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Tunnel to cisco router keeps doing HMAC validation failed

We have serious problems with keeping our VPN Tunnels up to our cisco devices.

After some time we see following errors in the fortigate log: Invalid ESP packet detected (HMAC validation failed).

cisco log: %CRYPTO-4-RECVD_PKT_MAC_ERR: decrypt: mac verify failed for connection id=3547 spi=8905364E seqno=00033572

Only solution is restarting the tunnel.

Fortigate is running 7.0.12

We tried upgrading our Cisco 2911 router firmware to 15.5 und disabled fortigate npu offloading with no success.

Attached you finde the Fortigate Tunnel config:

config vpn ipsec phase1-interface
edit Tunnel1
set interface "VLAN-XXX"
set local-gw xx.xx.xx.xx
set keylife 28800
set peertype any
set net-device disable
set exchange-interface-ip enable
set proposal aes256-sha256
set npu-offload disable
set dhgrp 21
set nattraversal disable
set remote-gw xx.xx.xx.xx

config vpn ipsec phase2-interface
edit "Tunnel1"
set phase1name "Tunnel1"
set proposal aes256-sha256
set dhgrp 21
set auto-negotiate enable
set keylifeseconds 3600


an the cisco configuration:

crypto isakmp policy 100
encr aes 256
hash sha256
authentication pre-share
group 21
lifetime 28800
crypto isakmp key xxx address xx.xx.xx.xx
crypto isakmp keepalive 10
crypto ipsec transform-set Tunnel-IPSEC esp-aes 256 esp-sha256-hmac
mode tunnel
crypto ipsec profile Tunnel-IPSEC
set transform-set Tunnel-IPSEC
set pfs group21




Similar issue is described in the kb below:


You may consider to try to apply steps 5 - 7 and check whether the issue persists.


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