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L2TP - IPSec Support for NAT and PAT Windows Clients

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L2TP—IPSec Support for NAT and PAT Windows Clients

Contents

Prerequisites for L2TP—IPSec Support for NAT and PAT Windows Clients

Restrictions for L2TP—IPSec Support for NAT and PAT Windows Clients

Information About L2TP—IPSec Support for NAT and PAT Windows Clients

How L2TP—IPSec Support for NAT and PAT Windows Clients Works

How to Enable L2TP—IPSec Support for NAT and PAT Windows Clients

Enabling L2TP—IPSec Support

Configuration Examples for L2TP—IPSec Support for NAT and PAT Windows Clients

Dynamic Map Configuration: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

set nat demux


L2TP—IPSec Support for NAT and PAT Windows Clients

The L2TP—IPSec Support for NAT and PAT Windows Clients feature allows more than one Windows client to connect to a Layer 2 Tunneling Protocol (L2TP) Network Server (LNS) at one time with IP Security (IPSec) enabled and a network address translation (NAT) or port address translation (PAT) server between the Windows client and LNS.

Currently, if one Windows client is connected to a Cisco IOS LNS router through a NAT or PAT server with IPSec enabled, and then another Windows client connects to the same Cisco IOS LNS router, the first client's connection is effectively terminated. Enabling L2TP—IPSec Support for NAT and PAT Windows Clients ensures that Windows client connections in this environment are established and maintained until the connection is closed.

History for the L2TP—IPSec Support for NAT and PAT Windows Clients Feature

Release
Modification

12.3(11)T4

This feature was introduced.

12.4(1)

This feature was integrated into Release 12.4(1).


Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

•Prerequisites for L2TP—IPSec Support for NAT and PAT Windows Clients

•Restrictions for L2TP—IPSec Support for NAT and PAT Windows Clients

•Information About L2TP—IPSec Support for NAT and PAT Windows Clients

•How to Enable L2TP—IPSec Support for NAT and PAT Windows Clients

•Configuration Examples for L2TP—IPSec Support for NAT and PAT Windows Clients

•Additional References

•Command Reference

Prerequisites for L2TP—IPSec Support for NAT and PAT Windows Clients

•You have an environment consisting of Windows clients and Cisco IOS LNS routers with IPSec enabled and a NAT or PAT server between the Windows client and LNS router.

•You must have a version of IPSec that contains the L2TP—IPSec Support for NAT and PAT Windows Clients feature.

•You must understand Windows 2000 concepts and configuration requirements.

•You must understand Cisco IOS LNS routers concepts and configuration requirements.

•You must understand NAT and PAT concepts and configuration requirements.

•You must understand IPSec concepts and configuration requirements.

•You must understand L2TP concepts and configuration requirements.

Restrictions for L2TP—IPSec Support for NAT and PAT Windows Clients

•Tested only with Windows 2000 L2TP/IPsec clients running hotfix 818043.

•Port translation is not a standard default behavior. Port translation is incompatible with standard IPSec because it changes the LNS header port information.

•L2TP requires the client to have Microsoft DUN configured. L2TP is supported solely by Windows 2000 MS-DUN (L2TP is not supported by Windows 95, Windows 98, or Windows NT).

Information About L2TP—IPSec Support for NAT and PAT Windows Clients

To use the L2TP—IPSec Support for NAT and PAT Windows Clients feature, the following concept should be understood:

•How L2TP—IPSec Support for NAT and PAT Windows Clients Works

How L2TP—IPSec Support for NAT and PAT Windows Clients Works

With the L2TP—IPSec Support for NAT and PAT Windows Clients feature not enabled, Windows clients lose connection with the Cisco IOS LNS router when another Windows client establishes an IPSec-protected L2TP tunnel to the Cisco IOS LNS router when IPSec is enabled and there is a NAT or PAT server between the Windows clients and the LNS.

Note If you do not have IPSec enabled, or you do not have a NAT or PAT server, you can have multiple Windows clients connect to a LNS without this command enabled.

Without L2TP—IPSec Support for NAT and PAT Windows Clients Feature Enabled

For example, Figure 1 shows two Windows 2000 clients that are trying to connect to the end host through the router running NAT or PAT and the same Cisco IOS LNS router. IPSec is enabled.

