Gateway protocols

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All gateways, whether interior or exterior, use protocols to communicate with each other. Here are brief descriptions of the more commonly used TCP/IP gateway protocols:

HELLO Protocol (HELLO): HELLO is one protocol that the interior gateways use to communicate among themselves. HELLO calculates the shortest path to other networks by determining the path that has the least delay time.
Routing Information Protocol (RIP): Routing Information Protocol is a protocol that the interior gateways use to communicate among themselves. Like the HELLO Protocol, RIP calculates the shortest path to other networks. Unlike HELLO, RIP estimates distance not by delay time, but by hop counts. Because the gated daemon stores all metrics internally as time delays, it converts RIP hop counts into time delays.
Routing Information Protocol Next Generation: RIPng is the RIP protocol that is enhanced to support IPv6.
Open Shortest Path First (OSPF): OPSF is a protocol that the interior gateways use to communicate among themselves. It is a link-state protocol that is better suited than RIP for complex networks with many routers. It provides equal cost multipath routing.
Exterior Gateway Protocol (EGP): The exterior gateways can use the Exterior Gateway Protocol to communicate among themselves. The EGP does not calculate the shortest path to other networks. Instead, it merely indicates whether a particular network is reachable or not.
Border Gateway Protocol (BGP): The exterior gateways can use this protocol to communicate among themselves. It exchanges reachability information between autonomous systems, but provides more capabilities than EGP. BGP uses path attributes to provide more information about each route as an aid in selecting the best route.
Border Gateway Protocol 4+: BGP4+ is the BGP protocol version 4, which supports IPv6 and has other enhancements over past versions of the protocol.
Intermediate System to Intermediate System (IS-IS): Interior gateways use IS-IS protocol to communicate among themselves. It is a link-state protocol that can route IP and ISO/CLNP packets and, like OSPF, uses a "shorter path first" algorithm to determine routes.