The Firewall Screen; Configuring The Firewall Screen; Figure 253 Using Virtual Interfaces To Avoid Asymmetrical Routes - ZyXEL Communications ZyWALL USG 100 Series User Manual

20.2 The Firewall Screen

Asymmetrical Routes
If an alternate gateway on LAN1 has an IP address in the same subnet as the ZyWALL's
LAN1 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 LAN1 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 LAN1. The following steps and figure
describe such a scenario.
1 A computer on 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 LAN1 in Subnet 1.

Figure 253 Using Virtual Interfaces to Avoid Asymmetrical Routes

20.2.1 Configuring the Firewall Screen

Click 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.
• If you enable intra-zone traffic blocking (see the chapter about zones), the firewall
automatically creates (implicit) rules to deny packet passage between the interfaces in the
specified zone.
ZyWALL USG 100/200 Series User's Guide
Chapter 20 Firewall
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