Cisco Router 850 Series Software Configuration Guide

							
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Configuring PPP over Ethernet with NAT
The Cisco 851 and Cisco 871access routers support  Point-to-Point Protocol over Ethernet (PPPoE) 
clients and network address translation (NAT). 
Multiple PCs can be connected to the LAN behind the  router. Before the traffic from these PCs is sent 
to the PPPoE session, it can be en crypted, filtered, and so forth. 
Figure 3-1 shows a typical deployment 
scenario with a PPPoE client and NA T configured on the Cisco router. 
Figure 3-1 PPP over Ethernet with NAT
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1Multiple networked devices—Desktops, laptop PCs, switches
2Fast Ethernet LAN interface (inside interface for NAT)
3PPPoE client—Cisco 851 or Cisco 871 access router
4Point at which NAT occurs
5Fast Ethernet WAN interface (outside interface for NAT)
6Cable modem or other server (for example, a Cisco 6400 server) that is connected to the Internet
7PPPoE session between the  client and a PPPoE server 
						

							 
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Chapter 3      Configuring PPP over Ethernet with NAT
  Configure the Virtual Private Dialup Network Group Number
PPPoE
The PPPoE Client feature on the router provides PPPoE client support on Ethernet interfaces. A dialer 
interface must be used for cloning virtual access. Multiple PPPoE client sessions can be configured on 
an Ethernet interface, but each session must use a separate dialer interface and a separate dialer pool. 
A PPPoE session is initiated on the client side by the Cisco 850 or Cisco 870 series router.An established 
PPPoE client session can be terminated in one of two ways:
 By entering the clear vpdn tunnel pppoe command. The PPPoE client session terminates, and the 
PPPoE client immediately tries to reestablish the session. This also occurs if the session has a 
timeout.
 By entering the no pppoe-client dial-pool number command to clear the session. The PPPoE client 
does not attempt to reestablish the session.
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 Virtual Private Dialup Network Group Number
 Configure the Fast Ethernet WAN Interfaces
 Configure the Dialer Interface
 Configure Network Address Translation
An example showing the results of these configuration tasks is shown in the “Configuration Example” 
section on page 3-9.
Configure the Virtual Private Dialup Network Group Number
Configuring a virtual private dialup network (VPDN) enables multiple clients to communicate through 
the router by way of a single IP address. 
Complete the following steps to configure a VPDN, starting from the global configuration mode. See the 
“Configure Global Parameters” section on page 1-5 for details about entering this mode.
Command or ActionPurpose
Step 1vpdn enable
Example:
Router(config)# vpdn enable
Router(config-vpdn)# 
Enables VPDN on the router.
Step 2vpdn group name
Example:
Router(config-vpdn)# vpdn group 1
Router(config-vpdn-grp)# 
Creates and associates a VPDN group with a 
customer or VPDN profile. 
						

							 
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Chapter 3      Configuring PPP over Ethernet with NAT
  Configure the Virtual Private Dialup Network Group Number
Step 3request-dialin
Example:
Router(config-vpdn-grp)# request-dialinRouter(config-vpdn-grp)# 
Creates a request-dialin VPDN subgroup, 
indicating the dialing direction, and initiates the 
tunnel.
Step 4initiate to ip ip-address
Example:
Router(config-vpdn-grp)# initiate to 
192.168.1.1
Router(config-vpdn-grp)# 
Specifies the address to which requests are 
tunneled.
For details about this command and additional 
parameters that can be set, see the 
Cisco IOS Dial 
Technologies Command Reference.
Step 5protocol {l2f | l2tp | pppoe | any}
Example:
Router(config-vpdn-grp)# protocol pppoe
Router(config-vpdn-grp)# 
Specifies the type of sessions the VPDN subgroup 
can establish.
Step 6exit
Example:
Router(config-vpdn-grp)# exitRouter(config-vpdn)# 
Exits VPDN group configuration.
Step 7exit
Example:
Router(config-vpdn)# exit
Router(config)# 
Exits VPDN configuration, returning to global 
configuration mode.
Command or Action Purpose 
						

							
 
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Chapter 3      Configuring PPP over Ethernet with NAT
  Configure the Fast Ethernet WAN Interfaces
Configure the Fast Ethernet WAN Interfaces
In this scenario, the PPPoE client (your Cisco  router) communicates over a 10/100 Mbps-Ethernet 
interface on both the inside and the outside. 
Perform these steps to configure the Fast Ethernet  WAN interfaces, starting in global configuration 
mode: 
CommandPurpose
Step 1interface  type number
Example:
Router(config)#  interface fastethernet 4
Router(config-if)# 
Enters interface configuration mode for a 
Fast
 Ethernet WAN interface.
Step 2pppoe-client dial-pool-number  number
Example:
Router(config-if)# pppoe-client 
dial-pool-number 1
Router(config-if)# 
Configures the PPPoE client and specifies the 
dialer interface to use for cloning. 
Step 3no shutdown
Example:
Router(config-if)#  no shutdownRouter(config-if)# 
Enables the Fast Ethernet interface and the 
configuration changes just made to it.
Step 4exit
Example:
Router(config-if)# exit
Router(config)# 
Exits configuration mode for the Fast Ethernet 
interface and returns to global configuration 
mode. 
						

