Cisco 6000 Series IP DSL Switches

Configuring a Cisco 827 Router With a Single IP Address, DHCP, and PPPoA

Document ID: 12880

Updated: Jan 25, 2008



This sample configuration shows a Cisco 827 Digital Subscriber Line (DSL) Router connecting to a Cisco 6130 Digital Subscriber Line Access Multiplexer (DSLAM) and terminating on a Cisco 6400 Universal Access Concentrator (UAC). The Cisco 827 router has been configured as a Dynamic Host Configuration Protocol (DHCP) server with Point-to-Point Protocol over ATM (PPPoA).

Before You Begin


For more information on document conventions, see the Cisco Technical Tips Conventions.


There are no specific prerequisites for this document.

Components Used

The information in this document is based on the software and hardware versions below.

  • Cisco 827-4V Customer Premises Equipment (CPE) IOS® Software Release 12.1(1)XB

  • Cisco 6400 UAC-Node Route Processor (NRP) IOS Software Release 12.0(7)DC

  • Cisco 6400 UAC-Node Switch Processor (NSP) IOS Software Release 12.0(4)DB

  • Cisco 6130 DSLAM-NI2 IOS Software Release 12.1(1)DA

The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.

Background Information

In this instance, the Internet Service Provider (ISP) has given the subscriber a single IP address ( for Internet connectivity, but the subscriber has a small network of PCs and wants to have Internet access for all devices.

The solution shown here implements Network Address Translation (NAT) on the Cisco 827 router. NAT is designed for IP address simplification and conservation; it enables private IP internetworks that use nonregistered IP addresses to connect to the Internet. NAT operates on a router, usually connecting two networks, and translates the private (in this case the network) addresses in the internal network to legal (in this case, addresses before packets are forwarded to another network. As part of this functionality, NAT can be configured to advertise only one address ( for the entire network. This provides additional security, effectively hiding the entire internal network behind a single IP address.

NAT serves the dual purposes of security and address conservation, and is typically implemented in remote access environments. In this example, an IP address of is manually configured on the Ethernet interface of the Cisco 827 router. The Cisco 827 router is configured to act as a DHCP server, and leases IP addresses to local LAN devices attached to its Ethernet network.

The following configuration shows NAT configured for the Ethernet and ATM interfaces. The Ethernet interface (designated Ethernet0 in the network diagram below) has an IP address of, with a subnet mask of NAT is configured for inside, which means that the interface is connected to the inside network that is subject to NAT translation. The ATM interface (designated Dialer0 in the network diagram below) has an IP address of and a subnet mask of NAT on Dialer0 is configured for outside, which means that the interface is connected to an outside network such as the Internet. For more information about end-to-end Asynchronous Digital Subscriber Line (ADSL) PPPoA architecture, refer to PPPoA Baseline Architecture.


In this section, you are presented with the information to configure the features described in this document.

Note: To find additional information on the commands used in this document, use the Command Lookup Tool (registered customers only) .

Network Diagram

This document uses the network setup shown in the diagram below.



This document uses the configuration shown below.

Cisco 827 Router
version 12.1
service timestamps debug datetime msec 
service timestamps datetime msec 
hostname R1 
ip subnet-zero 
ip dhcp excluded-address

!--- The DHCP pool does not lease this address; 
!--- it is used by interface E0.

ip dhcp pool <pool name>

!--- The default gateway is assigned to local devices.

interface Ethernet0 
 ip address 
 no ip directed-broadcast 
 ip nat inside
 no ip mroute-cache
interface ATM0 
 no ip address 
 no ip directed-broadcast 
 no ip mroute-cache 
 no atm ilmi-keepalive
 pvc 1/150 
  encapsulation aal5mux ppp dialer 
  dialer pool-member 1 
 hold-queue 224 in 
interface Dialer0 
  ip address    
  ip nat outside     
  no ip directed-broadcast 
  encapsulation ppp 
  dialer pool 1 
  dialer-group 2 
  ppp pap sent-username <username> password <password>
ip nat inside source list 1  interface Dialer0 overload 
ip classless 
ip route Dialer0 
no ip http server 
access-list 1 permit 
dialer-list 2 protocol ip permit
voice-port 1 
 timing hookflash-in 0
voice-port 2
 timing hookflash-in 0 
voice-port 3
 timing hookflash-in 0 
voice-port 4 
 timing hookflash-in 0 


There is currently no verification procedure available for this configuration.


There is currently no specific troubleshooting information available for this configuration.

Related Information

Updated: Jan 25, 2008
Document ID: 12880