Scenarios

Figure 4-3 illustrates the cable modem lab topology. A Cisco uBR7246 with an MC-16C cable modem card installed in slot 3 is used in the lab. With the MC-16C card, you get one downstream port and six upstream ports. In this lab, the downstream port is connected to the Wavecomm up-converter at 459 MHz, and upstream port 0 is used for the upstream transmission. Note that Dynamic Host Configuration Protocol (DHCP), Time of Day (ToD), and TFTP servers are required but are not shown in Figure 4-3.

Figure 4-3. Cable Modem Lab Topology

graphics/04fig03.gif

Scenario 4-1: Cisco CMTS Minimum Configuration Requirements

In this scenario, you will learn the minimum configuration requirements for the Cisco CMTS. You will also learn the configuration's syntax and commands to verify the configuration.

The Cisco CMTS minimum configuration requirements are as follows. They are required for link establishment between the CMTS and the cable modem:

Set the upstream frequency
Enable the upstream port
Configure the IP address
Configure the helper address

Setting the Upstream Frequency

You need to configure a fixed frequency of the upstream RF carrier for an upstream port. You should make sure that the upstream frequency of your RF output complies with the expected input frequency of your Cisco MC16C cable modem card. The valid range for a fixed upstream frequency is 5 to 42 MHz.

NOTE

You need to make sure that the upstream frequency you choose does not interfere with the frequencies used for any other upstream applications running in the cable plant. The cable interface will not operate until you either set a fixed upstream frequency or create and configure a spectrum group. This is covered later in this chapter.

Use the following commands to set the upstream frequency in cable interface configuration mode:

uBR7246(config-if)#interface cable slot/port
uBR7246(config-if)#cable upstream port frequency return frequency

In Example 4-1, the upstream frequency is set to 39 MHz. In Example 4-2, the command show controller cable 3/0 upstream 0 displays the upstream frequency. Note that Cisco cable interface line cards always program the upstream's center frequency in 16-KHz increments. This is the frequency displayed by the show controller cable upstream command. In Example 4-2, the actual center frequency is 38.992 MHz.

Example 4-1. Upstream Frequency Configuration

uBR7246(config-if)#interface cable 3/0
uBR7246(config-if)#cable upstream 0 frequency 39000000

Example 4-2. Verifying the Upstream Frequency

uBR7246#show controller cable 3/0 upstream 0
 Cable3/0 Upstream 0 is up
  Frequency 38.992 MHz, Channel Width 3.200 MHz, 16-QAM Symbol
  Rate 2.560 Msps
  SNR 28.6280 dB
  Nominal Input Power Level 0 dBmV, Tx Timing Offset 2744
  Ranging Backoff automatic (Start 0, End 3)
  Ranging Insertion Interval automatic (60 ms)
  Tx Backoff Start 0, Tx Backoff End 4
  Modulation Profile Group 5
  Concatenation is enabled
  part_id=0x3137, rev_id=0x03, rev2_id=0xFF
  nb_agc_thr=0x0000, nb_agc_nom=0x0000
  Range Load Reg Size=0x58
  Request Load Reg Size=0x0E
  Minislot Size in number of Timebase Ticks is = 8
  Minislot Size in Symbols = 128
  Bandwidth Requests = 0xAC3C
  Piggyback Requests = 0x84
  Invalid BW Requests= 0x22
  Minislots Requested= 0x3EAD8
  Minislots Granted  = 0x3EAD8
  Minislot Size in Bytes = 64
  Map Advance (Dynamic) : 2447 usecs
  UCD Count = 303031
 DES Ctrl Reg#0 = C000C043, Reg#1 = 0

Enabling the Upstream Port

Each upstream port must be activated to enable upstream data from the cable modems on the HFC network to the Cisco uBR7246.

To activate the upstream ports, use the following commands in global configuration mode:

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#no cable upstream port shutdown

Example 4-3 shows how to activate upstream port 0. Recall that an MC16C card is used in this lab. It is installed in slot 3. Upstream port 0 is used for upstream communication between the CMTS and the cable modem.

Example 4-3. Enabling the Upstream Port

uBR7246(config)#interface cable 3/0
uBR7246(config-if)#no cable upstream 0 shutdown

To verify whether the upstream ports are enabled or disabled, enter the show interface cable command for the upstream port you have configured, as shown in Example 4-4.

Example 4-4. Verifying the Upstream Port

uBR7246#show interface cable 3/0 upstream 0
Cable3/0: Upstream 0 is up
     Received 144 broadcasts, 12489 multicasts, 209258 unicasts
...

Configuring the IP Address and Helper Address

Configuring IP address in CMTS is the same way when you configure other Cisco IOS routers.