Figure 1 Multiple Windows 2000 Clients, NAT Router, and Cisco IOS LNS Router with IP Addresses

The Windows 2000 Client #1 establishes an IPSec-protected L2TP tunnel to the Cisco IOS LNS router. The Windows 2000 client and the Cisco IOS LNS router recognize that there is a router running NAT between them and IPSec and NAT-Traversal (NAT-T) are enabled. The Windows 2000 client attempts to establish an IPSec security association (SA) and requests transport mode (which it does by default) with proxies from 10.0.0.2, its local address, to 209.265.200.231, the Cisco IOS LNS router's address.

In transport mode NAT, running on the router, translates all outgoing connections (including 10.0.0.2) to its outside IP address (209.265.200.232),the address the traffic will come in on. However, NAT cannot modify the L2TP port designation (1701), which is protected by the IPSec encrypted area. So now, we have a local address of 209.265.200.231, a remote address of 209.265.200.232 and a remote port of 1701. All traffic is sent to the Windows 2000 Client #1 that matches the tunnel 209.265.200.231, port 1701.

Then Windows 2000 Client #2 establishes an IPSec-protected L2TP tunnel to the Cisco IOS LNS router, again in transport mode. And NAT, again, translates all outgoing connections to its outside IP address (209.265.200.232), but it cannot modify the L2TP port designation (1701). All traffic is now sent to Windows 2000 Client #2 that matches tunnel 209.265.200.231, port 1701. This second Windows client connection has effectively ended Windows Client #1's connection to the Cisco IOS LNS router since it is no longer receiving traffic.

With L2TP—IPSec Support for NAT and PAT Windows Clients Feature Enabled

With the L2TP—IPSec Support for NAT and PAT Windows Clients feature enabled, IPSec can translate the L2TP ports after decryption. This feature allows IPSec to map traffic from different hosts to different source ports. L2TP can now distinguish between traffic destined for multiple Windows 2000 clients.

So now, when an SA is created, a translated port will be assigned to it. This port is client-specific. The same port will be used for any new SA created by that client. When an encrypted request is received and decrypted, the source port is translated from the standard value, 1701, to a client specific value. The request with the translated port is then forwarded to L2TP.

As shown in Figure 1 with port translation enabled, the Windows 2000 Client #1 would have a translated port number of 1024 assigned and Windows 2000 Client #2 would have a translated port number of 1025 assigned.

When L2TP sends the reply packet, it uses the translated port number and creates a packet to that destination port. IPSec uses the destination port number to select the SA with which to encrypt the packet. Before encrypting the packet, IPSec translates the destination port back to the standard port number, 1701, which the Windows 2000 client expects. IPSec encrypts the packet, either with the SA to Windows 2000 Client #1 if the destination port was 1024 or with the SA to Windows 2000 Client #2 if the destination port was 1025. And now, all traffic is sent to the appropriate client and multiple Windows clients can be connected to a Cisco IOS LNS router through a NAT server at the same time.

The connection is maintained until one of the following actions occurs:

•The IPSec connection is closed.

•The NAT or PAT device ends the session.

•The LNS closes the session.

•The Windows client closes the session.

How to Enable L2TP—IPSec Support for NAT and PAT Windows Clients

This section contains the following procedure that allows you to enable NAT/PAT port translation:

•Enabling L2TP—IPSec Support

Enabling L2TP—IPSec Support

Use the following task to enable L2TP—IPSec Support for NAT and PAT Windows Clients for environments that have IPSec enabled and include multiple windows clients, a NAT or PAT server, L2TP, and a Cisco IOS LNS router.

SUMMARY STEPS

1. enable

2. configure terminal

3. crypto map map-name seq-num [ipsec-isakmp]

or

crypto dynamic-map dynamic-map-name dynamic-seq-num

4. set nat demux

5. exit

6. exit

7. show crypto map [interface interface | tag map-name]

or

show crypto dynamic-map [tag map-name]

8. show crypto ipsec sa

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

crypto map map-name seq-num [ipsec-isakmp]

Example:

Router(config)# crypto map STATIC_MAP 5

or

crypto dynamic-map dynamic-map-name dynamic-seq-num

Example:

Router(config)# crypto dynamic-map DYNAMIC_MAP 10

Names the static crypto map entry to create (or modify) and enters crypto map configuration mode.

or

Names the dynamic crypto map entry to create (or modify) and enters crypto map configuration mode.

