-Specific Information Field (VSIF) to specify a CLI command that should be executed after the Cisco uBR924 cable access router processes the DOCSIS configuration file and comes online.
Tip
Sample Configuration for DOCSIS-Compliant Bridging
The following shows a typical Cisco IOS configuration for a Cisco uBR924 router that is operating in "plug and play" DOCSIS-compliant bridging mode.
version 12.1service configno service padservice timestamps debug uptimeservice timestamps log uptimeno service password-encryption!hostname ubr924!clock timezone - 4ip subnet-zerono ip routing!voice-port 0!voice-port 1!interface Ethernet0no ip directed-broadcastno ip route-cachebridge-group 59bridge-group 59 spanning-disabled!interface cable-modem0ip address dhcpno ip directed-broadcastno ip route-cachebridge-group 59bridge-group 59 spanning-disabled!ip classlessno ip http serverno service finger!!line con 0transport input noneline vty 0 4!endThis configuration shows the following requirements for DOCSIS-compliant bridging:
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Configuring the Attached CPE Devices
In its "plug-and-play" bridging mode, the Cisco uBR924 router does not need any additional configuration to support the computers or other CPE devices that will access the Internet through the router's connection to the cable network. However, the PCs and CPE devices must be configured to support DHCP allocation of IP addresses.
Each computer and CPE device performs this configuration differently. For Windows 95, for example, you would open up the Network control panel, select the computer's TCP/IP Ethernet adapter, and set the IP address configuration to "Obtain an IP address automatically."
Reconfiguring DOCSIS-Compliant Bridging
To reconfigure the Cisco uBR924 router to support DOCSIS-compliant bridging after it has been configured for routing, log in to the Cisco uBR924 router, enter global configuration mode, and enter the following commands:
