
B-10
Cisco PIX Firewall and VPN Configuration Guide
78-15033-01
Appendix B Configuration Examples for Other Remote Access Clients
L2TP with IPSec in Transport Mode
information in the IP header. However, the Layer 4 header will be encrypted, limiting the examination
of the packet. Unfortunately, transmitting the IP header in clear text, transport mode allows an attacker
to perform some traffic analysis.
Figure B-4 IPSec in Tunnel and Transport Modes
IP HDR
23246
Data
Encrypted
Tunnel mode
IP HDR Data
Encrypted
IPSec HDRNew IP HDR
IP HDR Data
Transport mode
DataIPSec HDRIP HDR
Windows 2000 uses IPSec transport mode when tunneling L2TP data. Transport mode should be
configured on the PIX
Firewall to receive the L2TP IPSec transport mode data from a Windows 2000
client.
Configuring L2TP with IPSec in Transport Mode
To configure L2TP with IPSec in transport mode, perform the following steps:
Step 1 Specify IPSec to use transport mode rather than tunnel mode:
crypto ipsec transform-set trans_name mode transport
Step 2 Allow L2TP traffic to bypass conduit/access list checking:
sysopt connection permit-ipsec
sysopt connection permit-l2tp
Step 3 Instruct the PIX Firewall to accept L2TP dial-in requests:
vpdn group group_name accept dial-in l2tp
Step 4 Specify PPP protocol and authentication protocol (PAP, CHAP, or MS-CHAP):
vpdn group group_name ppp authentication pap/chap/mschap
Step 5 Specify the local address pool used to allocate the IP address to the client:
vpdn group group_name client configuration address local address_pool_name
Step 6 (Optional) Instruct the PIX Firewall to send DNS server IP addresses to the client:
vpdn group group_name client configuration dns dns_server_ip1 dns_server_ ip2
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