Routing loops involving IKEv2-based VPNs can be elusive and painful: traffic gets forwarded back and forth between endpoints, tunnels flap, or packets vanish into a cycle that consumes bandwidth and CPU. This article provides a practical, detail-rich approach to diagnosing and fixing IKEv2 routing loops for site operators, developers, and enterprise network teams. It focuses on root causes, observable symptoms, packet-level diagnostics, vendor-specific checks, and concrete remediation steps you can apply in production.

Why IKEv2 setups create routing loops

IKEv2 itself is a control-plane protocol to negotiate Security Associations (SAs) and child SAs for IPsec traffic. Routing loops are usually caused not by IKEv2 control messages directly, but by how IP routing and IPsec policy interact after a tunnel is established. Typical causes include:

  • Policy-based vs route-based VPN mismatches: When you mix policy-based IPsec (selectors defining which traffic gets encrypted) with route-based configurations (virtual interfaces like VTI/gre/virtual-tunnel), traffic may be duplicated or misrouted because selectors don’t match the routing table.
  • Asymmetric routing and ECMP: Traffic returning via a different path or next-hop than the tunnel endpoint can cause the tunnel to re-encapsulate packets that have already traversed the encrypted path.
  • Overlapping or ambiguous selectors: Broad IPsec selectors (0.0.0.0/0 source or destination) or overlapping subnets at both ends create ambiguity about which traffic belongs in the tunnel, resulting in re-encapsulation and loops.
  • MOBIKE and multi-homing: Mobility support (MOBIKE) or multiple WAN links can change the crypto endpoint mid-session; without correct route pinning, packets may bounce between endpoints during rekey or failover.
  • Routing policy and firewall interaction: Firewalls or NAT devices altering source/destination addresses (or policy routing) can push packets onto a path that returns them to the tunnel entry-point.
  • Misconfigured route advertisements: Dynamic routing protocols (BGP/OSPF) advertising the same networks from both sides or failing to withdraw routes during failover produce loops at L3.

Symptoms that indicate an IKEv2 routing loop

Recognizing the problem quickly helps target diagnostics. Look for these signs:

  • Sustained high CPU on VPN gateways while utilization on the WAN ports remains moderate.
  • Packet counters on IPsec interfaces increasing but application-layer traffic failing.
  • ICMP traceroutes showing alternating hops between the two VPN endpoints.
  • Frequent IKE child SA re-keys or tunnel teardown/bring-up events correlated with traffic bursts.
  • Asymmetric packet captures where a packet is seen entering the tunnel on one device and then reappears encapsulated coming back from the peer.

Practical diagnostic workflow

Follow an ordered approach: verify state, capture traffic, inspect policies, and test hypotheses. The steps below are designed to be repeatable and non-disruptive where possible.

1. Confirm IKEv2 state and SAs

On Linux with strongSwan: check “ipsec statusall” and “sudo ip xfrm state” and “sudo ip xfrm policy”. On Cisco/IOS-XE: use “show crypto ikev2 sa” and “show crypto ipsec sa”. On Juniper: “show security ike security-associations” and “show security ipsec sa”. Verify that both IKE SA and Child SAs exist and that encryption/authentication proposals and lifetimes match on both ends. Mismatched lifetimes or algorithms can trigger rekey storms and unpredictable behavior.

2. Capture traffic at multiple points

Use tcpdump or equivalent to capture packets on the physical WAN interface and the virtual/tunnel interface simultaneously. Example filters: capture ESP: “tcpdump -i eth0 proto 50 -w esp_wan.pcap” and on the tunnel interface “tcpdump -i vti0 -w vti.pcap”. For UDP-encapsulated ESP (NAT-T), use “udp port 4500”. Look for the same inner packet being encapsulated twice or bouncing between peers.

3. Inspect ip xfrm policy and routing table

On Linux, run “ip xfrm policy” to list policies that map selectors to SAs. Ensure that selectors match the intended source/destination prefixes. Then run “ip route show table main” and any policy routing tables. If a policy routes traffic to the tunnel endpoint and the tunnel endpoint’s own routing sends it back, a loop can ensue.

4. Test with targeted pings and traceroutes

Use ICMP with explicit source addresses to verify the path. On Linux: “traceroute -s -n ” and “ping -I “. If traceroute alternates between the two gateway IPs repeatedly, that’s a classic indication of forwarding loops.

5. Check for overlapping subnets and route leaks

Verify that internal LAN subnets are unique and not advertised on both sides. For BGP/OSPF check route tables and use “show ip bgp ” or “show route ” on Juniper. Route leaks where a prefix is learned via the tunnel and then advertised back create loops.