Step 4 

set nat demux

Example:

Router(config-crypto-map)# set nat demux


Enables L2TP—IPSec support.

Step 5 

exit

Example:

Router(config-crypto-map)# exit


Exits crypto map configuration mode and returns to global configuration mode.

Step 6 

exit

Example:

Router(config)# exit


Exits global configuration mode and returns to privileged EXEC mode.

Step 7 

show crypto map [interface interface | tag map-name]

Example:

Router# show crypto map


or


show crypto dynamic-map [tag map-name]

Example:

Router# show crypto dynamic-map

(Optional) Displays information about crypto map configuration.

or

(Optional) Displays information about dynamic crypto map configuration.

Step 8 

show crypto ipsec sa

Example:

Router# show crypto ipsec sa

(Optional) Displays the settings used by current SAs.

Configuration Examples for L2TP—IPSec Support for NAT and PAT Windows Clients

This section provides the following configuration example:

•Dynamic Map Configuration: Example

Dynamic Map Configuration: Example

The following example shows how to enable the L2TP—IPSec Support for NAT and PAT Windows Clients feature for a dynamic crypto map: 

!

version 12.3

service timestamps debug datetime msec

service timestamps log datetime msec

no service password-encryption

!

hostname 72_LNS

!

boot-start-marker

boot-end-marker

!

aaa new-model

!

!

aaa authentication ppp default local

aaa session-id common

ip subnet-zero

!

!

no ip cef

no ip domain lookup

ip domain name cisco.com

ip dhcp excluded-address 20.0.0.8

ip dhcp excluded-address 20.0.0.10

!

!

ip vrf VPN

 rd 1:1

!

!Enable virtual private networking. 

vpdn enable

vpdn ip udp ignore checksum

!

! Default L2TP VPDN group

vpdn-group L2TP

!

!Enables the LNS to accept dial in requests; specifies L2TP as the tunneling 
!protocol; specifies the number of the virtual templates used to clone 
!virtual-access interfaces

 accept-dialin

  protocol l2tp

  virtual-template 1

!Disables L2TP tunnel authentication.

no l2tp tunnel authentication

!

!

crypto keyring L2TP

  pre-shared-key address 0.0.0.0 0.0.0.0 key *****

!

!Defines an Internet Key Exchange (IKE) policy and assigns priority 1.

crypto isakmp policy 1

 encr 3des

 authentication pre-share

 group 2

 lifetime 3600

!

crypto isakmp key cisco hostname w2k01

crypto isakmp keepalive 3600

!

crypto ipsec security-association lifetime seconds 600

!

!Defines a transform set.

crypto ipsec transform-set TS1 esp-3des esp-sha-hmac

 mode transport

!

!Names the dynamic crypto map entry and enters crypto map configuration mode; Enables 
!L2TP—IPSec support; Specifies which transform sets can be used with the crypto map 
!entry

crypto dynamic-map DYN_MAP 10

 set nat demux

 set transform-set TS1!

!

crypto map CRYP_MAP 6000 ipsec-isakmp dynamic DYN_MAP

!

interface Loopback0

 ip address 12.0.0.8 255.255.255.255

!

interface FastEthernet0/0

 ip address 11.0.0.8 255.255.255.0

 no ip route-cache

 duplex full

 speed 100

 crypto map CRYP_MAP

!

interface FastEthernet0/1

 ip address 20.0.0.8 255.255.255.0

 duplex full

 speed 100

!

interface FastEthernet2/0

 ip address 172.19.192.138 255.255.255.0

 duplex full

!

interface Virtual-Template1

 ip unnumbered Loopback0

 peer default ip address pool POOL

 ppp mtu adaptive

 ppp authentication chap ms-chap

!

router ospf 1

 log-adjacency-changes

 redistribute static subnets

 network 11.0.0.0 0.0.0.255 area 0

!

ip local pool POOL 20.0.0.100 20.0.0.110

ip classless

ip route 171.0.0.0 255.0.0.0 172.19.192.1

!

no ip http server

no ip http secure-server

!

