Troubleshooting Tip: How to prevent a down service on VPNs before to upgrade from v6.0.x to v6.4.x or above

[bkarl]

Description	This article describes the workaround for an IP's configured pool with ARP reply enabled and an upgrade from v6.0.X to v6.4.0 or above is processed, but VPNs do not up at v6.4.0.
Scope	FortiGate v6.4.X above.
Solution	To avoid down services on VPNs when the ISP is advising prefixes by BGP, it is suggested to add a secondary IP address in order to keep VPNs always UP after the Upgrade finishes. The following behavior will happen if a secondary IP address is not added to the WAN interface. The VPNs are down. So before upgrading, it is better to add a secondary IP on the WAN interface when on v6.0.x. Consider there is an IP pool with ARP reply enabled on v6.0.x. The result must be like this: