Priority Group Activation | Priority Group Activation of F5 BIG-IP LTM Load Balancer

Priority group activation allows the BIG-IP administrator to designate and backup sets of pool members with in a pool. This feature is often used to assist meeting client traffic demand when too many servers are unavailable.

Take a look at this diagram. As you can see all six servers are members of a single pool.

With priority group activation set to 2, and 3 of highest priority member available, lower priority member are not used.

We decides two available server is enough to handle client traffic loads. Just to be sure though we decides to use three. These three servers then should be the first to receive requests. Priority group activation makes this behavior possible.

We configures three servers with a priority of 10 and configures the others with a priority of 5. if priority group activation is set to BIG-IP will use available members with the highest priority number first. It will then add all pool member at the next lower priority until a total of two available member is obtained.

In this case, if atleast two priority 10 pool member are available, those servers will receive all the requests.

But lets suppose two of priority 10 pool members are marked offline. Since this means only one priority 10 members is left, we have now fallen below our 2 available pool members.

BIG-IP LTM then adds all available members at the next lower priority number atleast  2 are obtained. In this scenario load balancing occurs as shown. If another priority 10 members becomes available, BIG-IP no longer uses the priority 5 member.

Fall back Host (http only)

The fall back host feature, which is designed for the http protocol only comes into play if all members in a pool are unavailable.

If all members fail then client can be sent an http redirect.

 Using this fall back host feature BIG-IP sends an HTTP redirect to the client rather then sending no response at all.