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Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol inter network.
Why we need GRE Tunnel:
GRE is a preferred method of enabling multicasting capabilities to IPSec tunnels. If you are not defining your neighbors in your dynamic routing protocols then you would need GRE to run over IPSec tunnels. GRE tunnels can carry multicast packets just like real network interfaces as opposed to using IPSec by itself, which can't encrypt multicast traffic.
To Creat a VPN that also allow routing protocols traffic
