EGP

The Exterior Gateway Protocol (EGP) was used to interconnect autonomous systems (IP-network, which is seen as a unit and uses one or more routing protocols internally). Information of which networks are accessible at the moment is passed from network to network. This information is later translated into internal routing information by the gateways of the autonomous systems.
The Exterior Gateway Protocol was developed by BBN employee Eric Rosen in October 1982. EGP is specified in RFC 827 and it is used as a gateway protocol between different networks.

An EGP has three basic functions. Neighbours should be found. Therefore gateways of two separate networks communicate and determine, if they become EGP partners. Additionally the EGP detects the availability of its neighbours for which it verifies if certain earlier determined EGP-partners are still available or not. But the main function is detecting the availability of whole networks. Thus, all EGP-partners get a list of available networks in there neighbours’ autonomous system on request.