Zywall / USG - WAN Trunk Load Balancing

Trunk Overview
Use trunks for WAN traffic load balancing to increase overall network throughput and reliability. Load balancing divides traffic loads between multiple interfaces. This allows you to improve the quality of service and maximize bandwidth utilization for multiple ISP links.

Maybe you have two Internet connections with different bandwidths. You could set up a trunk that uses spillover or weighted round-robin load balancing, so time-sensitive traffic (like video) usually goes through the higher-bandwidth interface. You might want to use the least load first load balancing to distribute the traffic load for other traffic.

Load Balancing Algorithms

The following sections describe the load balancing algorithms the Zyxel Device can use to decide which interface the traffic (from the LAN) should use for a session.

Least Load First

The least load first algorithm uses the current (or recent) outbound bandwidth utilization of each trunk member interface as the load balancing index(es) when deciding which interface a new session will be distributed. The outbound bandwidth utilization is defined as the measured outbound throughput over the available outbound bandwidth.

Here the Zyxel Device has two WAN interfaces connected to the Internet. The configured available outbound bandwidths for WAN 1 and WAN 2 are 512K and 256K, respectively.

The outbound bandwidth utilization is used as the load balancing index. In this example, the measured (current) outbound throughput of WAN 1 is 412K, and WAN 2 is 198K. The Zyxel Device calculates the load balancing index, as shown in the table below.

Since WAN 2 has a smaller load balancing index (meaning that it is less utilized than WAN 1), the Zyxel Device will send the subsequent new session traffic through WAN 2.

Weighted Round Robin

Round Robin scheduling services queues on a rotating basis and is activated only when an interface has more traffic than it can handle.

A queue is given an amount of bandwidth irrespective of the incoming traffic on that interface. This queue then moves to the back of the list. The next queue is given an equal amount of bandwidth and then moves to the end of the list, and so on, depending on the number of queues being used. This works in a looping fashion until a queue is empty.

The Weighted Round Robin (WRR) algorithm is best suited for situations when the two WAN interfaces' bandwidths are different. The WAN interfaces are assigned weights, and the interface with a larger weight gets more chances to transmit traffic than an interface with a smaller weight.

For example, in the figure below, the configured available bandwidth of WAN1 is 1M, and WAN2 is 512K. You can set the Zyxel Device to distribute the network traffic between the two interfaces by setting the weight of wan1 and wan2 to 2 and 1, respectively. The Zyxel Device assigns the traffic of two sessions to wan1 and one session's traffic to wan2 in each round of 3 new sessions.

Spillover

The spillover load balancing algorithm sends network traffic to the first interface in the trunk member list until the interface’s maximum allowable load is reached, sending the excess network traffic of new sessions to the next interface in the trunk member list. This continues as long as there are more member interfaces and traffic to be sent through them.

Suppose the first trunk member interface uses an unlimited access Internet connection, and the second is billed by usage. Spillover load balancing only uses the second interface when the traffic load exceeds the first interface threshold. This fully utilizes the first interface's bandwidth to reduce Internet usage fees and avoid overloading the interface.

In this example figure, the upper threshold of the first interface is set to 800K. The Zyxel Device sends network traffic of new sessions that exceed this limit to the secondary WAN interface.

• Trunk Summary Screen
  
  

Click Configuration > Network > Interface > Trunk to open the Trunk screen.
The Trunk Summary screen lists the configured trunks and the load balancing algorithm configured to use.

• Configuring a User-Defined Trunk
  
  

Click Configuration > Network > Interface > Trunk. In the User Configuration table, click the Add (or Edit) icon to open the following screen.
Use this screen to create or edit a WAN trunk entry.

• Configuring the System Default Trunk
  
  

In the Configuration > Network > Interface > Trunk screen and the System Default section, select the default trunk entry and click Edit to open the following screen.
Use this screen to change the load balancing algorithm and view the bandwidth allocations for each member interface.

Note: The available bandwidth is allocated to each member interface equally and cannot be changed for the default trunk.