Windows error 13821 (ERROR_IPSEC_IKE_SIMULTANEOUS

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: Both peers attempted to rekey the security association at the same time.

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ERROR_IPSEC_IKE_NO_PRIVATE_KEY	ERROR_IPSEC_IKE_DH_FAIL

ERROR_IPSEC_IKE_SIMULTANEOUS_REKEY

ERROR_IPSEC_IKE_SIMULTANEOUS_REKEY means both endpoints initiated rekeying concurrently. This can be transient and may recover automatically, but repeated occurrences can expose incompatible lifetimes, packet loss, or unstable peers.

What to check

Check whether the tunnel recovered and a new SA was established before treating the event as an outage.
Compare key lifetimes and rekey policy on both peers.
If it repeats, capture the rekey exchange and check for delayed or lost delete/notify messages.

Get-NetIPsecMainModeSA
Get-NetIPsecQuickModeSA

Microsoft: Get-NetIPsecMainModeSA

Microsoft: Get-NetIPsecQuickModeSA

Microsoft: IPsec IKE system error codes

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