User`s guide

Chapter 15 The Firewall Screens
User’s Guide
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15.4.3 The “Triangle Route” Problem
A traffic route is a path for sending or receiving data packets between two Ethernet devices.
You may have more than one connection to the Internet (through one or more ISPs). If an
alternate gateway is on the LAN (and its IP address is in the same subnet as the WiMAX
Modem’s LAN IP address), the “triangle route” (also called asymmetrical route) problem may
occur. The steps below describe the “triangle route” problem.
1 A computer on the LAN initiates a connection by sending out a SYN packet to a
receiving server on the WAN.
2 The WiMAX Modem reroutes the SYN packet through Gateway A on the LAN to the
WA N.
3 The reply from the WAN goes directly to the computer on the LAN without going
through the WiMAX Modem.
As a result, the WiMAX Modem resets the connection, as the connection has not been
acknowledged.
Figure 89 “Triangle Route” Problem
15.4.3.1 Solving the “Triangle Route” Problem
If you have the WiMAX Modem allow triangle route sessions, traffic from the WAN can go
directly to a LAN computer without passing through the WiMAX Modem 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 WiMAX Modem supports up to three logical
LAN interfaces with the WiMAX Modem 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 WiMAX Modem 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 WiMAX Modem reroutes the packet to Gateway A, which is in Subnet 2.
3 The reply from the WAN goes to the WiMAX Modem.
4 The WiMAX Modem then sends it to the computer on the LAN in Subnet 1.