Figure 92 Ideal Firewall Setup; Figure 93 "Triangle Route" Problem - ZyXEL Communications P-660HN-F1 User Manual

Figure 92 Ideal Firewall Setup

9.5.4.1 The "Triangle Route" Problem
A traffic route is a path for sending or receiving data packets between two Ethernet devices.
You may have more than one connection to the Internet (through one or more ISPs). If an
alternate gateway is on the LAN (and its IP address is in the same subnet as the ZyXEL
Device's LAN IP address), the "triangle route" (also called asymmetrical route) problem may
occur. The steps below describe the "triangle route" problem.
1 A computer on the LAN initiates a connection by sending out a SYN packet to a
receiving server on the WAN.
2 The ZyXEL Device reroutes the SYN packet through Gateway A on the LAN to the
WAN.
3 The reply from the WAN goes directly to the computer on the LAN without going
through the ZyXEL Device.
As a result, the ZyXEL Device resets the connection, as the connection has not been
acknowledged.

Figure 93 "Triangle Route" Problem

9.5.4.2 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.
P-660HN-F1 User's Guide
LAN
1
2
LAN
1
2
3
WAN
WAN
ISP 1
ISP 2
A
Chapter 9 Firewalls
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