D-Link DFL-210-WCF-12 Product Manual - Page 477
Server Health Monitoring, ICMP Ping, TCP Connection
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10.4.5. Server Health Monitoring Chapter 10. Traffic Management Figure 10.11. Stickiness and Round-Robin 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. Figure 10.12. Stickiness and Connection-rate 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 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. 477