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Troubleshooting IPsec Tunnels: Common Problems and Fixes

· 1 min read · Peer1IX Team

Troubleshooting IPsec Tunnels: Common Problems and Fixes

When a tunnel will not come up or traffic stalls, these are the usual suspects.

Tunnel will not establish

  • IKE proposals: encryption, integrity, DH group and lifetime must match on both sides.
  • Authentication: check pre-shared keys or certificates and identities.
  • Reachability: ensure UDP 500 and 4500 are open between peers.
  • NAT traversal: if either side is behind NAT, NAT-T must be enabled.

Tunnel up, traffic not flowing

  • Traffic selectors / proxy IDs must match for policy-based tunnels.
  • Routing: confirm routes point into the tunnel on both sides.
  • Firewall rules may block traffic after decryption.

Large transfers stall

IPsec adds overhead, reducing the effective MTU. Adjust TCP MSS clamping (commonly to around 1360 bytes) or lower the tunnel MTU.

Tunnel drops periodically

Check rekey lifetimes, dead peer detection settings and the stability of the underlying internet link.

IPsec troubleshootingVPN issuesMTU

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