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#ip address IP address IP subnet mask

The helper address provides a way for packets from the cable modem and the PC to locate their supporting DHCP server, from which they receive their IP address and the address of their supporting TFTP and ToD servers.

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable helper-address IP address

Example 4-5 shows the syntax to configure the DHCP server's IP address.

Example 4-5. Configuring the Helper Address

uBR7246(config)#interface cable 3/0
uBR7246(config-if)#cable helper-address 10.1.1.5

NOTE

DOCSIS mandates that the DHCP, ToD, and TFTP servers be part of the cable access solutions. Cisco Network Registrar software can be used as the DHCP and TFTP servers, or you can configure the DHCP, ToD, and TFTP services on Cisco's CMTS.

Scenario 4-2: Cisco CMTS Optional Configuration

In this scenario, you will learn how to configure some of the optional configuration for Cisco CMTS and command syntax. Some parameters don't have to be modified, but they are listed here for your reference.

Setting the Upstream Input Power Level

The uBR7246 controls the cable modems' output power levels to meet the desired upstream input power level. The default setting of 0 dBmV is the optimal setting for the upstream power level.

NOTE

If you increase the input power level, the cable modems on your HFC network increase their transmit power level. This might cause an increase in the network's carrier-to-noise ratio (CNR). Be careful if you adjust this parameter. You might violate the upstream return laser design parameters.

To set the upstream input power level, use the following commands in cable interface configuration mode:

uBR7246(config-if)#interface cable slot/port
uBR7246(config-if)#cable upstream port power-level dBmV

In Example 4-6, the power level is set to 0 dBmV for upstream channel 0. Again, it is the default setting and is the optimal setting for the upstream power level.

Example 4-6. Configuring the Upstream Input Power Level

uBR7246(config-if)#interface cable 3/0
uBR7246(config-if)#cable upstream 0 power-level 0

To verify the current value of the upstream input power level, enter the show controller cable command for the upstream port you just configured, as shown in Example 4-7.

Example 4-7. Verifying the Upstream Input Power Level

uBR7246#show controller cable 3/0 upstream 0
 Cable3/0 Upstream 0 is up
  Frequency 38.992 MHz, Channel Width 3.200 MHz, 16-QAM Symbol
  Rate 2.560 Msps
  Spectrum Group 20
  SNR 28.6280 dB
  Nominal Input Power Level 0 dBmV, Tx Timing Offset 2744
  Ranging Backoff automatic (Start 0, End 3)
  Ranging Insertion Interval automatic (60 ms)
  Tx Backoff Start 0, Tx Backoff End 4
  Modulation Profile Group 5
  Concatenation is enabled
  part_id=0x3137, rev_id=0x03, rev2_id=0xFF
  nb_agc_thr=0x0000, nb_agc_nom=0x0000
  Range Load Reg Size=0x58
  Request Load Reg Size=0x0E
  Minislot Size in number of Timebase Ticks is = 8
  Minislot Size in Symbols = 128
  Bandwidth Requests = 0xAC3C
  Piggyback Requests = 0x84
  Invalid BW Requests= 0x22
  Minislots Requested= 0x3EAD8
  Minislots Granted  = 0x3EAD8
  Minislot Size in Bytes = 64
  Map Advance (Dynamic) : 2447 usecs
  UCD Count = 303031
  DES Ctrl Reg#0 = C000C043, Reg#1 = 0

Configuring the Upstream Channel Bandwidth

By default, the upstream RF bandwidth is set to 1600 KHz. The command to configure the upstream channel bandwidth is as follows:

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable upstream port channel-width [200000 | 400000 | 800000 |
  1600000 |3200000]

Example 4-8 shows you how to configure the channel width for upstream port 0. You can also use the show controller cable command to view the channel width configuration of the upstream port you just configured, as shown in Example 4-9.

Example 4-8. Configuring the Upstream Channel Bandwidth

uBR7246(config)#interface cable 3/0
uBR7246(config-if)#cable upstream 0 channel-width 3200000

NOTE

Before increasing the channel width or modulation, you should perform a thorough analysis of your upstream spectrum using a spectrum analyzer to find a wide-enough band with adequate CNR. Failure to do so can potentially affect other services in your cable network.

