Configuring PPP over Ethernet with NAT
This chapter provides an overview of Point-to-Point Protocol over Ethernet (PPPoE) clients and Network Address Translation (NAT) that can be configured on the Cisco 819 Integrated Services Routers (ISRs).
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 encrypted, filtered, and so forth. Figure 11-1 shows a typical deployment scenario with a PPPoE client and NAT configured on the Cisco router.
Figure 11-1 PPP over Ethernet with NAT
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Multiple networked devices—Desktops, laptop PCs, switches |
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Fast Ethernet LAN interface (inside interface for NAT) |
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PPPoE client—Cisco 819 ISRs |
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Point at which NAT occurs |
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Fast Ethernet WAN interface (outside interface for NAT) |
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Cable modem or other server that is connected to the Internet |
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PPPoE session between the client and a PPPoE server |
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 819 ISRs. 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 re-establish 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 re-establish 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:
An example showing the results of these configuration tasks is shown in the “Configuration Example” section.
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.
SUMMARY STEPS
1. vpdn enable
2. vpdn-group name
3. request-dialin
4. protocol {l2tp | pppoe}
5. exit
6. exit
DETAILED STEPS
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Step 1 |
vpdn enable Example:
Router(config)#
vpdn enable
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Enables VPDN on the router. |
Step 2 |
vpdn-group name Example:
Router(config)#
vpdn-group 1
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Creates and associates a VPDN group with a customer or VPDN profile. |
Step 3 |
request-dialin Example:
Router(config-vpdn)#
request-dialin
Router(config-vpdn-req-in)#
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Creates a request-dialin VPDN subgroup, indicating the dialing direction, and initiates the tunnel. |
Step 4 |
protocol { l2tp | pppoe } Example:
Router(config-vpdn-req-in)#
protocol pppoe
Router(config-vpdn-req-in)#
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Specifies the type of sessions the VPDN subgroup can establish. |
Step 5 |
exit Example:
Router(config-vpdn-req-in)#
exit
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Exits request-dialin VPDN group configuration. |
Step 6 |
exit Example:
Router(config-vpdn)#
exit
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Exits VPDN configuration, returning to global configuration mode. |
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:
SUMMARY STEPS
1. interface type number
2. pppoe-client dial-pool-number number
3. no shutdown
4. exit
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Step 1 |
interface type number Example:
Router(config)#
interface fastethernet 4
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Enters interface configuration mode for a Fast Ethernet WAN interface. |
Step 2 |
pppoe-client dial-pool-number number Example:
Router(config-if)#
pppoe-client dial-pool-number 1
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Configures the PPPoE client and specifies the dialer interface to use for cloning. |
Step 3 |
no shutdown Example:
Router(config-if)#
no shutdown
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Enables the Fast Ethernet interface and the configuration changes just made to it. |
Step 4 |
exit Example:
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Exits configuration mode for the Fast Ethernet interface and returns to global configuration mode. |
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.
SUMMARY STEPS
1. interface dialer dialer-rotary-group-number
2. ip address negotiated
3. ip mtu bytes
4. encapsulation encapsulation-type
5. ppp authentication { protocol1 [ protocol2 ...]}
6. dialer pool number
7. dialer-group group-number
8. exit
9. dialer-list dialer-group protocol protocol-name {permit | deny | list access-list-number | access-group}
10. ip route prefix mask { interface-type interface-number }
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:
SUMMARY STEPS
1. ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length }
2. ip nat inside source {list access-list-number } {interface type number | pool name } [overload]
3. interface type number
4. ip nat {inside | outside}
5. no shutdown
6. exit
7. interface type number
8. ip nat {inside | outside}
9. no shutdown
10. exit
11. access-list access-list-number {deny | permit} source [ source-wildcard ]
DETAILED STEPS
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Step 1 |
ip 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 255.255.252.0
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Creates pool of global IP addresses for NAT. |
Step 2 |
ip nat inside source {list access-list-number} {interface type number | pool name} [overload] Example:
Router(config)#
ip nat inside source list 1
interface dialer 0 overload
or Example:
Router(config)#
ip nat inside source list acl1 pool
pool1
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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 Cisco IOS IP Command Reference, Volume 1 of 4: Addressing and Services. |
Step 3 |
interface type number Example:
Router(config)#
interface
vlan 1
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Enters configuration mode for the VLAN (on which the Fast Ethernet LAN interfaces [FE0–FE3] reside) to be the inside interface for NAT. |
Step 4 |
ip nat { inside | outside } Example:
Router(config-if)#
ip nat inside
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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 Cisco IOS IP Command Reference, Volume 1 of 4: Addressing and Services. |
Step 5 |
no shutdown Example:
Router(config-if)#
no shutdown
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Enables the configuration changes just made to the Ethernet interface. |
Step 6 |
exit Example:
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Exits configuration mode for the Fast Ethernet interface. |
Step 7 |
interface type number Example:
Router(config)#
interface fastethernet
4
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Enters configuration mode for the Fast Ethernet WAN interface (FE4) to be the outside interface for NAT. |
Step 8 |
ip nat { inside | outside } Example:
Router(config-if)#
ip nat outside
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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 Cisco IOS IP Command Reference, Volume 1 of 4: Addressing and Services. |
Step 9 |
no shutdown Example:
Router(config-if)#
no shutdown
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Enables the configuration changes just made to the Ethernet interface. |
Step 10 |
exit Example:
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Exits configuration mode for the Fast Ethernet interface. |
Step 11 |
access-list access-list-number {deny | permit} source [source-wildcard] Example:
Router(config)#
access-list 1 permit 192.168.1.0 255.255.255.0
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Defines a standard access list indicating which addresses need translation. Note All other addresses are implicitly denied. |
Note If you want to use NAT with a virtual-template interface, you must configure a loopback interface. See the “Basic Router Configuration” section for information on configuring a loopback interface.
For complete information on the NAT commands, see the Cisco IOS Release 12.3 documentation set. For more general information on NAT concept, see the “Cisco IOS Software Basic Skills” section.
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
Note Commands marked by “(default)” are generated automatically when you run the show running-config command.
vpdn enable
vpdn-group 1
request-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 4
no ip address
no ip directed-broadcast (default)
ip nat outside
pppoe enable group global
pppoe-client dial-pool-number 1
no sh
!
interface dialer 0
ip address negotiated
ip mtu 1492
encapsulation ppp
ppp authentication chap
dialer 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 10.10.25.2 255.255.255.255 dialer 0
ip nat pool pool1 192.168.1.0 192.168.2.0 netmask 255.255.252.0
ip nat inside source list acl1 pool pool1
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)
CEF Translated packets: 0, CEF Punted packets: 0
[Id: 1] access-list 1 interface Dialer0 refcount 0