I have followed the below article: https://networklessons.com/uncategorized/cisco-3g-configuration-example/
and I have one question for you; I need to connect a cisco 810 series router to WAN interface of a firewall FortiGate; I need a public ip on WAN interface for use DDNS, I want to wake up an IPsec tunnel on a firewall and use router for internet access.
How can I configure Cisco Router for getting a dynamic public ip on WAN interface of the firewall?
If I got this right, your router is in between your ISP and the Fortinet firewall?
Cisco IOS routers have “routed” interfaces so each interface has a different IP address / subnet. However, it is possible to bridge interfaces. If you bridge two interfaces then the interface will work similar to switchports.
Here is a simple example how you can bridge two Gigabit interfaces:
bridge 1 protocol ieee
With this configuration, these Gigabit interfaces act like switch ports. If Gi0/1 is connected to the ISP and Gi0/2 to the Fortinet firewall, then your firewall will be in the same subnet as the ISP router.