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Chapter 3      Configuring PPP over Ethernet with NAT
  Configuration Example
For complete information on the NAT commands, see the Cisco IOS Release 12.3 documentation set. 
For more general information on NAT concepts, see 
Appendix B, “Concepts.”
Configuration Example
The following configuration example shows a portion of the configuration file for the PPPoE scenario 
described in this chapter.
The VLAN interface has an IP address of 192.168.1.1 with a subnet mask of 255.255.255.0. NAT is 
configured for inside and outside.
NoteSince the VLAN interface is on LAN, we have used a private IP address.
NoteCommands marked by “(default)” are generated automatically when you run the show running-config 
command.
!
vpdn enable
vpdn-group 1request-dialin
protocol pppoe
!interface vlan 1
ip address 192.168.1.1 255.255.255.0
no ip directed-broadcast (default)ip nat inside
!
interface FastEthernet 0ip address 192.1.12.2 255.255.255.0
no ip directed-broadcast (default)
ip nat outside!
interface dialer 1
ip address negotiated
ppp authentication chapdialer pool 1
dialer-group 1
!dialer-list 1 protocol ip permit
ip nat inside source list 1 interface dialer 0 overload
ip classless (default)ip route 10.10.25.2 0.255.255.255 dialer 0
! 
						

							 
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Chapter 3      Configuring PPP over Ethernet with NAT
  Configuration Example
Verifying Your Configuration
Use the show ip nat statistics command in privileged EXEC mode to verify NAT configuration. You 
should see verification output similar to the following example:
Router# show ip nat statistics 
Total active translations: 0 (0 static, 0 dynamic; 0 extended)
Outside interfaces:  FastEthernet4
Inside interfaces:
  Vlan1Hits: 0  Misses: 0
CEF Translated packets: 0, CEF Punted packets: 0
Expired translations: 0Dynamic mappings:
-- Inside Source
[Id: 1] access-list 1 interface Dialer0 refcount 0Queued Packets: 0 
						

							
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Configuring PPP over ATM with NAT
The Cisco 1801, Cisco 1802, and Cisco 1803 access routers support Point-to-Point Protocol over 
Asynchronous Transfer Mode (PPPoA) client s and network address translation (NAT). 
Multiple PCs can be connected to the LAN behind the router. Before traffic from the PCs is sent to the 
PPPoA session, it can be encrypted, filtered, and so  forth. PPP over ATM provides a network solution 
with simplified address handling and straight user verification like a dial network. 
Figure 4-1 shows a 
typical deployment scenario with  a PPPoA client and NAT configured on  the Cisco router. This scenario 
uses a single static IP ad dress for the ATM connection.
Figure 4-1 PPP over ATM with NAT
92340
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4
ISP
1Small business with multiple networked devices—desktops, laptop PCs, switches
2Fast Ethernet LAN interface (insid e interface for NAT, 192.168.1.1/24)
3PPPoA Client—Cisco 1801, Cisco 1802, or Cisco 1803 router
4Point at which NAT occurs
5ATM WAN interface (outside interface for NAT)
6PPPoA session between the client and a PPPoA server at the ISP 
						

							 
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Chapter 4      Configuring PPP over ATM with NAT
  
In this scenario, the small business or remote user on the Fast Ethernet LAN can connect to an Internet 
Service Provider (ISP) using the following protocols on the WAN connection:
 Asymmetric digital subscriber line (ADSL) over plain old telephone service (POTS) using the 
Cisco
 1801 router
 ADSL over integrated services digital network (ISDN) using the Cisco 1802 router
 Single-pair high-speed digital subscriber line (G.SHDSL) using the Cisco 1803 router
The Fast Ethernet interface carries the data packet through the LAN and off-loads it to the PPP 
connection on the ATM interface. The ATM traffic is encapsulated and sent over the ADSL, ISDN, or 
G.SHDSL lines. The dialer interface is used to connect to the ISP.
PPPoA
The PPPoA Client feature on the router provides PPPoA client support on ATM interfaces. A dialer 
interface must be used for cloning virtual access. Multiple PPPoA client sessions can be configured on 
an ATM interface, but each session must use a separate dialer interface and a separate dialer pool. 
A PPPoA session is initiated on the client side by the Cisco 1800 series router.
NAT
NAT (represented as the dashed line at the edge of the Cisco router) signifies two addressing domains 
and the inside source address. The source list defines how the packet travels through the network.
Configuration Tasks
Perform the following tasks to configure this network scenario:
 Configure the Dialer Interface
 Configure the ATM WAN Interface
 Configure DSL Signaling Protocol
 Configure Network Address Translation
An example showing the results of these configuration tasks is shown in the section “Configuration 
Example.” 
						

