next up previous contents index
Next: Routing Policy Up: Configuration Guide Previous: Route Preference

Protocol Overview

Protocol Overview


Routing protocols determine the "best" route to each destination and distribute routing information among the systems on a network. Routing protocols are divided into two general groups: interior and exterior protocols. GateD software combines management of the interior and exterior routing protocols in one software daemon.

Interior Routing Protocols


Interior protocols are used to exchange reachability information within an autonomous system (AS). They are referred to as a class by the acronym igp. There are several interior protocols:


The Routing Information Protocol, Version 1 and Version 2, is the most commonly used interior protocol. RIP selects the route with the lowest metric as the best route. The metric is a hop count representing the number of gateways through which data must pass to reach its destination. The longest path that RIP accepts is 15 hops. If the metric is greater than 15, a destination is considered unreachable and GateD discards the route. RIP assumes the best route is the one that uses the fewest gateways i.e., the shortest path, not taking into account congestion or delay on route.

The RIP version 1 protocol is described in [RFC 1058]. and the RIP version 2 protocol is described in [RFC 1388].


HELLO , another interior protocol, uses delay as the deciding factor in choosing the best route. Round-trip time is the length of time it takes a datagram to travel from the source and destination. HELLO is historically significant for the Internet as it was the protocol used among the original prototype NSFNET backbone fuzzball gateways. Today, like fuzzballs, HELLO is a little-used protocol.

An earlier version of the HELLO protocol is described in [RFC 891].


Open Shortest Path First is a link-state protocol. OSPF is better suited than RIP for complex networks with many routers. OSPF provides equal cost multipath routing.

OSPF is described in [RFC 1583], the MIB is defined in [RFC 1253]. Other related documents are [RFC 1245], [RFC 1246] and [RFC 1370].


Intermediate System to Intermediate System (ISIS) is a link state interior gateway protocol (IGP) originally developed for routing ISO/CLNP (International Organization for Standardization/Connectionless Network Protocol) packets. The version distributed with GateD can route IP packets as well.

Exterior Routing Protocols


Exterior protocols are used to exchange routing information between autonomous systems. Exterior protocols are only required when an autonomous system must exchange routing information with another autonomous system. Routers within an autonomous system run an interior routing protocol like RIP. Only those gateways that connect an autonomous system to another autonomous system need to run an exterior routing protocol. There are two exterior protocols currently supported by GateD:


Exterior Gateway Protocol: Originally EGP reachability information was passed into ARPANET/MILNET "core" gateways where the best routes were chosen and passed back out to all connected autonomous systems. As the Internet moved toward a less hierarchical architecture, EGP, an exterior routing protocol which assumes a hierarchical structure, became less effective.

The EGP protocol is described in [RFC 827] and [RFC 904].


Border Gateway Protocol is replacing EGP as the exterior protocol of choice. BGP 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. Path attributes may include, for example, administrative preferences based on political, organizational, or security (policy) considerations in the routing decision. BGP supports non-hierarchical topologies and can be used to implement a network structure of equivalent autonomous systems.

BGP version 1 is described in [RFC 1105], version 2 in [RFC 1163], version 3 in [RFC 1267] and version 4 in [RFC 1771]. The version 3 MIB is described in [RFC 1269]. The three documents, [RFC 1164], [RFC 1268] and [RFC 1772], describe the application of version 2, 3 and 4 in the internet. A protocol analysis of and experience with BGP version 3 are available in [RFC 1265] and [RFC 1266], [RFC 1397] talks about advertising a default route in BGP version 2 and 3. And finally, [RFC 1403] describes BGP - OSPF interaction.

Other Routing Protocols

Router Discovery

  The Router Discovery protocol is used to inform hosts of the availability of hosts it can send packets to and is used to supplement a statically configured default router. This is the preferred protocol for hosts to run, they are discouraged from wiretapping routing protocols. Router Discovery is described in [RFC 1256].

next up previous contents index
Next: Routing Policy Up: Configuration Guide Previous: Route Preference

Laurent Joncheray
Wed Jun 12 15:35:22 EDT 1996