--> Whenever the FQDN in the DNS Query matches the Wide IP then it selects the Pool using Pool level Load balancing then after that it selects the Virtual server within the pool and reply with the Virtual Server IP address in the response to the client.
1) Round Robin Load Balancing Method
--> It is recommended to use the round-robin load-balancing method when all the Virtual Servers need to serve the traffic equally.
--> Distributes the DNS Query requests equally to all the Virtual Servers configured within the Pool.
--> The Virtual Server IP address which is unavailable/down will not be taken into consideration.
2) Ratio Load Balancing Method
--> It is recommended to use the ratio load balancing method when you want some of the virtual servers to receive the traffic more than other virtual servers within the pool.
--> By default, The ratio value 1 is assigned to all the virtual servers configured within the Pool.
--> You need to configure higher ratio value on the virtual server to receive more traffic than others.
--> This Method is useful when you have the servers with high processing capabilities and servers with lower processing capabilities in the same pool.
3) Topology Load Balancing
--> Topology Load Balancing is used to select the virtual server IP address in the DNS response depends upon the location/closeness of the Local DNS Server.
--> Requires Topology records need to be created on the BIG IP DNS.
--> This Method is useful when you have the multiple Data Center in multiple locations for the same applications and want to return the Virtual Server IP address which is closer to the Local DNS.
4) Static Persist
--> It is recommended to use the Static Persist Load Balancing algorithm when you want the traffic from a particular Local DNS Server always resolves to only one Virtual Server.
--> Similar to Global Availability Load Balancing Algorithm, It forwards the first Virtual Server IP address which is available in the DNS Query Response.
--> After that the BIG IP DNS creates the hash value of LDNS and maps the Virtual Server IP address with hash value so that the BIG IP DNS always return the same virtual server IP address in the DNS response.
--> If the selected virtual server is unavailable then the BIG IP DNS selects the other available virtual server and sends the same in the DNS Query response.
--> When the actual selected virtual server becomes available then the BIG IP DNS again provides the original virtual server in the response.
--> This Method is useful when you have stateful applications such as online/ecommerce/trading which needs to have the client access the same IP address always in the same session.
5) Fallback IP
--> Fallback IP address is provided as the DNS Query Response to the Local DNS Server.
--> Fallback IP address generally the DR Virtual Server IP address.
6) Drop Packet
--> It is recommended to use the Drop Packet Load balancing method when you do not want to return any virtual server IP address in the DNS response.
--> BIG IP DNS receives the DNS Query from Local DNS Server and then the BIG IP DNS simply drops the DNS query.
--> After some time the LDNS Server will try for another DNS Server to get the response as it did not get any response from the BIG IP DNS.
--> This Method is useful when you want the BIG IP DNS not to respond for the DNS Query.
7) Global Availability
--> It is recommended to use the Global Availability Load Balancing algorithm when you want the traffic to be sent to only one virtual server always.
--> Uses the order of the virtual server configured within the pool.
--> This method is useful when you want the traffic to be only forwarded to Primary Data Center and if the Primary Data Center goes down then forward the traffic to Secondary Data Center or DR.
--> Once the Primary Data Center comes up then all the traffic is forwarded to Primary Data Center from Secondary Data Center
8) Return to DNS
--> Return to DNS Load balancing makes the BIG IP DNS to follow the order of DNS Express/DNS Cache/Bind/Load Balancing Pool depends upon what is enabled on the DNS profile.
9) None
--> None Load Balancing makes the BIG IP DNS not to perform any kind of Load Balancing.
Ref: F5com
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