ZyXEL Communications P-2612HW-F1 User Manual page 250

Chapter 12 Firewall
"triangle route" (also called asymmetrical route) problem may occur. The steps
below describe the "triangle route" problem.
A computer on the LAN initiates a connection by sending out a SYN packet to a
1
receiving server on the WAN.
The ZyXEL Device reroutes the SYN packet through Gateway A on the LAN to the
2
WAN.
The reply from the WAN goes directly to the computer on the LAN without going
3
through the ZyXEL Device.
As a result, the ZyXEL Device resets the connection, as the connection has not
been acknowledged.
Figure 136 "Triangle Route" Problem
12.5.4.2 Solving the "Triangle Route" Problem
If you have the ZyXEL Device allow triangle route sessions, traffic from the WAN
can go directly to a LAN computer without passing through the ZyXEL Device and
its firewall protection.
Another solution is to use IP alias. IP alias allows you to partition your network
into logical sections over the same Ethernet interface. Your ZyXEL Device supports
up to three logical LAN interfaces with the ZyXEL Device being the gateway for
each logical network.
It's like having multiple LAN networks that actually use the same physical cables
and ports. By putting your LAN and Gateway A in different subnets, all returning
network traffic must pass through the ZyXEL Device to your LAN. The following
steps describe such a scenario.
A computer on the LAN initiates a connection by sending a SYN packet to a
1
receiving server on the WAN.
250
LAN
1
2
3
WAN
ISP 1
ISP 2
A
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