Asymmetrical Routes; Asymmetrical Routes And Ip Alias; Firewall Default Rule (Router Mode); Figure 90 Using Ip Alias To Solve The Triangle Route Problem - ZyXEL Communications ZYWALL P1 User Manual

Chapter 9 Firewall

9.6 Asymmetrical Routes

If an alternate gateway on the LAN has an IP address in the same subnet as the ZyWALL's
LAN IP address, return traffic may not go through the ZyWALL. This is called an
asymmetrical or "triangle" route. This causes the ZyWALL to reset the connection, as the
connection has not been acknowledged.
You can have the ZyWALL permit the use of asymmetrical route topology on the network
(not reset the connection).
Allowing asymmetrical routes may let traffic from the WAN go directly to the LAN without
passing through the ZyWALL. A better solution is to use IP alias to put the ZyWALL and the
backup gateway on separate subnets.

9.6.1 Asymmetrical Routes and IP Alias

You can use IP alias instead of allowing asymmetrical routes. IP Alias allow you to partition
your network into logical sections over the same interface.
By putting your LAN and Gateway A in different subnets, all returning network traffic must
pass through the ZyWALL to your LAN. The following steps describe such a scenario.
1 A computer on the LAN initiates a connection by sending a SYN packet to a receiving
server on the WAN.
2 The ZyWALL reroutes the packet to Gateway A, which is in Subnet 2.
3 The reply from the WAN goes to the ZyWALL.
4 The ZyWALL then sends it to the computer on the LAN in Subnet 1.

Figure 90 Using IP Alias to Solve the Triangle Route Problem

9.7 Firewall Default Rule (Router Mode)

Click SECURITY > FIREWALL to open the Default Rule screen.
Use this screen to configure general firewall settings when the ZyWALL is set to router mode.
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ZyWALL P1 User's Guide