ZyXEL Communications eircom D1000 User Manual page 163

the "triangle route" (also called asymmetrical route) problem may occur. The steps below describe
the "triangle route" problem.
A computer on the LAN initiates a connection by sending out a SYN packet to a receiving server on
1
the WAN.
The Device reroutes the SYN packet through Gateway A on the LAN to the WAN.
2
The reply from the WAN goes directly to the computer on the LAN without going through the
3
Device.
As a result, the Device resets the connection, as the connection has not been acknowledged.
Figure 110 "Triangle Route" Problem
13.6.4.2 Solving the "Triangle Route" Problem
If you have the Device allow triangle route sessions, traffic from the WAN can go directly to a LAN
computer without passing through the Device and its firewall protection.
Another solution is to use IP alias. IP alias allows you to partition your network into logical sections
over the same Ethernet interface. Your Device supports up to three logical LAN interfaces with the
Device being the gateway for each logical network.
It's like having multiple LAN networks that actually use the same physical cables and ports. By
putting your LAN and Gateway A in different subnets, all returning network traffic must pass
through the Device to your LAN. The following steps describe such a scenario.
A computer on the LAN initiates a connection by sending a SYN packet to a receiving server on the
1
WAN.
The Device reroutes the packet to Gateway A, which is in Subnet 2.
2
The reply from the WAN goes to the Device.
3
The Device then sends it to the computer on the LAN in Subnet 1.
4
eircom D1000 modem User's Guide
LAN
1
2
3
A
WAN
ISP 1
ISP 2
Chapter 13 Firewall
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