!

control-plane

!

gatekeeper

 shutdown!

!

line con 0

 exec-timeout 0 0

 logging synchronous

 stopbits 1

line aux 0

 stopbits 1

line vty 0 4

!

end

Additional References

The following sections provide references related to L2TP—IPSec Support for NAT and PAT Windows Clients.

Related Documents

Related Topic
Document Title

IP Security and Encryption Overview

Cisco IOS Security Configuration Guide, Release 12.3

Configuring IPSec Network Security

Cisco IOS Security Configuration Guide, Release 12.3


Standards

Standard
Title

None

-


MIBs

MIB
MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

None

-


Technical Assistance

Description
Link

The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/techsupport


Command Reference

This section documents the following command only.

•set nat demux

set nat demux

To enable L2TP—IPSec support for NAT or PAT Windows clients, use the set nat demux command in crypto map configuration mode. To disable L2TP—IPSec support, use the no form of this command.

set nat demux

no set nat demux

Syntax Description

This command has no arguments or keywords.

Command Default

With this command disabled, Windows clients lose connection when another Windows client establishes an IP Security (IPSec) protected Cisco IOS Layer 2 Tunneling Protocol (L2TP) tunnel to the same Cisco IOS L2TP Network Server (LNS) when there is a network address translation (NAT) or port address translation (PAT) server between the Windows clients and the LNS.

Command Modes

Crypto map configuration

Command History

Release
Modification

12.3(11)T4

This command was introduced.

12.4(1)

This command was integrated into Release 12.4(1).


Usage Guidelines

Use this command if you have an environment with IPSec enabled and consisting of an LNS, and a network address translation (NAT) or port address translation (PAT) server between the Windows clients and the LNS.

This command has been tested only with Windows 2000 L2TP/IPsec clients running hotfix 818043.

You must enter the crypto map command if you are using static crypto maps or the crypto dynamic-map command if you are using dynamic crypto maps before issuing the set nat demux command.

Note If you do not have IPSec enabled, or you do not have a NAT or PAT server, you can have multiple Windows clients connect to a LNS without this command enabled.

Examples

The following example shows how to enable L2TP—IPSec support for NAT or PAT Windows clients for a dynamic crypto map: 

.

.

.

!Enable virtual private networking. 

vpdn enable


! Default L2TP VPDN group

vpdn-group 1

!

!Enables the LNS to accept dial in requests; specifies L2TP as the tunneling 
protocol; specifies the number of the virtual templates used to clone 
virtual-access interfaces; specifies an alternate IP address for a VPDN tunnel

accept-dialin. 

  protocol l2tp

  virtual-template 1

  source-ip 10.0.0.1

!

!Disables Layer 2 Tunneling Protocol (L2TP) tunnel authentication.

no l2tp tunnel authentication

!

!Defines an Internet Key Exchange (IKE) policy and assigns priority 1.

crypto isakmp policy 1

 encr 3des

 group 2

!

crypto isakmp policy 2

 encr 3des

 authentication pre-share

 group 2

!

!Defines a transform set.

crypto ipsec transform-set vpn esp-3des esp-md5-hmac

 mode transport

crypto mib ipsec flowmib history tunnel size 2

crypto mib ipsec flowmib history failure size 2

!

!Names the dynamic crypto map entry to create (or modify) and enters crypto map 
configuration mode.

crypto dynamic-map dyn_map 1

!Specifies which transform sets can be used with the crypto map entry

 set transform-set vpn

!Enables L2TP—IPSec support.

 set nat demux

.

.

.

Related Commands

Command
Description

crypto dynamic-map

Names the dynamic crypto map entry to create (or modify) and enters crypto map configuration mode.

crypto map

Names the static crypto map entry to create (or modify) and enters crypto map configuration mode.

show crypto dynamic-map

Displays information about dynamic crypto maps.

show crypto ipsec sa

Displays the settings used by current SAs.

show crypto map

Displays information about static crypto maps.


Copyright © 2005 Cisco Systems, Inc. All rights reserved