ZyXEL Communications P-335WT - Manual (Page 219)

P-335 Series User’s Guide
Chapter 17 VPN Screens 219
17.6 Keep Alive
When you initiate an IPSec tunnel with keep alive enabled, the Prestige automatically
renegotiates the tunnel when the IPSec SA lifetime period expires (
the IPSec Algorithms
section 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.
Note: When there is outbound traffic with no inbound traffic, the
Prestige automatically drops the tunnel after two minutes.
Note:
17.7 NAT Traversal
NAT traversal allows you to set up a VPN connection when there are NAT routers between
IPSec routers A and B.
Figure 99 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.
17.7.1 NAT Traversal Configuration
For NAT traversal to work you must:
• Use ESP security protocol (in either transport or tunnel mode).