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How to configure a cable modem for automatic configuration of the NAT pool

Core Issue

Network Address Translation (NAT) replaces IP addresses within a packet with different IP addresses. NAT is useful for conserving IP addresses and connecting a private network using unregistered addresses to a public network like the Internet.

Cable modems (like the uBR900 series devices) are used to provide Internet access to users through a cable network by connecting to a Cable Modem Termination System (CMTS) device. The cable modems also connect to the Customer Premises Equipment (CPE), such as the PC over the LAN interface. The uBR900 series cable modem can operate in either bridging or routing mode. NAT is used on the cable modem when the CPE devices that use an unregistered address need to connect to the Internet. This is done by using the valid address provided by the service provider. Use Port Address Translation (PAT) to bind the single valid address to the various unregistered addresses, which are used by the CPE devices.

Resolution

When configuring PAT, a NAT pool has to be defined with the valid addresses made available by the provider. To automatically get the address from the provider and use it in the NAT pool, the NAT pool can be configured manually or the cable modems can be configured as a DHCP proxy agent.

The addresses are provided by a DHCP server. This server can run either on the CMTS or on another device that can be accessed by the CMTS.

To configure NAT, perform these steps:

  1. Before configuring NAT, enable routing on the cable modem by issuing the no cable-modem compliant bridge command under the cable interface.  

  2. Remove any bridge groups under the cable and LAN interfaces by issuing the no bridge-group command.    

    Note: These two commands disable the bridging functionality.

       
  3. Route IP traffic by issuing the ip routing command in global configuration mode.  

  4. Configure an IP address under the LAN interface by issuing the ip address command.  

  5. Enable the DHCP proxy agent for NAT configuration by issuing the http://www.cisco.com/en/US/customer/docs/ios/cable/command/reference/cmts_cable_cref_a_to_cable-modem.html#wp1011538cable-modem dhcp-proxy pool-name command under the cable interface.   
      
  6. To permit the addresses of the CPE devices that need to be translated, define an Access Control List (ACL) by issuing the access-list command.  
       
  7. Bind the pool and the list of hosts to be translated by issuing the ip nat inside source pool-name overload command in global configuration mode.   
  8. Issue the ip nat command under the cable interface and the ip nat inside command under the LAN interface.   

  9. To reload the cable modem or reset the cable interface, issue the reload or shutdown and no shutdown commands.  

    This step is necessary for the cable modem to fetch the IP address through DHCP and use it in the pool. A loopback interface is created and the IP address assigned through DHCP is automatically used on this interface. The configuration also shows the same address, which is defined as part of the pool associated with the configured DHCP proxy command.

For more information, refer to Cable-modem dhcp-proxy nat on Cisco Cable Modem.

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