User's Manual

Chapter 16 IPSec VPN 125
Transport mo
de ESP with authentication is not compatible with NAT.
Table 71 VPN and NA
T
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport N
ESP Tunnel Y
16.3.6 VPN, NAT, and NAT Traversal
NAT is incompatible with the AH protocol in both transport and tunnel mode. An IPSec VPN using
the AH protocol digitally signs the outbound packet, both data payload and headers, with a hash
value appended to the packet, but a NAT device between the IPSec endpoints rewrites the source or
destination address. As a result, the VPN device at the receiving end finds a mismatch between the
hash value and the data and assumes that the data has been maliciously altered.
NAT is not normally compatible with ESP in tr
ansport mode either, but the Router’s NAT Traversal
feature provides a way to handle this. NAT traversal allows you to set up an IKE SA when there are
NAT routers between the two IPSec routers.
Figure 81 N
AT Router Between IPSec Routers
A
B
Normally you cannot set up an IKE SA with a NAT router between the two IPSec routers because the
NAT router changes the header of the IPSec packet. NAT traversal solves the problem by adding a
UDP port 500 header to the IPSec packet. The NAT router forwards the IPSec packet with the UDP
port 500 header unchanged. In the above figure, when IPSec router A tries to establish an IKE SA,
IPSec router B checks the UDP port 500 header, and IPSec routers A and B build the IKE SA.
For NAT traversal to work, you must:
• Use ESP security protocol (in either transport or tunnel mode).
• Use IKE keying mode.
• Enable NAT traversal on both IPSec endpoints.
• Set the NAT router to forward UDP port 500 to IPSec router A.