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 protocols and exterior protocols.
GateD software combines management of the interior and exterior routing protocols
in one software daemon.
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 two interior protocols currently supported
by this version of GateD:
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RIP
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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.
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OSPF
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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.
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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:
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EGP
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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.
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BGP
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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 nonhierarchical 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 versions 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.
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Router Discovery
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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