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We received a Facebook question on Sunday asking about BIG-IP Pool Priority.  The scenario is this:  you have a pool with 9 pool members and three priority groups defined (three pool members per priority group) and minimum active members set to 2 (see details below).  If the pool members start going down (disabled for some reason), then how does the BIG-IP handle the load balancing for the remaining active pool members?  In this edition of Lightboard Lessons, we dive into the details of how all this happens.  

Here's the configuration in the question:

\npool my_pool {\n\nlb_mode fastest\n\nmin active members 2\n\nmember A-IP priority 1\n\nmember B-IP priority 1\n\nmember C-IP priority 1\n\nmember D-IP priority 2\n\nmember E-IP priority 2\n\nmember F-IP priority 2\n\nmember G-IP priority 3\n\nmember H-IP priority 3\n\nmember J-IP priority 3\n\n}

We received a Facebook question on Sunday asking about BIG-IP Pool Priority.  The scenario is this:  you have a pool with 9 pool members and three priority groups defined (three pool members per priority group) and minimum active members set to 2 (see details below).  If the pool members start going down (disabled for some reason), then how does the BIG-IP handle the load balancing for the remaining active pool members?  In this edition of Lightboard Lessons, we dive into the details of how all this happens.  

Here's the configuration in the question:

\npool my_pool {\n\nlb_mode fastest\n\nmin active members 2\n\nmember A-IP priority 1\n\nmember B-IP priority 1\n\nmember C-IP priority 1\n\nmember D-IP priority 2\n\nmember E-IP priority 2\n\nmember F-IP priority 2\n\nmember G-IP priority 3\n\nmember H-IP priority 3\n\nmember J-IP priority 3\n\n}

Very clear john , thks again 

Question: If all but one pool member is up, and the minimum active pool members is 2, wouldn't the LTM mark the entire virtual server the pool is assigned to as down? I wouldn't want traffic to go just one pool member if the pool member could not handle the load.

@RandyTittleman, great question! The BIG-IP tends to err on the side of availability here, so it will still send traffic to a single pool member even if the minimum active members is set to 2.

@John Wagnon,

Good Explanation.

Rgds, Thx, Neeraj

HI John Wagnon,

lets example:If i have two members with highest priority and active less than value is equal 1 and admin disabled one pool members but monitor status is still up. Will BIGIP add another pool in rotation or not?

Second question: Will BIGIP add whole pool in load balancing rotation or only one member. I think it should add whole pool with same priority otherwise how it would consider which pool member i have to add with same priority.

I asked this question because bootcamp have wrong example on priority based activation.

@keshavArora, thanks for the question! If the min active members is set to 1 and you still have one available pool member in the highest priority group, then the BIG-IP will not add another pool member into the rotation. Because it has 1 pool member available (and the min active is set to 1), then it will use that pool member for all the traffic. \n

If the situation arises where an entire priority group is disabled and the BIG-IP needs to enable a pool member from another priority group to achieve the min active number, then it will enable all the pool members of the next-highest priority group (the entire priority group). I hope this helps!

Hi,

Great explanation, but it could be just amazing :-) Missing part for me is some description of opposite process, I mean what will happen when some pool members in different groups will be restored, especially when it happens in groups with lower priority than 3. So for example we have 1 pool member with priority 3 up, and one with priority 1 and then two members with priority 2 are up. I assume that priority 1 members will be excluded from LB but am I right?

Piotr

@Piotr...great question! As pool members are restored in priority groups, the BIG-IP will send traffic to the highest priority group that has pool members available and that satisfies the minimum active threshold. In your example, the BIG-IP would send traffic to groups 2 and 3 because group 3 is the highest priority (but it only has 1 pool member available and the min active is 2), and group 2 because it has at least one member available to fill out the min active number of pool members (min active is 2). In this case, priority group 1 would not serve traffic.

Thanks a lot for clarification, I should think twice before getting to the conclusion, it's obvious now that traffic will not be directed only to priority 3 grup before it will have at least two members. Old rule works \"Think twice before doing something\" :-)

Piotr

@John Wagnon...Thanks for the Answer. What about administrator disabled members but monitor up. Will it count as down or UP.