ZyXEL Communications NBG410W3G - Manual (Page 173)

Chapter 9 Firewall
NBG410W3G Series User’s Guide
173
9.6 Asymmetrical Routes
If an alternate gateway on the LAN has an IP address in the same subnet as the ZyXEL
Device’s LAN IP address, return traffic may not go through the ZyXEL Device. This is called
an asymmetrical or “triangle” route. This causes the ZyXEL Device to reset the connection, as
the connection has not been acknowledged.
You can have the ZyXEL Device 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 ZyXEL Device. A better solution is to use IP alias to put the ZyXEL
Device 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 ZyXEL Device 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 ZyXEL Device reroutes the packet to Gateway A, which is in Subnet 2.
3 The reply from the WAN goes to the ZyXEL Device.
4 The ZyXEL Device then sends it to the computer on the LAN in Subnet 1.
Figure 101 Using IP Alias to Solve the Triangle Route Problem
9.7 Firewall Default Rule
Click SECURITY > FIREWALL to open the Default Rule screen.
Use this screen to configure general firewall settings.
1
2
Internet
3
LAN
A
ISP 1
ISP 2
4
WAN
Subnet 1
Subnet 2