F5 LTM VE – Load balancing methods

So now that we are getting comfortable with the LTM, we should discuss the various types of load balancing methods.  The LTM provides for lots of different types as shown below…

F5 breaks them into two groups, static and dynamic.  Let’s go through them one at a time and discuss each.

Round Robin (Static)
Its the default load balancing type.  Everything we’ve seen up to this point has been round robin.  It provides an even distribution between all available pool members.

Ratio (Static)
Ratio mode allows you to define a ratio for each server.  This is a good mode to use when servers have varying degrees of processing power.  A ratio of…
WebHost1 – 3
WebHost2 – 1
WebHost3 – 1
Would result in the host being directed to WebHost1, WebHost1, WebHost1, Webhost2, WebHost3, Webhost1, Webhost1, Webhost1, Webhost2, etc.

Least Connections (Dynamic)
This mode distributes connections between web servers based on the current connection count between the F5 and the web server.  Its not possible for the F5 to know of any other connections coming from other hosts so it only uses its own connection table to determine where to send the next request.  So let’s test this one out to see what happens.  I’m going to open a http request to one of the web servers.  To do this, I’m actually going to download a large file from both of them to keep the sessions open long enough to test.  With the file being downloaded we can see that we have a open connection to Webhost1…


Any future requests will take this into account.  Browsing to that VIP will point us to the remaining two web servers.  Try hitting the VIP and see what happens…





As you can see, we aren’t getting any requests going to Webhost1 at this point.

Fastest distributes connections based on the server response time.  This would be useful in a situation where previous requests may put varying degrees of a load on a server, hence slowing down future requests.

Least Sessions
Much like least connections but based on the session (F5 calls a persistence record that’s valid a session) count.  The member with the least amount of sessions will get the next connection.

Weighted Least Connections
In order to use this method each member must have a defined (not 0) connection limit.  The LTM then calculates the member capacity by as a ratio of current connections to the connection limit.  The server with the smallest percentage of used capacity gets the next session.

This is sort of an odd one.  The system assigns ratios to the members dynamically.  The LTM checks the connection count on each member every second.  The average is calculated and members with less than average connection counts get assigned a ratio of 3 and the those with a greater than average connection count get assigned a ratio of 2.

Same as observed but uses 4 rather than 3 for less then average and 1 rather then 2 for greater than average.

Dynamic Ratio
A much more complex method of load balancing where particular brands of servers can be configured with special monitors and the load balancing method is based on various calculations of these metrics.

So that’s all of them.  I tried to show examples of some but couldn’t show all of them in a lab setup.  Next up is persistence.  I might talk about profiles before that, not sure yet.

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