ZyXEL p-334 Manual Do Utilizador

Prestige 334 User’s Guide

Chapter 15 VPN Screens

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When you initiate an IPSec tunnel with keep alive enabled, the Prestige automatically 
renegotiates the tunnel when the IPSec SA lifetime period expires (

 for more on the IPSec SA lifetime). In effect, the IPSec tunnel becomes an “always 

on” connection after you initiate it. Both IPSec routers must have a Prestige-compatible keep 
alive feature enabled in order for this feature to work.

If the Prestige has its maximum number of simultaneous IPSec tunnels connected to it and 
they all have keep alive enabled, then no other tunnels can take a turn connecting to the 
Prestige because the Prestige never drops the tunnels that are already connected. 

NAT traversal allows you to set up a VPN connection when there are NAT routers between 
IPSec routers A and B.

Figure 61   NAT Router Between IPSec Routers

Normally you cannot set up a VPN connection with a NAT router between the two IPSec 
routers because the NAT router changes the header of the IPSec packet. In the previous figure, 
IPSec router A sends an IPSec packet in an attempt to initiate a VPN. The NAT router changes 
the IPSec packet’s header so it does not match the header for which IPSec router B is 
checking. Therefore, IPSec router B does not respond and the VPN connection cannot be built.

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. IPSec router 
B checks the UDP port 500 header and responds. IPSec routers A and B build a VPN 
connection.

For NAT traversal to work you must:

• Use ESP security protocol (in either transport or tunnel mode).
• Use IKE keying mode.

Note: When there is outbound traffic with no inbound traffic, the 
Prestige automatically drops the tunnel after two minutes.