							 
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Chapter 4      Configuring PPP over ATM with NAT
  Configure the Dialer Interface
Configure the Dialer Interface
The dialer interface indicates how to handle traffic from the clients, including, for example, default 
routing information, the encapsulation protocol, and the dialer pool to use. It is also used for cloning 
virtual access. Multiple PPPoA client sessions can be configured on an ATM interface, but each session 
must use a separate dialer interface and a separate dialer pool.
Perform these steps to configure a dialer interface for the ATM interface on the router, starting in global 
configuration mode. 
CommandPurpose
Step 1interface dialer dialer-rotary-group-number
Example:
Router(config)# interface dialer 0
Router(config-if)# 
Creates a dialer interface (numbered 0–255), and 
enters into interface configuration mode.
Step 2ip address negotiated
Example:
Router(config-if)# ip address negotiatedRouter(config-if)# 
Specifies that the IP address for the dialer 
interface is obtained through PPP/IPCP (IP 
Control Protocol) address negotiation.
Step 3ip mtu bytes
Example:
Router(config-if)# ip mtu 4470
Router(config-if)# 
Sets the size of the IP maximum transmission unit 
(MTU). The default minimum is 128
 bytes. The 
maximum for ATM is 4470
 bytes.
Step 4encapsulation encapsulation-type
Example:
Router(config-if)# encapsulation ppp
Router(config-if)# 
Sets the encapsulation type to PPP for the data 
packets being transmitted and received.
Step 5ppp authentication {protocol1 [protocol2...]}
Example:
Router(config-if)# ppp authentication chapRouter(config-if)# 
Sets the PPP authentication method.
The example applies the Challenge Handshake 
Authentication Protocol (CHAP).
For details about this command and additional 
parameters that can be set, see the
 Cisco IOS 
Security Command Reference.
Step 6dialer pool number
Example:
Router(config-if)# dialer pool 1
Router(config-if)# 
Specifies the dialer pool to use to connect to a 
specific destination subnetwork. 
						

							 
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  Configure the Dialer Interface
Repeat these steps for any additional dialer interfaces or dialer pools needed.
Step 7dialer-group group-number
Example:
Router(config-if)# dialer-group 1Router(config-if)# 
Assigns the dialer interface to a dialer group 
(1–10).
TipUsing a dialer group controls access to 
your router.
Step 8exit
Example:
Router(config-if)# exit
Router(config)# 
Exits the dialer 0 interface configuration.
Step 9dialer-list dialer-group protocol protocol-name 
{permit | deny | list access-list-number | 
access-group} 
Example:
Router(config)# dialer-list 1 protocol ip 
permit
Router(config)# 
Creates a dialer list and associates a dial group 
with it. Packets are then forwarded through the 
specified interface dialer group. 
For details about this command and additional 
parameters that can be set, see the 
Cisco IOS Dial 
Technologies Command Reference.
Step 10ip route prefix mask {interface-type 
interface-number}
Example:
Router(config)# ip route 10.10.25.0 
255.255.255.0 dialer 0
Router(config)# 
Sets the IP route for the default gateway for the 
dialer 0 interface.
For details about this command and additional 
parameters that can be set, see the
 Cisco IOS IP 
Command Reference, Volume 1 of 4: Routing 
Protocols. 
Command Purpose 
						

							 
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  Configure the ATM WAN Interface
Configure the ATM WAN Interface
Perform these steps to configure the ATM interface, beginning in global configuration mode.
CommandPurpose
Step 1interface type number
Example:
Router(config)# interface atm 0Router(config-if)# 
Enters interface configuration mode for the ATM 
interface (labeled ADSLoPOTS or G.SHDSL on 
the back of your router).
NoteThis interface was initially configured 
during basic router configuration. See 
“Configure WAN Interfaces” section on 
page 1-7.
Step 2pvc vpi/vci
Example:
Router(config-if)# pvc 8/35
Router(config-if-atm-vc)# 
Creates an ATM PVC for each end node (up to ten) 
with which the router communicates. Enters ATM 
virtual circuit configuration mode.
When a PVC is defined, AAL5SNAP 
encapsulation is defined by default. Use the 
encapsulation command to change this, as shown 
in 
Step 3. The VPI and VCI arguments cannot be 
simultaneously specified as zero; if one is 0, the 
other cannot be 0. 
For details about this command and additional 
parameters that can be set, see the
 Cisco IOS 
Wide-Area Networking Command Reference.
Step 3encapsulation {aal5auto | aal5autoppp 
virtual-template number [group group-name] | 
aal5ciscoppp virtual-template number | 
aal5mux protocol | aal5nlpid | aal5snap} 
Example:
Router(config-if-atm-vc)# encapsulation 
aal5mux ppp dialer
Router(config-if-atm-vc)# 
Specifies the encapsulation type for the PVC and 
points back to the dialer interface. 
For details about this command and additional 
parameters that can be set, see the
 Cisco IOS 
Wide-Area Networking Command Reference.
Step 4dialer pool-member number
Example:
Router(config-if-atm-vc)# dialer 
pool-member 1
Router(config-if-atm-vc)# 
Specifies the ATM interface as a member of a 
dialer profile dialing pool. The pool number must 
be in the range of 1–255. 
						

