Chapter 12 IPSec VPN

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 Figure 137 on page 271, 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.

Table 78 VPN and NAT

SECURITY

MODE

NAT

PROTOCOL

 

 

AH

Transport

N

 

 

 

AH

Tunnel

N

 

 

 

ESP

Transport

Y*

 

 

 

ESP

Tunnel

Y

 

 

 

Y* - This is supported in the P-2812HNU-51c if you enable NAT traversal.

12.5.7 ID Type and Content

With aggressive negotiation mode (see Section 12.5.4 on page 270), the P- 2812HNU-51c identifies incoming SAs by ID type and content since this identifying information is not encrypted. This enables the P-2812HNU-51c 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 P-2812HNU-51c does not allow you to save multiple active rules with overlapping local and remote IP addresses.

With main mode (see Section 12.5.4 on page 270), the ID type and content are encrypted to provide identity protection. In this case the P-2812HNU-51c can only distinguish between up to 12 different incoming SAs that connect from remote

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P-2812HNU-51c User’s Guide