							 
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Chapter 3      Configuring PPP over Ethernet 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. The dialer interface is also 
used for cloning virtual access. Multiple PPPoE client sessions can be configured on a Fast Ethernet 
interface, but each session must use a separate dialer interface and a separate dialer pool.
Complete the following steps to configure a dialer interface for one of the Fast Ethernet LAN interfaces 
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 interface configuration mode.
Step 2ip address negotiated
Example:
Router(config-if)# ip address negotiatedRouter(config-if)# 
Specifies that the IP address for the interface is 
obtained through PPP/IPCP (IP Control Protocol) 
address negotiation.
Step 3ip mtu bytes
Example:
Router(config-if)# ip mtu 1492
Router(config-if)# 
Sets the size of the IP maximum transmission unit 
(MTU). The default minimum is 128
 bytes. The 
maximum for Ethernet is 1492
 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 to 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
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 10.10.25.2 
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 2; Routing 
Protocols. 
Command Purpose 
						

							 
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Chapter 3      Configuring PPP over Ethernet with NAT
  Configure Network Address Translation
Configure Network Address Translation
Network Address Translation (NAT) translates packets from addresses that match a standard access list, 
using global addresses allocated by the dialer interface. Packets that enter the router through the inside 
interface, packets sourced from the router, or both are checked against the access list for possible address 
translation. You can configure NAT for either static or dynamic address translations.
Perform these steps to configure the outside Fast Ethernet WAN interface with dynamic NAT, beginning 
in global configuration mode:
CommandPurpose
Step 1ip nat pool name start-ip end-ip {netmask 
netmask | prefix-length prefix-length}
Example:
Router(config)# ip nat pool pool1 
192.168.1.0 192.168.2.0 netmask 0.0.0.255
Router(config)# 
Creates pool of global IP addresses for NAT. 
Step 2ip nat inside source {list access-list-number} 
{interface type number | pool name} [overload]
Example 1:
Router(config)# ip nat inside source list 1 
interface dialer 0 overload
or
Example 2:
Router(config)# ip nat inside source list 
acl1 pool pool1
Enables dynamic translation of addresses on the 
inside interface.
The first example shows the addresses permitted 
by the access list 1 to be translated to one of the 
addresses specified in the dialer interface 0.
The second example shows the addresses 
permitted by access list acl1 to be translated to one 
of the addresses specified in the NAT pool pool1.
For details about this command and additional 
parameters that can be set, as well as information 
about enabling static translation, see the 
Cisco IOS IP Command Reference, Volume 1 of 4: 
Addressing and Services.
Step 3interface type number
Example:
Router(config)# interface vlan 1
Router(config-if)# 
Enters configuration mode for the VLAN (on 
which the Fast Ethernet LAN interfaces 
[FE0–FE3] reside) to be the inside interface for 
NAT.
Step 4ip nat {inside | outside}
Example:
Router(config-if)# ip nat insideRouter(config-if)# 
Identifies the specified VLAN interface as the 
NAT inside interface.
For details about this command and additional 
parameters that can be set, as well as information 
about enabling static translation, see the 
Cisco IOS IP Command Reference, Volume 1 of 4: 
Addressing and Services. 
						

							 
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Chapter 3      Configuring PPP over Ethernet with NAT
  Configure Network Address Translation
NoteIf you want to use NAT with a virtual-template interface, you must configure a loopback interface. See 
Chapter 1, “Basic Router Configuration,” for information on configuring a loopback interface.
Step 5no shutdown
Example:
Router(config-if)# no shutdownRouter(config-if)# 
Enables the configuration changes just made to the 
Ethernet interface.
Step 6exit
Example:
Router(config-if)# exit
Router(config)# 
Exits configuration mode for the Fast Ethernet 
interface.
Step 7interface type number
Example:
Router(config)# interface fastethernet 4
Router(config-if)# 
Enters configuration mode for the Fast Ethernet 
WAN interface (FE4) to be the outside interface 
for NAT.
Step 8ip nat {inside | outside}
Example:
Router(config-if)# ip nat outside
Router(config-if)# 
Identifies the specified WAN interface as the NAT 
outside interface.
For details about this command and additional 
parameters that can be set, as well as information 
about enabling static translation, see the 
Cisco IOS IP Command Reference, Volume 1 of 4: 
Addressing and Services.
Step 9no shutdown
Example:
Router(config-if)# no shutdown
Router(config-if)# 
Enables the configuration changes just made to the 
Ethernet interface.
Step 10exit
Example:
Router(config-if)# exitRouter(config)# 
Exits configuration mode for the Fast Ethernet 
interface.
Step 11access-list access-list-number {deny | permit} 
source [source-wildcard]
Example:
Router(config)# access-list 1 permit 
192.168.1.0 0.0.0.255
Defines a standard access list indicating which 
addresses need translation.
NoteAll other addresses are implicitly denied.
Command Purpose 
						

							 
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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.
NoteCommands marked by “(default)” are generated automatically when you run the show running-config 
command.
!
vpdn enablevpdn-group 1
request-dialin
protocol pppoe!
interface vlan 1
ip address 192.168.1.1 255.255.255.0no ip directed-broadcast (default)
ip nat inside
!interface FastEthernet 4
ip address 192.168.12.2 255.255.255.0
no ip directed-broadcast (default)
ip nat outside!
interface dialer 1
ip address negotiatedppp authentication chap
dialer pool 1
dialer-group 1!
dialer-list 1 protocol ip permit
ip nat inside source list 1 interface dialer 0 overloadip classless (default)
ip route 10.10.25.2 0.255.255.255 dialer 0
! 
						

							 
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  Configuration Example
Verifying Your Configuration
Use the show ip nat statistics command in privileged EXEC mode to verify the PPPoE with 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