Installation guide

Also, the active LVS router dynamically monitors the overall health of the specific services on the real
servers through simple send/expect scripts. To aid in detecting the health of services that require
dynamic data, such as HTTPS or SSL, you can also call external executables. If a service on a real
server malfunctions, the active LVS router stops sending jobs to that server until it returns to normal
operation.
The backup LVS router performs the role of a standby system. Periodically, the LVS routers exchange
heartbeat messages through the primary external public interface and, in a failover situation, the
private interface. Should the backup LVS router fail to receive a heartbeat message within an
expected interval, it initiates a failover and assumes the role of the active LVS router. During failover,
the backup LVS router takes over the VIP addresses serviced by the failed router using a technique
known as ARP spoofing — where the backup LVS router announces itself as the destination for IP
packets addressed to the failed node. When the failed node returns to active service, the backup LVS
router assumes its backup role again.
The simple, two-tier configuration in Figure 1.17, “Two-Tier LVS Topology” is suited best for clusters
serving data that does not change very frequently — such as static web pages — because the
individual real servers do not automatically synchronize data among themselves.
1.8.2. T hree-T ier LVS T opology
Figure 1.18, “ Three-Tier LVS Topology” shows a typical three-tier LVS configuration. In the example,
the active LVS router routes the requests from the public network (Internet) to the second tier — real
servers. Each real server then accesses a shared data source of a Red Hat cluster in the third tier
over the private network.
Red Hat Ent erprise Linux 5 Clust er Suit e Overview
30