EGP (Exterior Gateway Protocols)
Exterior Gateway Protocols, such as BGP (Border Gateway Protocol), are designed to serve as a conduit for communication between autonomous systems. BGP is the most popular inter-autonomous system (or Inter-AS) routing protocol used throughout the Internet community. EGP protocols connect separate AS' together providing a transit path between and through these AS' to facilitate traffic forwarding across the Internet (see Figure 3.17).
Figure 3.17 Multiple AS' (AS 1 and AS 2) are connected by border routers running both IGP and EGP routing protocols. The EGP protocol used in this example is BGP (Border Gateway Protocol).
EGP protocols only recognize the AS within the routing hierarchy, ignoring the IGPs. Border routers from different AS' typically run some type of IGP over the interfaces within their home AS and BGP or some other EGP over the external interface used to connect this AS to a remote AS. A full discussion of BGP and inter-autonomous system routing can be found in Chapter 7, "Border Gateway Protocol (BGP)."
Routers running multiple routing protocols for the purpose of connecting routing domains or AS' are referred to as Border routers.