Introduction

Enterprises increasingly rely on persistent remote connectivity to support mobile workforce, branch offices, and cloud-hosted resources. An Always-On IKEv2 VPN provides a reliable, secure and manageable solution by establishing a persistent IPsec tunnel between endpoints and the corporate network. This guide walks through the architecture, security considerations, deployment patterns, and operational practices to build a scalable Always-On IKEv2 VPN for enterprise environments.

Why IKEv2 for Always-On VPN?

IKEv2 is the modern Internet Key Exchange protocol that negotiates IPsec Security Associations (SAs). It is preferred for Always-On deployments because it supports:

  • Mobility and multihoming (MOBIKE) for seamless roaming between networks and IP changes.
  • Fast rekeying and resiliency to recover from transient network interruptions.
  • Efficient packet encapsulation and NAT Traversal (NAT-T).
  • Flexible authentication methods: certificates, EAP, Pre-Shared Keys (PSKs).

High-Level Architecture

A robust Always-On IKEv2 deployment typically consists of:

  • Redundant VPN gateways (virtual or physical) in a high-availability pair or cluster.
  • A strong Public Key Infrastructure (PKI) for certificate issuance and lifecycle management.
  • Endpoint management (MDM/EMM) to enforce device configuration and distribute profiles.
  • Centralized authentication: RADIUS/AAA servers, optionally integrated with LDAP/Active Directory.
  • Monitoring and logging systems for telemetry, session visibility and incident response.

Design Considerations

Authentication and Identity

For enterprise-grade security, prefer certificate-based authentication for both server and client. Certificates issued by an internal or trusted CA reduce reliance on user-memorized secrets. In scenarios requiring conditional access, combine certificates with EAP (e.g., EAP-TLS) or certificate + MFA via RADIUS to enforce multi-factor policies.

IP Addressing and Routing

Decide whether to use a centrally-assigned enterprise subnet (managed via IP pools) or split tunneling. For sensitive data and internal-only applications, prefer full tunnel (all traffic routed through VPN). If bandwidth or latency to cloud services matters, implement selective split tunneling based on destination prefixes and application-aware routing at the gateway.

Security Policies and Cipher Suites

Enforce contemporary cryptographic suites. Recommended proposals include:

  • IKEv2 exchanges using ECC (P-256/P-384) or ECDH for key exchange, fallback to strong DH groups (e.g., 19/20/21).
  • Integrity and encryption: AES-GCM (AES-256-GCM) or AES-CBC with HMAC-SHA2 (e.g., AES-256-CBC with SHA-256). Prefer AEAD ciphers (GCM) when supported.
  • Use SHA-2 family for PRFs and integrity algorithms.

Keep SA lifetimes conservative enough for security but not too frequent to cause churn. Typical lifetimes: IKE SA 24 hours, Child SA 1–8 hours depending on traffic profile; rekey triggers can be traffic volume or time-based.

Core Configuration and Operational Details

NAT Traversal and Fragmentation

Always-On clients may be behind NATs. Ensure VPN gateway supports NAT-T (UDP encapsulation on port 4500). IKEv2 also handles fragmentation of large IKE messages; enable fragmentation for environments where MSS or MTU issues are common. On the client side, policies should set appropriate MTU/MSS clamping to avoid packet drops.

Mobility and Recovery

Enable MOBIKE to let clients change IP addresses without reauthenticating. Also configure Dead Peer Detection (DPD) and IKEv2’s built-in mechanisms to detect and clear stale SAs quickly. Recommended DPD values: interval 20–30s, retry 3. Combine DPD with fast rekeying and session resiliency logic to reduce user-visible disruptions.

Traffic Selectors and Split Tunneling

Ikev2 uses traffic selectors (TSi/TSr) to define what traffic flows over the IPsec tunnel. For Always-On setups, define clear selectors for:

  • Corporate subnets and service prefixes that must always be routed through the tunnel.
  • Cloud provider prefixes or SaaS destinations eligible for direct access (if split tunnel enabled).

When using split tunneling, implement strict security controls—DNS leak prevention, web filtering, and endpoint posture checks—to avoid bypassing corporate inspection.

Endpoint Configuration and Auto-Connect

On managed devices, provision Always-On profiles using MDM/EMM (Apple Configuration Profiles, Windows MDM, Android Enterprise managed configs). Profiles should:

  • Enable auto-connect on network change and device boot.
  • Pin the VPN gateway certificate or CA to prevent MITM.
  • Enforce kill-switch behavior: if the VPN disconnects unexpectedly, block traffic according to policy.