Common low-level packet patterns and their meanings

Understanding what you see in captures is crucial.

  • Inner packets appearing inside ESP multiple times: indicates re-encapsulation due to policy mismatch or packet returning to the encrypting gateway.
  • ESP packets with incrementing SPI but same inner IPs: frequent rekeying that might correlate with timers—check CHILD_SA lifetimes.
  • ESP inbound followed by outbound UDP/4500 from the same source: NAT traversal bouncing between encapsulation modes or MOBIKE endpoint change.

Fixes you can apply (ordered by safety and impact)

Start with non-intrusive configuration changes, then progress to more structural adjustments.

1. Tighten IPsec selectors

Avoid 0.0.0.0/0 selectors unless intended. Use specific source/destination prefixes for both policy-based IPsec and ACLs. On strongSwan, define leftsubnet and rightsubnet narrowly. On Cisco, specify crypto ACLs that only match the intended flows.

2. Prefer route-based tunnels for complex routing

When you need flexible routing, use VTIs (Linux), tunnel interfaces on Cisco (tunnel mode ipsec ipv4), or route-based IPSec on Juniper. These make routing explicitly control which traffic enters the tunnel and avoid selector ambiguities.

3. Adjust routing to pin outbound traffic

Use policy-based routing to ensure return traffic goes out the same interface that it arrived on. On Linux, ip rule + ip route table entries can direct selected source/destination pairs to the correct next hop. On Cisco, use route-maps with PBR.

4. Resolve overlapping subnets and route advertisements

Change addressing to eliminate overlap or use NAT on the tunnel (NAT-T / IP masquerading) as a last resort to distinguish traffic. In dynamic routing, use route filters to prevent re-advertisement of learned routes across the tunnel.

5. Configure Dead Peer Detection and MOBIKE settings carefully

Enable DPD with sane intervals and retries. For multi-homed endpoints, tune MOBIKE to prevent flapping when the best egress changes. On strongSwan, configure charon DPD and MOBIKE parameters; on Cisco, use “ikev2 dpd periodic 10 3”.

6. Match lifetimes and rekey parameters

Ensure IKE SA and CHILD SA lifetimes are consistent and not too short. Very short lifetimes can trigger frequent rekeys; mismatched lifetimes can lead to race conditions that temporarily route traffic incorrectly.

7. Use NAT keepalives and UDP encapsulation properly

If NAT devices exist between peers, ensure NAT-T (UDP/4500) and keepalives are enabled so that stateful NAT mappings don’t expire and cause asymmetric paths.

8. Monitor and control ECMP behavior

If multiple equal-cost paths exist to the peer, use routing metrics or per-flow hashing to ensure stable path selection. Use “ip route replace … nexthop via …” with explicit metrics or configure your router’s ECMP hashing policy.

Vendor-specific checks and commands

Cisco IOS/IOS-XE

show crypto ikev2 sa
show crypto ipsec sa
show run | section crypto map
show route
Use “debug crypto ikev2” cautiously in lab or off-peak windows to capture rekey and DPD events.

Juniper

show security ike security-associations
show security ipsec sa
show route advertising-protocol bgp
Investigate “show security flow session” to see how sessions are forwarded through the device.

Linux / strongSwan

ipsec statusall
sudo ip xfrm state; sudo ip xfrm policy
ip route show; ip rule show
tcpdump -i any ‘ip proto 50 or udp port 4500’ -w /tmp/ikev2.pcap
Check /var/log/syslog or charon logs for rekey and DPD messages (increase logging if needed).

When to redesign the VPN topology

If loops persist after tightening selectors and adjusting routing, consider a topology change: consolidate dynamic routing to one domain, move to route-based tunnels, or use a central hub-and-spoke model to remove direct bilateral tunnels that lead to circular advertisements. Also evaluate whether application traffic can be segmented across dedicated tunnels to avoid complex selector overlaps.

Checklist for preventing future routing loops

  • Use specific, non-overlapping subnets for internal networks.
  • Prefer route-based IPsec where complex routing is required.
  • Keep IKE/CHILD SA lifetimes consistent across peers.
  • Enable DPD and tune MOBIKE properly for multi-homed endpoints.
  • Filter dynamic routing advertisements to avoid route reflection across a tunnel.
  • Monitor captures systematically: collect WAN and tunnel-side traces during incidents.

Routing loops with IKEv2 are often fixable with disciplined policy configuration, precise selectors, and aligned routing. Start with low-impact diagnostics and tighten policies incrementally; if necessary, shift to route-based designs and clear route advertisement boundaries. Proper logging and packet captures will reveal the majority of root causes.

For more VPN engineering guides and configuration examples, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.