While troubleshooting an Application Centric network cluster, an engineer sees that an APIC has gone offline.
What is the minimum number of APICs required for a production ACI fabric to continue to operate?
A. 1
B. 2
C. 3
D. 4
What is the minimum number of APICs required for a production ACI fabric to continue to operate?
To understand why three APICs is the recommended minimum you must understand how the APICs distribute information between the three. All parts of ACI are datasets generated and processed by the Distributed Policy Repository and that data for those APICs functions are partitioned into logically banded subsets called shards (like DB shard). a Shard is then broken into three replicas or copies. each APIC has a replica for every shard but only 1 APIC is the master for a particular replica/shard. This is a way to distribute the workload evenly and load balance processing across the cluster of 3 as well as a fail safe in case an APIC goes down.
Now that the theory is out of the way, imagine one of your three APICs goes down. the remaining two will negotiate who will now be the master for the shards that the down APIC was in charge of. Workload is then load balance to the two and the cluster becomes fully fit again. Working with 2 APICs is really unadvised due to the split brain condition. This occurs when APIC 1 and APIC 2 thing they are both leaders for a shard and cannot agree so the shard is in contention and the cluster is unfit/”data layer partially diverged”. with the cluster in this state it is unadvised to make changes in the GUI, i don’t remember if its even allowed.
With the case of only 1 APIC, that APIC does all the work, it is the leader for all shards but if it goes down then you can not make any changes at all. data plane will continue forwarding but since no APIC, theres no way to create new policies or changes.
https://community.cisco.com/t5/application-centric/apic-cluster-why-minimum-of-3-controllers/td-p/2614489
should be 1. (min APIC for production, recommend APIC is 3)