Server Health Checks - D-Link DFL-1600 User Manual

266 Chapter 24. Server Load Balancing (SLB)
An example is shown in the figures below (Figure 24.2, Figure 24.3, and
Figure 24.4). In this example, the firewall is responsible for balancing
connections from 3 clients with different addresses to 2 servers. Stickiness is
set. When Round Robin algorithm is used, the first arriving requests R1
and R2 from Client 1 are both assigned to one sever, say Server 1,
according to stickiness. The next request R3 from Client 2 is then routed to
Server 2. When R4 from Client 3 arrives, Server 1 gets back its turn again
and will be assigned with R4. If Connection Rate is applied instead, R1
and R2 will be sent to the same server because of stickiness, but the
subsequent requests R3 and R4 will be routed to another server since the
number of new connections on each server within the Window Time span is
counted in for the distribution.
Figure 24.2: A SLB distribution algorithm example.
Regardless which algorithm is chosen, if a server goes down, traffic will be
sent to other servers. And when the sever comes back online, it can
automatically be placed back into the server farm and start getting requests
again.
24.2.3

Server Health Checks

Performing various checks to determine the "health" condition of servers is
one of the most important benefits of the SLB. At different OSI layers,
D-Link firewalls can carry out certain network-level checks.
When a server fails, the firewall removes it from the active server list, and
will not route any packet to this server until it resumes back. An ICMP
D-Link Firewalls User's Guide