This article describes a scenario where an IPsec VPN tunnel does not establish even though the VPN configurations on both peers are identical and basic connectivity between the devices is confirmed. IKE traceoptions show that the initiating device does not receive a response from the remote peer during IKE negotiation. The article provides the investigative steps and resolution for one of this behaviour.

• The IPsec VPN tunnel fails to establish between the peer devices.
• VPN configurations on both peers appear to be identical and correctly configured.
• Basic connectivity between the VPN endpoints is verified, with no underlying network reachability issues observed.
• IKE traceoptions indicate that IKE negotiation packets are being transmitted, but no response is received from the remote peer device.

1. The status of the VPN tunnel was first verified using the show security ipsec inactive-tunnel command, which indicated that IPsec negotiation had failed due to no response from the peer.
2. Further investigation of the IKE Phase 1 negotiation confirmed that the SRX device was successfully transmitting the initial IKE packets to the peer. Analysis showed that the peer device received these packets and generated responses; however, the SRX was not processing the return traffic.
3. Flow traceoptions on the SRX to examine the handling of incoming packets. The traces confirmed that the IKE response packets were reaching the SRX but were not being processed further.
4. Additional review of the configuration revealed that a firewall filter had been applied to the loopback interface in the input direction. The filter was blocking UDP ports 500 (IKE) and 4500 (NAT Traversal), both of which are required for successful IPsec VPN negotiation.
5. The firewall filter was modified to permit UDP traffic on ports 500 and 4500. After committing the configuration changes, the IPsec VPN tunnel successfully established.
6. For more detailed troubleshooting steps and additional information on resolving IPsec tunnel issues, refer to KB10101 [juniper.net].