Example 4-9. Verifying Upstream Channel Width

uBR7246#show controller cable 3/0 upstream 0
 Cable3/0 Upstream 0 is up
  Frequency 38.992 MHz, Channel Width 3.200 MHz, 16-QAM Symbol
  Rate 2.560 Msps
  Spectrum Group 20
  SNR 28.6280 dB
  Nominal Input Power Level 0 dBmV, Tx Timing Offset 2744
  Ranging Backoff automatic (Start 0, End 3)
  Ranging Insertion Interval automatic (60 ms)
  Tx Backoff Start 0, Tx Backoff End 4
  Modulation Profile Group 5
  Concatenation is enabled
  part_id=0x3137, rev_id=0x03, rev2_id=0xFF
  nb_agc_thr=0x0000, nb_agc_nom=0x0000
  Range Load Reg Size=0x58
  Request Load Reg Size=0x0E
  Minislot Size in number of Timebase Ticks is = 4
  Minislot Size in Symbols = 32
  Bandwidth Requests = 0xAC3C
  Piggyback Requests = 0x84
  Invalid BW Requests= 0x22
  Minislots Requested= 0x3EAD8
  Minislots Granted  = 0x3EAD8
  Minislot Size in Bytes = 64
  Map Advance (Dynamic) : 2447 usecs
  UCD Count = 303031
  DES Ctrl Reg#0 = C000C043, Reg#1 = 0

Configuring Spectrum Management

Spectrum management is a way to improve performance on upstream signal traffic and to compensate for noise and interference. The spectrum manager monitors the upstream frequencies. If there is too much noise or interference in an upstream channel, the spectrum manager reassigns the upstream channel to a different upstream frequency. Spectrum management is configured and activated using spectrum groups. A spectrum group is a table of frequencies that upstream ports can use to implement a frequency-hopping policy. The commands to configure spectrum management are as follows:

uBR7246(config)#cable spectrum-group group-number [time day hh:mm:ss] frequency
  upstream-frequency [dBmV]
uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable upstream port spectrum-group group-number

In Example 4-10, three fixed frequencies29 MHz, 33 MHz, and 39 MHzare configured under spectrum group 20. Spectrum group 20 is then assigned to upstream port 0.

Example 4-10. Configuring a Spectrum Group and Associating It with the Upstream Port

uBR7246(config)#cable spectrum-group 20 frequency 29000000
uBR7246(config)#cable spectrum-group 20 frequency 33000000
uBR7246(config)#cable spectrum-group 20 frequency 39000000
uBR7246(config)#interface cable 3/0
uBR7246(config-if)#cable upstream 0 spectrum-group 20

You can use the show cable spectrum-group command to display the current allocation table and frequency assignment, as shown in Example 4-11.

Example 4-11. Displaying the Spectrum Group Configuration

uBR7246#show cable spectrum-group
Group  Frequency      Upstream     Weekly Scheduled            Power   Shared
No.    Band           Port         Availability                Level   Spectrum
       (Mhz)                       From Time:    To Time:      (dBmV)
1      29.000                                                  0       No
10     29.000                                                  0       No
20     29.000                                                  0       No
20     33.000                                                  0       No
20     39.000                                                  0       No
20     38.992 [3.20]  Cable3/0 U0                              0
20     29.008 [1.60]  Cable3/0 U1                              0
20     29.008 [1.60]  Cable3/0 U2                              0
20     29.008 [1.60]  Cable3/0 U3                              0
20     29.008 [1.60]  Cable3/0 U4                              0
20     29.008 [1.60]  Cable3/0 U5                              0

Example 4-12 displays the current frequency assignment and spectrum group 20 for upstream port 0.

Example 4-12. Verifying the Upstream Frequency and Its Associated Spectrum Group Configuration

uBR7246#show controller cable 3/0 upstream 0
 Cable3/0 Upstream 0 is up
  Frequency 38.992 MHz, Channel Width 3.200 MHz, 16-QAM Symbol
  Rate 2.560 Msps
  Spectrum Group 20
  SNR 28.6280 dB
  Nominal Input Power Level 0 dBmV, Tx Timing Offset 2744
  Ranging Backoff automatic (Start 0, End 3)
  Ranging Insertion Interval automatic (60 ms)
  Tx Backoff Start 0, Tx Backoff End 4
  Modulation Profile Group 5
  Concatenation is enabled
  part_id=0x3137, rev_id=0x03, rev2_id=0xFF
  nb_agc_thr=0x0000, nb_agc_nom=0x0000
  Range Load Reg Size=0x58
  Request Load Reg Size=0x0E
  Minislot Size in number of Timebase Ticks is = 4
  Minislot Size in Symbols = 32
  Bandwidth Requests = 0xAC3C
  Piggyback Requests = 0x84
  Invalid BW Requests= 0x22
  Minislots Requested= 0x3EAD8
  Minislots Granted  = 0x3EAD8
  Minislot Size in Bytes = 64
  Map Advance (Dynamic) : 2447 usecs
  UCD Count = 303031
  DES Ctrl Reg#0 = C000C043, Reg#1 = 0

NOTE

The Cisco uBR MC16S Spectrum Management card and Cisco IOS Release 12.1(7)CX together provide advanced spectrum management features such as intelligent frequency hopping, dynamic upstream modulation, and proactive channel management.

