Server Health Monitoring - D-Link DFL-1660 User Manual

10.4.5. Server Health Monitoring

If the connection-rate algorithm 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.
Regardless which algorithm is chosen, if a server goes down, traffic will be sent to other servers.
And when the server comes back online, it can automatically be placed back into the server farm
and start getting requests again.
10.4.5. Server Health Monitoring
SLB uses Server Health Monitoring to continuously check the condition of the servers in an SLB
configuration. SLB can monitor different OSI layers to check the condition of each server.
Regardless of the algorithms used, if a server is deemed to have failed, SLB will not open any more
connections to it until the server is restored to full functionality.
D-Link Server Load Balancing provides the following monitoring modes:
ICMP Ping
TCP Connection
Figure 10.11. Stickiness and Round-Robin
Figure 10.12. Stickiness and Connection-rate
This works at OSI layer 3. SLB will ping the IP address of each individual
server in the server farm. This will detect any failed servers.
This works at OSI layer 4. SLB attempts to connect to a specified port on
each server. For example, if a server is specified as running web services on
port 80, the SLB will send a TCP SYN request to that port. If SLB does not
receive a TCP SYN/ACK back, it will mark port 80 on that server as down.
SLB recognizes the conditions no response, normal response or closed port
response from servers.
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Chapter 10. Traffic Management