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Cisco ASA Series CLI Configuration Guide
Chapter 1 Configuring IPsec and ISAKMP
Configuring ISAKMP
Using NAT-T
To use NAT-T, you must perform the following site-to-site steps in either single or multiple context
mode:
Step 1 Enter the following command to enable IPsec over NAT-T globally on the ASA:
crypto isakmp nat-traversal natkeepalive
The range for the natkeepalive argument is 10 to 3600 seconds. The default is 20 seconds.
For example, enter the following command to enable NAT-T and set the keepalive value to one hour.
hostname(config)# crypto isakmp nat-traversal 3600
Step 2 Select the before-encryption option for the IPsec fragmentation policy by entering this command:
hostname(config)# crypto ipsec fragmentation before-encryption
This option lets traffic travel across NAT devices that do not support IP fragmentation. It does not impede
the operation of NAT devices that do support IP fragmentation.
Enabling IPsec with IKEv1 over TCP
IPsec/IKEv1 over TCP enables a Cisco VPN client to operate in an environment in which standard ESP
or IKEv1 cannot function or can function only with modification to existing firewall rules. IPsec over
TCP encapsulates both the IKEv1 and IPsec protocols within a TCP-like packet and enables secure
tunneling through both NAT and PAT devices and firewalls. This feature is disabled by default.
Note This feature does not work with proxy-based firewalls.
IPsec over TCP works with remote access clients. You enable it globally, and it works on all
IKEv1-enabled interfaces. It is a client to the ASA feature only. It does not work for LAN-to-LAN
connections.
The ASA can simultaneously support standard IPsec, IPsec over TCP, NAT-Traversal, and IPsec over
UDP, depending on the client with which it is exchanging data. IPsec over TCP, if enabled, takes
precedence over all other connection methods.
The VPN 3002 hardware client, which supports one tunnel at a time, can connect using standard IPsec,
IPsec over TCP, NAT-Traversal, or IPsec over UDP.
You enable IPsec over TCP on both the ASA and the client to which it connects.
You can enable IPsec over TCP for up to 10 ports that you specify. If you enter a well-known port, for
example port 80 (HTTP) or port 443 (HTTPS), the system displays a warning that the protocol associated
with that port no longer works on the public interface. The consequence is that you can no longer use a
browser to manage the ASA through the public interface. To solve this problem, reconfigure the
HTTP/HTTPS management to different ports.
The default port is 10000.
You must configure TCP port(s) on the client as well as on the ASA. The client configuration must
include at least one of the ports you set for the ASA.
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