Configuring the Downstream Cable Interface

If the external up-converter is used, the downstream frequency is an information-only command. It should reflect the digital carrier frequency, which is the center frequency of the downstream RF carrier for that downstream port. The configuration controlling the digital carrier frequency is done in the IF-to-RF up-converter that must be installed in the downstream path from the Cisco uBR7246. The commands to configure the downstream frequency are as follows:

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable downstream frequency 54000000-1000000000 Broadcast
  Frequency - Hz

NOTE

The cable downstream frequency command currently has no effect on external up-converters; it is information only.

Annex B is the MPEG framing format used in North America. By default, the downstream carrier MPEG frame format is set to Annex B. Under normal circumstances, this setting does not have to be changed. The commands to configure the framing format are as follows:

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable downstream annex B

Interleaving is used to improve the bit error rate. Larger interleaving values increase noise stability but at the cost of potentially increased transmission time. DOCSIS specifies that the operator can select the interleave depth for best operational throughput. The default value is 32. Optional values are 8, 16, 32, 64, and 128. Under normal circumstances, this setting does not have to be changed. The commands to configure the downstream interleave depth are as follows:

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable downstream interleave-depth [8 | 16 | 32 | 64 | 128]

The default modulation mode for downstream is 64-QAM. You can use the following commands to set the downstream modulation to either 64-QAM or 256-QAM:

uBR7246(config)#interface cable slot/port
uBR7246(config-if)#cable downstream modulation [64qam | 256qam]

To summarize what you have learned, Example 4-13 shows the basic CMTS downstream configuration. Use the command shown in Example 4-14 to verify the downstream configuration.

Example 4-13. Cisco CMTS Downstream Configuration

uBR7246(config)#interface cable 3/0
uBR7246(config-if)#cable downstream annex B
uBR7246(config-if)#cable downstream modulation 64qam
uBR7246(config-if)#cable downstream interleave-depth 32
uBR7246(config-if)#cable downstream frequency 459000000

Example 4-14. Displaying the Downstream Characteristics

uBR7246#show controller cable 3/0 downstream
 Cable3/0 Downstream is up
  Frequency 459.0000 MHz, Channel Width 6 MHz, 64-QAM, Symbol
  Rate 5.056941 Msps
  FEC ITU-T J.83 Annex B, R/S Interleave I=32, J=4
  Downstream channel ID: 0

Scenario 4-3: Cisco Cable Modem Bridging and Routing Configuration

You saw the basic Cisco CMTS configuration in the previous scenarios. Now it is time to learn the configuration of the Cisco cable access router. This scenario presents two types of configuration:

DOCSIS-compliant bridging
Routing

DOCSIS-compliant bridging is also known as plug-and-play bridging. It is a default configuration for most Cisco cable access routers, such as uBR924 and uBR925. In bridging mode, a Cisco cable access router performs as a DOCSIS 1.0 cable modem and should work with any DOCSIS-qualified CMTS. If you don't intend to implement any advanced data features, such as IPSec or a firewall, bridging mode is easier to configure. You need to configure routing mode for a Cisco cable access router if advanced data features are required. This chapter explains how you can configure basic routing mode for a Cisco cable access router. If you need to configure NAT or IPSec, Chapters 12, "Scaling IP Addressing with Network Address Translation," and 14, "Securing Remote Access Networks," provide you with more information on how to configure these features.

Cisco Cable Access Router DOCSIS-Compliant Bridging Configuration

As mentioned earlier, this is the default mode of operation for a Cisco cable access router. The cable access router functions in its plug-and-play DOCSIS-compliant bridging mode and performs as a DOCSIS-compliant two-way cable modem with this configuration. A Cisco DOCSIS-compliant cable modem supports the following minimum set of features:

It downloads the DOCSIS configuration file from the CMTS or dedicated server at the headend. It provisions and configures itself automatically.
It operates in bridge mode and provides Internet connectivity to the CPE devices.

NOTE

The DOCSIS specification requires that a DOCSIS-compliant cable modem download a DOCSIS configuration file during its power-on or reset sequence. Cisco provides a DOCSIS cable modem configuration tool at www.cisco.com/support/toolkit/CableModem. You need a CCO account to access this tool.

