Figure 185 Triangle Route: Using Virtual Interfaces; Alerts; Asymmetrical Routes; Virtual Interfaces And Asymmetrical Routes - ZyXEL Communications Unified Security Gateway ZyWALL 300 User Manual

19.4 Alerts

You can choose to generate an alert or log when a rule is matched and have the ZyWALL send
an immediate e-mail message to you. Otherwise, see the logs created (for the categories you
specified) in the View Log screen. Refer to the chapter on logs for details.

19.5 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 virtual interfaces to put the
ZyWALL and the backup gateway on separate subnets.

19.5.1 Virtual Interfaces and Asymmetrical Routes

You can use virtual interfaces instead of allowing asymmetrical routes. Virtual interfaces
allow you to partition your network into logical sections over the same interface. See the
chapter about interfaces for more information.
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 185 Triangle Route: Using Virtual Interfaces

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