Chapter 20 VPN
VMG8924-B10A and VMG8924-B30A Series User’s Guide
233
Figure 141 NAT Router Between IPSec Routers
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.
Finally, NAT is compatible with ESP in tunnel mode because integrity checks are performed over the
combination of the "original header plus original payload," which is unchanged by a NAT device. The
compatibility of AH and ESP with NAT in tunnel and transport modes is summarized in the following
table.
Y* - This is supported in the Device if you enable NAT traversal.
20.5.7 ID Type and Content
With aggressive negotiation mode (see Section 20.5.4 on page 231), the Device identifies incoming
SAs by ID type and content since this identifying information is not encrypted. This enables the
Device to distinguish between multiple rules for SAs that connect from remote IPSec routers that
have dynamic WAN IP addresses.
Regardless of the ID type and content configuration, the Device does not allow you to save multiple
active rules with overlapping local and remote IP addresses.
With main mode (see Section 20.5.4 on page 231), the ID type and content are encrypted to
provide identity protection. In this case the Device can only distinguish between up to 12 different
incoming SAs that connect from remote IPSec routers that have dynamic WAN IP addresses. The
Device can distinguish up to 48 incoming SAs because you can select between three encryption
algorithms (DES, 3DES and AES), two authentication algorithms (MD5 and SHA1) and eight key
groups when you configure a VPN rule (see Section 20.2 on page 221). The ID type and content act
as an extra level of identification for incoming SAs.
Table 106 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport Y*
ESP Tunnel Y
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