If the Cisco cable access router is configured in routing mode, the following steps are necessary to convert it back to bridging mode:

Step 1. Disable IP routing on the cable access router:

uBR925(config)#no ip routing

Step 2. Remove the IP address from both the Ethernet and cable interfaces:

uBR925(config)# interface Ethernet0
uBR925(config-if)# no ip address
uBR925(config-if)# interface cable-modem0
uBR925(config-if)# no ip address

Assign both the Ethernet and cable interfaces to a bridge spanning group. For the bridge-group number, you can choose any integer from 1 to 63. Also disable spanning tree on the cable interface.

uBR925(config)#interface Ethernet0
uBR925(config-if)# bridge-group bridge-group
uBR925(config-if)# bridge-group bridge-group spanning-disabled
uBR925(config-if)# interface cable-modem0
uBR925(config-if)# bridge-group bridge-group
uBR925(config-if)# bridge-group bridge-group spanning-disabled

Step 3. Enable DOCSIS-compliant bridging:

uBR925(config)# interface cable-modem0
uBR925(config-if)# cable modem compliant bridge

cable modem compliant bridge is the default configuration under interface cable-modem0. Therefore, it doesn't show up in the configuration when you enter that command.

NOTE

With Cisco IOS Release 12.0(5)T and later, a Cisco cable access router such as uBR925 supports a maximum of 254 CPE devices in bridging mode. No limit exists when the cable access router is operating in routing mode.

The MAX CPE field in the DOCSIS configuration file is used to determine the maximum number of CPE devices that can connect the cable network. The default value is 1.

Figure 4-4 illustrates a typical bridging topology, and Example 4-15 displays the basic plug-and-play bridging configuration. Cisco cable access routers do not need additional configuration to provide Internet access for PCs and other customer premises equipment (CPE) devices. However, the PCs and CPE devices must be configured to support DHCP allocation of IP addresses.

Figure 4-4. Typical Cisco Cable Modem DOCSIS-Compliant Bridging Topology

graphics/04fig04.gif

Example 4-15. Cisco Cable Access Router DOCSIS-Compliant Bridging Configuration

no ip routing

interface Ethernet0
 no ip address
 bridge-group 59
 bridge-group 59 spanning-disabled
!
interface cable-modem0
 ip address docsis
 bridge-group 59
 bridge-group 59 spanning-disabled

Cisco Cable Access Router Routing Configuration

Figure 4-5 shows a typical routing topology. If you plan to use advanced features such as IPSec and a firewall, a Cisco cable access router needs to be configured for routing mode. All the CPE devices need to be on a different subnet than the subnet used by the CMTS. For routing protocols, you can configure RIP version 2 or just use the default route.

Figure 4-5. Typical Cisco Cable Modem Routing Topology

graphics/04fig05.gif

Keep in mind that the default configuration is bridging mode. To configure routing mode, follow these steps:

Step 1. Enable IP routing:

uBR925(config)# ip routing

Step 2. Disable DOCSIS-compliant bridging on the cable interface:

uBR925(config)# interface cable-modem0
uBR925(config-if)# no cable-modem compliant bridge

Step 3. Remove the bridge group on the cable and Ethernet interfaces:

uBR925(config)# interface Ethernet0
uBR925(config-if)# no bridge group number
uBR925(config-if)# interface cable-modem0
uBR925(config-if)# no bridge group number

Step 4. Configure the cable interface to receive an IP address from the DHCP server:

uBR925(config)# interface cable-modem0
uBR925(config-if)# ip address docsis

Step 5. Enter the Ethernet interface's IP address and subnet mask:

uBR925(config)# interface Ethernet0
uBR925(config-if)# ip address ip-address subnet-mask

Step 6. Configure the routing protocol (RIP version 2) or configure the default route:

uBR925(config)# router rip
uBR925(config-router)# version 2
uBR925(config-router)# network cable-network-number
uBR925(config-router)# network Ethernet-network-number

uBR925(config)# ip route 0.0.0.0 0.0.0.0 ip-address

where ip-address is the IP address for the CMTS.

Example 4-16 illustrates the cable modem routing configuration. RIP version 2 is used as the routing protocol. Two network statements exist under RIP configuration:

172.16.0.0 is for the Ethernet interface and CPE devices.
10.0.0.0 is for the cable modem interface and the CMTS.

In most cases the default route configuration should be enough.

Example 4-16. Cisco Cable Access Router Routing Configuration

ip routing
...
interface Ethernet0
 ip address 172.16.1.1 255.255.255.0
!
interface cable-modem0
 ip address docsis
 no cable-modem compliant bridge
 ...
router rip
 version 2
 network 10.0.0.0
 network 172.16.0.0

[ LiB ]