Site to Site VPN Tunnel traffic flow problems

Written by Simon Eady
Published on 29/8/2012 - Read in about 2 min (296 words)

Firewalls being used – Sonicwall 3500 & Cisco 506e

Several months ago we relocated and it was then necessary to setup a Site to Site VPN tunnel with another network. (In this instance the other network was not directly managed by us)

Upon the creation of the tunnel and after successful traffic tests all looked well. However after several hours or less in some cases traffic stopped flowing yet both firewalls reported the tunnel as “up”. We reviewed the first and second phase settings and tweaked the Sonicwall VPN settings to hopefully remedy.

Options on the Sonicwall such as “Enable IKE Dead Peer Detection” & “Enable Keep Alive” were enabled and disabled to try and find a fix for the VPN traffic flow problem.

What was interesting during the troubleshooting process, we found that if we manually restarted the VPN tunnel it would resume with no issue, but obviously this was hardly a practical fix for our issues.

Liaising with the other site we also experimented with Phase 1 and Phase 2 Life Time settings with no success.

It was then we had a small eureka moment, we decided to check the time servers each firewall referenced. It transpired the Time Server being referenced by the Cisco Firewall was out of sync (it was an internally hosted NTS)

After the offending NTS had been re-sync’d we decided to completely recreate the VPN tunnel double checking the settings as we went along. The VPN Tunnel came up with no issues and has been stable ever since.

I would add if we encounter a problem like this again I would simply point both Firewalls to the same NTS but as one of the firewalls in this case was managed by a third party this was not an option.

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