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Subnets selection

Feature availability

EKS KOPS GKE AKS
Random subnets selection (from available set) + + - -
Subnets selection by usage* +** - - -

* Select the subnet with the greatest number of free IP addresses

** Available if AWS cloud CNI is used with specific settings

Available subnets detection

Cluster subnets with subnet IP CIDR and availability zone are synced periodically with CAST AI. The autoscaler based on various rules decides from which subnets to choose when constructing in-memory nodes for autoscaling. The selection is influenced by:

  • Pod node selector for topology.cast.ai/subnet-id label.
  • Pod node affinity for topology.cast.ai/subnet-id label.
  • Availability zone on in-memory node(would choose only subnets in the same zone).
  • Chose zone based on constraints from other parts - e.g., Persistent Volume zone is affecting in-memory node zone.
  • Choose the least allocated zone.
  • Choose a random zone.

If subnet calculation is supported and we detect that all the available subnets are full, the pod will get a pod event with a message there is no subnet with enough available IP addresses.

Subnets usage calculation

Subnets usage calculation is available only for EKS and when AWS cloud CNI is used for networking.

Subnets usage is calculated based on CNI settings and instance type networking capabilities(max ENI count on instance type and ipv4 count per ENI).

CNI settings used to calculate used IP addresses:

Name Description Default
WARM_ENI_TARGET How many free ENIs should be attached as reserve. 1
WARM_IP_TARGET How many free secondary IPs should be kept as reserve.
MINIMUM_IP_TARGET Minimum IP count to be requested when adding node.
MAX_ENI Additional caping on instance type max ENI count.

CNI settings that disable subnets usage calculation:

Name Supported values
AWS_VPC_K8S_CNI_CUSTOM_NETWORK_CFG None or false
ENABLE_POD_ENI None or false
ENABLE_PREFIX_DELEGATION None or false
ENABLE_IPv6 None or false

How does the calculation work

The source of documentation is here.

Each instance type in AWS has limits on how many ENIs can be attached and how many IPs each ENI can have, those numbers are synchronized with CAST AI periodically and used by this algorithm.

Some key points:

  • Each ENI uses 1 IP for itself and all other IPs are secondary and can be used for pods, so always (max IPs for pods in ENI = max IPs per ENI - 1).
  • If we just attach ENI, 1 IP will always be used regardless of CNI settings.
  • If WARM_IP_TARGET is specified WARM_ENI_TARGET is not used.
  • If MAX_ENI < instances max ENI count, it works as an override for instance setting, otherwise instance setting is used.
  • All the pods that have hostNetwork:true don't get secondary IP and host IP is used for communication (as an example AWS CNI and kube-proxy) PS. They are still counted as PODs and are capped by podCount constraint on node.

Here is a detailed description of how WARM_ENI_TARGET, WARM_IP_TARGET, and MINIMUM_IP_TARGET work.

Useful commands for investigations

Command to get subnet IPs allocation - we consider that subnet is used only for this K8s cluster (some worker groups or security groups might use some IPs if they were created with this subnet and this could result in few IPs difference between calculation and actual allocation, bringing failed node creation instead of pod event in some edge cases), using same subnets for anything else than this cluster will make this feature work incorrectly.

aws ec2 describe-network-interfaces --filters Name=subnet-id,Values=subnet_id > subnet_id.yaml

Command to print all pods with IPs information, sorted by node.

kubectl get pod -o=custom-columns=NAME:.metadata.name,STATUS:.status.phase,NODE:.spec.nodeName,POD-IP:.status.podIP,HOST-IP:.status.hostIP --sort-by=.spec.nodeName  --all-namespaces