Load Balancing over Multiple Gateways

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Revision as of 18:05, 21 December 2005 by Rieks (talk | contribs)
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The typical situation where you got one router and want to connect to two ISPs:

<Picture comes here>

Of course, you want to do load balancing. There are several ways how to do it. Depending on the particular situation, you may find one best suited for you.

Policy Routing based on Client IP Address

If you have a number of hosts, you may group them by IP addresses. Then, send the traffic out through Gateway 1 or 2 depending on the source IP address.

Let us assume we use for our workstations IP addresses from network The IP addresses are assigned as follows:

  • are used for Group A workstations
  • are used for Group B workstations
  • is ised for the router.

All workstations have IP configuration with the IP address from the relevant group, they all have network mask, and the default gateway

We will talk about DNS servers later.

Next, we need to configure policy routing. We have workstations devided into groups, where we can reffer to them using subnet addressing:

  • Group A is, i.e., addresses
  • Group B is, i.e., addresses

If you do not understand this, take the TCP/IP Basics course, or, look for some resources about subnetting on the Internet!

Adding Mangle Rules

We need to add two IP Firewall Mangle rules to mark the packets originated from Group A or B workstations.

For Group A, start with specifying Chain forward and Src. Address

Dual gw 20.jpg

Next, specify the Action mark routing and add New Routing Mark GroupA.

Dual gw 22.jpg

It is a good practice to add a comment as well. Your mangle rules might be interesting for someone else and for yourself as well after some time.

For Group B, specify

  • Chain forward and Src. Address
  • Action mark routing and New Routing Mark GroupB

Dual gw 24.jpg

All IP traffic coming from workstations is marked with the routing marks GroupA or GroupB. We can use these marks in the routing table.

Next, we should specify two default routes (destination with appropriate routing marks and gateways:

Dual gw 26.jpg

This thing is not going to work, unless you do masquerading for your LAN! The simpliest way to it is by adding one NAT rule for Src. Address and Action masquerade:

Dual gw 28.jpg

Test the setup by using traceroute to some IP address on the Internet!

From a workstation of Group A, it should go like this:

C:\>tracert -d

Tracing route to over a maximum of 30 hops

  1     2 ms     2 ms     2 ms
  2    10 ms     4 ms     3 ms

From a workstation of Group B, it should go like this:

C:\>tracert -d

Tracing route to over a maximum of 30 hops

  1     2 ms     2 ms     2 ms
  2    10 ms     4 ms     3 ms

You can specify the the DNS server for workstations quite freely, just make it can be reached (test it by using traceroute to the IP address of DNS server you are using)!