ENILimitReachedException
AWS ENILimitReachedException means Lambda could not create an elastic network interface because ENI limits were reached during VPC setup (HTTP 502).
Last reviewed: February 12, 2026|Editorial standard: source-backed technical guidance
What Does ENI Limit Reached Exception Mean?
Lambda cannot allocate additional ENIs for VPC-connected execution, so scaling and cold starts fail until network interface capacity pressure is reduced.
Common Causes
- -VPC-enabled Lambda functions exhausted ENI quota in account/VPC context.
- -Concurrency spikes require more ENIs than allowed by current limits.
- -Multiple functions share constrained subnets/security groups, increasing ENI contention.
- -ENI cleanup lag plus burst scaling creates temporary exhaustion windows.
How to Fix ENI Limit Reached Exception
- 1Reduce burst concurrency or limit invocations while ENI pressure is high.
- 2Distribute functions across additional subnets/AZs and rebalance networking.
- 3Request relevant quota increases where applicable and validated.
- 4Retry with bounded backoff after ENI capacity recovers.
Step-by-Step Diagnosis for ENI Limit Reached Exception
- 1Inspect Lambda concurrency and ENI utilization metrics for affected functions.
- 2Review subnet/security-group assignments across all VPC-enabled Lambda workloads.
- 3Correlate ENI-limit errors with deploy bursts and scale events.
- 4Validate idle ENI cleanup and networking behavior in the target VPC.
ENI Capacity and Quota Analysis
- -Measure ENI consumption versus service quotas in affected VPC/subnets (example: burst scaling consumes available ENI slots faster than release cycle).
- -Correlate Lambda concurrency spikes with ENI allocation failures (example: sudden SQS fan-out drives VPC function cold starts beyond ENI headroom).
Subnet/IP and Concurrency Shaping
- -Audit subnet CIDR/IP capacity and distribution across functions (example: small /28 subnet exhausts assignable IPs during traffic burst).
- -Apply reserved concurrency and paced event-source scaling (example: cap function concurrency to keep ENI demand below safe ceiling).
How to Verify the Fix
- -Confirm Lambda VPC invokes succeed without ENI allocation failures.
- -Validate ENI utilization and concurrency headroom stay within safe operating range.
- -Re-run burst tests to ensure scaling no longer exhausts ENI capacity.
How to Prevent Recurrence
- -Set account/VPC ENI capacity alerts tied to Lambda concurrency growth.
- -Distribute high-concurrency functions across multiple subnets and AZs.
- -Load-test VPC-enabled Lambda scaling behavior before major traffic launches.
Pro Tip
- -maintain per-function ENI headroom budgets and block traffic ramp plans that would exceed modeled ENI demand in the target subnet set.
Decision Support
Compare Guide
429 Too Many Requests vs 503 Service Unavailable
Use 429 for caller-specific throttling and 503 for service-wide outages, so retry behavior, escalation paths, and incident ownership stay correct.
Compare Guide
AWS ThrottlingException vs GCP RESOURCE_EXHAUSTED
Compare AWS ThrottlingException and GCP RESOURCE_EXHAUSTED to separate rate limiting from quota/resource exhaustion and choose the remediation path.
Playbook
Rate Limit Recovery Playbook (429 / ThrottlingException / RESOURCE_EXHAUSTED)
Use this playbook to separate transient throttling from hard quota exhaustion and apply retry, traffic-shaping, and quota-capacity fixes safely.
Official References
Provider Context
This guidance is specific to AWS services. Always validate implementation details against official provider documentation before deploying to production.