ZyXEL Communications ZyWALL 110 Series, User Manual, Page: 268 / 562

Chapter 19 Firewall
ZyWALL 110/310/1100 Series User’s Guide
268
19.2 The Firewall Screen
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). However, 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. 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 LAN 1 and the alternate gateway (A in the figure) in different subnets, all returning
network traffic must pass through the ZyWALL to the LAN. The following steps and figure describe
such a scenario.
1
A computer on the LAN1 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 LAN1 in Subnet 1.
Figure 162
Using Virtual Interfaces to Avoid Asymmetrical Routes
19.2.1 Configuring the Firewall Screen
Click Configuration > Firewall to open the Firewall screen. Use this screen to enable or disable
the firewall and asymmetrical routes, set a maximum number of sessions per host, and display the
configured firewall rules. Specify from which zone packets come and to which zone packets travel to
display only the rules specific to the selected direction. Note the following.
LAN1