Currently, the following Three-Tier LB has been setup:
Preferred: Global Availability
Fallback: Drop Packet
And two pools ae configured in the Member Order:
0 - Pool A
1 - Pool B
Referring to the following document, is the sentence marked in bold desribing the behavior of GTM.
And if so, when Fallback is set to "Drop Packet" does this mean that the next server would be selected. For example, if Pool A becomes unavailable, GTM would not select one from Pool B.
The Global Availability load balancing method distributes DNS name resolution requests based on the order of resources in a list. Using global availability, BIG-IP® DNS sends a request to the first available resource in a list. Only when a resource becomes unavailable does BIG-IP DNS send requests to the next resource in the list. You can only select the next resource in the list if the Fallback load balancing method is set to None. Over time, the first resource in the list receives the most requests and the last resource in the list receives the least requests.
Regarding your questions "For example, if Pool A becomes unavailable, GTM would not select one from Pool B", please check the below.
using the global availability, will respond with pool B if only pool A is down, and this is how the global availability works. regarding the fallback method, it will be used only if the prefered and alternate method were not used. which means fallaback will be used and GTM will drop your request if both pools are down for some reason.
here is an example:
GTM sent the second IP since the first member was unavaulable. and when both members are down, the request will timeout because GTM is not responding any more and will drop the request (because fallback is set to drop)
[root@DMS-X root]# dig @192.168.7.111 www.auction.com
; <<>> DiG 9.2.0 <<>> @192.168.7.111 www.auction.com
;; global options: printcmd
;; connection timed out; no servers could be reached
Regarding the bold sentence that tou were referring to, I'm not sure what exactly the situation which will reflect on the dns resolution but from the same link, it is mentioned "BIG-IP DNS distributes DNS name resolution requests to the first available virtual server in a pool. BIG-IP DNS starts at the top of a manually configured list of virtual servers and sends requests to the first available virtual server in the list. Only when the virtual server becomes unavailable does BIG-IP DNS send requests to the next virtual server in the list." which is the expected behaviour and similar to the above test results.
Thank you Mohamed.
The confusion does kick in when reading the previous posted: "You can only select the next resource in the list if the Fallback load balancing method is set to None." Depending on the level of reading comprehension, one could come to the concuclsion that the Global Availability load balancing method does only select a member of Pool B [in case members in Pool A are not respnding anymore], if the Fallback load balancing method is is to None.
However, following up on your test, you simulated the unavailability of Pool A, and as expected, GTM select one from Pool B - correct, despite the fact that Fallback method is set to "Drop Packet."
Therefore the question remains, what the author of this paragraph is actually referring to?
I would appreciate if an F5 employee could comment on my question to get a better understanding.