Scaling and High Availability

Load Balancing and Session Persistence

Scale gateways horizontally by placing them behind a load balancer that performs TCP/UDP health checks. Because IKEv2 includes stateful SAs, ensure either:

  • Source IP affinity (client session stickiness) at the load balancer to the same backend.
  • Centralized session replication or shared storage for SA state (less common and complex).

For enterprises, deploying multiple gateways in active-active mode with a shared authentication backend (RADIUS/LDAP) provides capacity and resilience.

High-Availability and Redundancy

Use VRRP/HA protocols for gateway IP failover or implement floating IPs in cloud environments. Test failover scenarios regularly to ensure ongoing sessions reestablish without user action—MOBIKE helps here by rekeying with new endpoint IPs.

Certificates and PKI Best Practices

Manage certificates through an enterprise PKI or use external trusted CAs when appropriate. Key points:

  • Issue client certificates with short validity (e.g., 1 year) and automate renewal via SCEP/EST or ACME workflows.
  • Use CRL or OCSP stapling and ensure gateway validates revocation status to quickly disable compromised devices.
  • Rotate CA and gateway certificates according to policy; maintain an offline root CA and online issuing CAs for security.

Authentication Integration

Common enterprise setups integrate IKEv2 with a RADIUS server for centralized authentication and accounting. Patterns include:

  • Certificate authentication validated by the gateway and client certificate checks mapped to directory identities.
  • EAP-TLS or EAP-MSCHAPv2 via RADIUS to support MFA and conditional access.
  • Accounting and session logging to feed SIEM/analytics systems for auditing and anomaly detection.

Monitoring, Logging, and Incident Response

Comprehensive observability is crucial. Implement:

  • Real-time session metrics: active connections, bytes in/out, rekey events, and error rates.
  • Detailed logs of IKE negotiations, child SA lifecycle events, and authentication attempts forwarded to a centralized log collector (syslog/ELK/Graylog).
  • Alerting on unusual patterns: bursty reconnections, frequent authentication failures, or geographic anomalies.

Include packet capture capabilities in a controlled manner to troubleshoot complex issues; ensure capture storage and access controls comply with privacy policies.

Vendor and Platform Considerations

IKEv2 support spans many vendors and platforms. Notable options include strongSwan (Linux), Libreswan, Cisco ASA/Firepower, Palo Alto, and built-in clients in Windows, macOS and major mobile OSs. When selecting a product, verify:

  • Support for required cipher suites and MOBIKE.
  • Interoperability with MDM profiles and certificate enrollment (SCEP/EST).
  • Scalability, HA features, and logging/monitoring integrations.

Deployment Checklist

  • Define security posture: full tunnel vs split tunnel and required traffic selectors.
  • Design PKI lifecycle and automation for certificate issuance/renewal.
  • Configure gateway crypto proposals: ECDH/DH groups, AES-GCM, SHA-2 PRF.
  • Integrate with RADIUS/AAA and MFA if required.
  • Provision clients via MDM with Always-On profiles and certificate pinning.
  • Deploy HA/load balancing with health checks and stickiness or session replication strategy.
  • Enable monitoring, logging, and incident response playbooks.
  • Test failover, roaming, battery/idle scenarios and perform regular audits.

Common Pitfalls and Troubleshooting Tips

Be aware of these frequent issues:

  • MTU and fragmentation problems causing dropped IKE messages: enable fragmentation and MSS clamping.
  • NAT-related failures: ensure NAT-T is enabled and esnure UDP port 4500 is reachable.
  • Certificate validation failures: check time synchronization (NTP), trust chains and CRL/OCSP accessibility.
  • Intermittent disconnects during roaming: enable MOBIKE and tune DPD/keepalive settings.

Conclusion

Always-On IKEv2 VPNs deliver a powerful combination of security, mobility, and manageability for modern enterprises. By applying robust PKI practices, carefully chosen cryptographic parameters, and solid operational controls—HA, load balancing, monitoring—you can implement a secure, scalable Always-On VPN environment. Remember to automate certificate management, integrate with centralized authentication and endpoint management, and routinely test failover and roaming scenarios.

For additional resources and managed deployment guidance, visit Dedicated-IP-VPN.