Generic Routing Encapsulation (GRE) is a tunneling protocol that was first developed by Cisco. It can encapsulate a wide variety of network layer protocols within virtual point-to-point or point to multipoint links over an IP network.

Some characteristics of a GRE tunnel include:

  • supports multicast
  • has no inherent security features
  • is a fundamental component of DMVPN
  • operates as a point to point tunnel or as a multipoint tunnel (mGRE)
  • can carry a wide variety of protocols including