Generic Encapsulation Tunnels is a tunneling protocol that has the capabilities of encapsulating many network layer routing protocols, such as, RIP, EIGRP, or OSPF.
A GRE tunnel interface can support a header for passenger protocols for the following:
- IPv4 or IPv6
- GRE itself
- The transport delivery protocol is usually IP
GRE tunnels do have their own unique characteristics:
- GRE provides no security
- GRE has no support for IP multicast or IP broadcast
So, using GRE tunnels, together with IPSec, allows you to run routing protocols, IP multicast, as well as multiprotocol traffic across your network.
