HP-UX Reference (11i v3 07/02) - 4 File Formats (vol 8)

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gated.conf(4) gated.conf(4)
The EGP protocol is described in RFC 827 and RFC 904.
BGP
Border Gateway Protocol is replacing EGP as the exterior protocol of choice. BGP exchanges reacha-
bility 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, organiza-
tional, or security (policy) considerations in the routing decision. BGP supports nonhierarchical topolo-
gies 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. The version 3
MIB is described in RFC 1269. The two documents, RFC 1164 and RFC 1268 describe the application
of version 2 and three 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 ver-
sion 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.
Routing Information Protocol (RIP)
One of the most widely used interior gateway protocols is the Routing Information Protocol (RIP). RIP is an
implementation of a distance-vector, or Bellman-Ford routing protocol for local networks. It classifies
routers as active and passive (silent). Active routers advertise their routes (reachability information) to oth-
ers; passive routers listen and update their routes based on advertisements, but do not advertise. Typically,
routers run RIP in active mode, while hosts use passive mode.
A router running RIP in active mode broadcasts updates at set intervals. Each update contains paired
values where each pair consists of an IP network address and an integer distance to that network. RIP uses
a hop count metric to measure the distance to a destination. In the RIP metric, a router advertises directly
connected networks at a metric of 1. Networks which are reachable through one other gateway are two
hops etc. Thus, the number of hops or hop count along a path from a given source to a given destination
refers to the number of gateways that a datagram would encounter along that path. Using hop counts to
calculate shortest paths does not always produce optimal results. For example, a path with hop count 3 that
crosses three Ethernets may be substantially faster that a path with a hop count 2 that crosses two slow-
speed serial lines. To compensate for differences in technology many routers advertise artificially high hop
counts for slow links.
As delivered with most UNIX systems, RIP is run by the routing daemon, routed (pronounced route-"d"). A
RIP routing daemon dynamically builds on information received through RIP updates. When started up, it
issues a REQUEST for routing information and then listens for responses to the request. If a system
configured to supply RIP hears the request, it responds with a RESPONSE packet based on information in
its routing database. The RESPONSE packet contains destination network addresses and the routing
metric for each destination.
When a RIP RESPONSE packet is received, the routing daemon takes the information and rebuilds the
routing database adding new routes and "better" (lower metric) routes to destinations already listed in the
database. RIP also deletes routes from the database if the next router to that destination says the route
contains more than 15 hops, or if the route is deleted. All routes through a gateway are deleted if no
updates are received from that gateway for a specified time period. In general, routing updates are issued
every 30 seconds. In many implementations, if a gateway is not heard from for 180 seconds, all routes from
that gateway are deleted from the routing database. This 180 second interval also applies to deletion of
specific routes.
RIP version 2 (more commonly known as RIP II) add additional capabilities to RIP. Some of these capabili-
ties are compatible with RIP I and some are not. To avoid supplying information to RIP I routes that could
be misinterpreted, RIP II can only use noncompatible features when its packets are multicast. On inter-
faces that are not capable of IP multicast, RIP I compatible packets are used that do not contain potentially
confusing information.
Some of the most notable RIP II enhancements are:
HP-UX 11i Version 3: February 2007 − 12 − Hewlett-Packard Company 139