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IPSec phase-1 tunnel becomes inactive


We have a strange phenomenon.

We have a VPN IPSec tunnel established between two Cisco Routers. Every night (around 01:00am), we receive a snmp trap indicating that the IPSec Phase-1 tunnel becomes inactive. It last about 01 hour and then, we receive another snmp trap indicating that the IPSec Phase-1 tunnel becomes active again. Very strange beacuse, when we consult the show log of these two routers, there are no messages related to this IPSec Phase-1 tunnel (for the determined date and hour). Very strange also because, the SNMP traffic is not interrupted between the two cisco routers (we dont't receive SNMP timeout from SNMP manager when it tries to reach the SNMP agent behind these two routers). Normally when the IPSec Phase-1 tunnel becomes inactive, the tunnel is no more established and available and the SNMP traffic must be interrupted between these two routers ==> SNMP Timeout between SNMP manager and SNMP agents.

What is the just explanation for continuation of traffic between these two routers (with Phase-1 tunnel inactive) ?

Yours sincerely,

Olivier MONTEE.