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Configuring PPP over Ethernet with NAT

Configuration Example

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4

Configuring PPP over ATM with NAT

The Cisco 857, Cisco 876, Cisco 877, and Cisco 878 access routers support Point-to-Point Protocol over

Asynchronous Transfer Mode (PPPoA) clients 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 address for the ATM connection.

Figure 4-1

PPP over ATM with NAT

2

3

5

1

6

4

ISP

1

Small business with multiple networked devices—desktops, laptop PCs, switches

2

Fast Ethernet LAN interface (inside interface for NAT, 192.168.1.1/24)

3

PPPoA Client—Cisco 857, Cisco 876, Cisco 877, or Cisco 878 router

4

Point at which NAT occurs

5

ATM WAN interface (outside interface for NAT)

6

PPPoA session between the client and a PPPoA server at the ISP

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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 857 or Cisco 877 router

•

ADSL over integrated services digital network (ISDN) using the Cisco 876 router

•

Single-pair high-speed digital subscriber line (G.SHDSL) using the Cisco 878 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 850 or Cisco 870 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

“Configuration Example”

section on page 4-11

.

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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.

Command

Purpose

Step 1

interface 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 2

ip address negotiated

Example:

Router(config-if)#

ip address negotiated

Router(config-if)#

Specifies that the IP address for the dialer

interface is obtained through PPP/IPCP (IP

Control Protocol) address negotiation.

Step 3

ip 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 4

encapsulation

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 5

ppp authentication

{

protocol1

[

protocol2

...]}

Example:

Router(config-if)#

ppp authentication chap

Router(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 6

dialer 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 7

dialer-group

group-number

Example:

Router(config-if)#

dialer-group 1

Router(config-if)#

Assigns the dialer interface to a dialer group

(1–10).

Tip

Using a dialer group controls access to

your router.

Step 8

exit

Example:

Router(config-if)#

exit

Router(config)#

Exits the dialer 0 interface configuration.

Step 9

dialer-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 10

ip route

prefix mask

{

interface-type

interface-number

}

Example:

Router(config)#

ip route 10.10.25.2

0.255.255.255 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