							
 
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Chapter 4      Configuring PPP over ATM with NAT
  Configure DSL Signaling Protocol
Configure DSL Signaling Protocol
DSL signaling must be configured on the ATM interface for connection to your ISP. The Cisco 1801 
supports ADSL signaling over POTS, the Cisco
 1802 supports ADSL signaling over ISDN, and the 
Cisco 1803 supports SHDSL signaling.
Based on the router you are configuring, see one of the following sections to configure the appropriate 
DSL signaling protocol.
 Configuring ADSL
 Configuring SHDSL
Configuring ADSL
The default configuration for ADSL signaling is shown in  Ta b l e 4-1. 
Ta b l e 4-1 Default ADSL Configuration
AttributeDescriptionDefault Value
Operating modeSpecifies the operating mode of the digital 
subscriber line (DSL) for an ATM interface.
 ADSL over POTS—ANSI or ITU full 
rate, or automatic selection.
 ADSL over ISDN—ITU full rate, ETSI, 
or automatic selection.
Auto
Loss of marginSpecifies the number of times a loss of margin 
may occur.
Training logToggles between enabling the training log and 
disabling the training log.Disabled
Step 5no shutdown
Example:
Router(config-if-atm-vc)#  no shutdownRouter(config-if)# 
Enables interface and configuration changes just 
made to the ATM interface.
Step 6exit
Example:
Router(config-if)# exit
Router(config)# 
Exits configuration mode for the ATM interface.
Command Purpose 
						

							 
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  Configure DSL Signaling Protocol
If you wish to change any of these settings, use one of the following commands in global configuration 
mode.
 dsl operating-mode (from the ATM interface configuration mode)
 dsl lom integer
 dsl enable-training-log
See the Cisco IOS Wide-Area Networking Command Reference for details of these commands.
Verify the Configuration
You can verify that the configuration is set the way you want using the show dsl interface atm 0 
command from privileged EXEC mode.
Configuring SHDSL
Complete the following steps to configure the DSL controller in your router to use SHDSL signaling, 
beginning in global configuration mode.
CommandPurpose
Step 1controller dsl port
Example:
Router(config)# controller dsl 0
Router(config-controller)# 
Enters the configuration mode for the DSL 
controller.
Step 2line-term {co | cpe}
Example:
Router(config-controller)# line-term co
Router(config-controller)# 
Specifies if the DSL line is terminated at a central 
office (CO) or at customer premises equipment 
(CPE).
Step 3exit
Example:
Router(config-controller)# exitRouter(config)# 
Exits controller configuration mode, returning to 
global configuration mode.
Step 4mode protocol
Example:
Router(config)# mode atm
Router(config-controller)# 
Specifies the mode of the DSL controller and 
enters controller configuration mode. 
						

							 
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  Configure DSL Signaling Protocol
NoteIf you are integrating your Cisco router into a European network, please use one of the following 
commands:
 
 
For CO mode, use the dsl dsl-mode shdsl symmetric annex {A | B | B-ANFP} command to choose 
annex B or B-ANFP. 
 
 
For CPE mode, use the dsl dsl-mode shdsl symmetric annex {A | A-B | A-B-ANFP | B | B-ANFP} to 
choose any option except option A.
 
 
The router uses annex A by default (United States).
Verify the Configuration
You can verify that the configuration is set the way you want using the show controllers dsl command 
from privileged EXEC mode.
Router# show controllers dsl 0
DSL 0 controller UP
SLOT 0: Globespan xDSL controller chipsetLine Mode: Four Wire Standard Mode
DSL mode: SHDSL Annex A
Frame mode: UtopiaConfigured Line rate: Auto
Line Re-activated 6 times after system bootup
LOSW Defect alarm: ACTIVECRC per second alarm: ACTIVE
Line termination: CPE
Current 15 min CRC: 0
Current 15 min LOSW Defect: 0
Step 5line-mode {4-wire enhanced | 4-wire standard | 
2-wire}
Example:
Router(config-controller)# line-mode 4-wire 
standard
Router(config-controller)# 
Specifies whether this DSL connection is 
operating in 2-wire, 4-wire standard, or 4-wire 
enhanced mode.
Noteline mode 4-wire will default to 4-wire 
enhanced mode.
Step 6ignore-error-duration number
Example:
Router(config-controller)# 
ignore-error-duration 15
Router(config-controller)# 
Specifies how long, 15 to 30 seconds, to ignore 
errors.
Step 7exit
Example:
Router(config-controller)# exit
Router(config)# 
Exits controller configuration mode, returning to 
global configuration mode.
Command Purpose