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Configuring Server Load Balancing

Server Load Balancing Overview

OmniSwitch AOS Release 6 Network Configuration Guide

page 44-7

You can configure up to 16 Server Load Balancing (SLB) clusters on a switch. Each cluster may consist 
of 32 servers, which allows for configuration of up to 256 physical servers per switch. The following 
sections describe SLB operational theory (see 

), an SLB example (

), and server health 

monitoring (see 

).

Note. Alcatel-Lucent also offers link aggregation, which combines multiple Ethernet links into one virtual 
channel. Please refer to 

for more information on 

link aggregation and dynamic link aggregation, and to 

 for information on static (OmniChannel) link aggregation.

An SLB cluster consists of a group of physical servers, also known as a server farm. The SLB cluster 
appears as one large virtual server, which is identified using one of the following methods:

• Virtual IP (VIP)—An IP address is assigned to the cluster (virtual server). Client requests destined for 

this VIP are routed (Layer-3 mode) to the servers that are members of the VIP cluster. Note that it is 
necessary to configure cluster servers with a loopback interface.

• Condition—A QoS policy condition name is assigned to the cluster (virtual server). Client requests that 

meet the criteria of the policy condition are bridged (Layer-2 mode) or routed (Layer-3 mode) to the 
servers that are members of the condition cluster. Note that it is not necessary to configure cluster 
servers with a loopback interface.

Note. See 

 for more information on configuring servers. See 

 for more information on configuring VIP and 

condition clusters.

The cluster mode refers to whether client requests are bridged (Layer-2 mode) or routed (Layer-3 mode) 
by the switch to the appropriate SLB cluster. A VIP cluster only supports Layer-3 mode, so request 
packets are always routed to the cluster. A condition cluster supports both Layer-2 and Layer-3 modes.

When the Layer-3 mode is active (VIP or condition clusters), routed packets are modified as follows:

• The source MAC address is changed to the MAC address for the switch router interface.

• The destination MAC address is changed to the MAC address of the destined server.

• The TTL value is decremented.

When the Layer-2 mode is active (condition clusters only), request packets are not modified and are only 
switched within the same VLAN domain. The Layer-2 or Layer-3 mode is selected when the condition 
cluster is configured on the switch. See 

 for more information.