ZyXEL Communications USG40 User Manual page 348

Load Balancing Algorithms
The following sections describe the load balancing algorithms the ZyWALL/USG can use to decide
which interface the traffic (from the LAN) should use for a session. In the load balancing section, a
session may refer to normal connection-oriented, UDP or SNMP2 traffic
you configure on the ZyWALL/USG refers to the actual bandwidth provided by the ISP and the
measured bandwidth refers to the bandwidth an interface is currently using.
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 making decisions about to which
interface a new session is to be distributed. The outbound bandwidth utilization is defined as the
measured outbound throughput over the available outbound bandwidth.
Here the ZyWALL/USG has two WAN interfaces connected to the Internet. The configured available
outbound bandwidths for WAN 1 and WAN 2 are 512K and 256K respectively.
Figure 244 Least Load First Example
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 ZyWALL/USG
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
ZyWALL/USG will send the subsequent new session traffic through WAN 2.
Table 123 Least Load First Example
OUTBOUND
INTERFACE
AVAILABLE (A)
WAN 1 512 K
WAN 2 256 K
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 bandwidths set
for the two WAN interfaces are different. Similar to the Round Robin (RR) algorithm, the Weighted
Round Robin (WRR) algorithm sets the ZyWALL/USG to send traffic through each WAN interface in
Chapter 10 Interfaces
MEASURED (M)
412 K
198 K
ZyWALL/USG Series User's Guide
348
The available bandwidth
.
LOAD BALANCING INDEX
(M/A)
0.8
0.77