Autonomous Systems; Interior And Exterior Gateway Protocols; Routing Tables - Juniper J2300 User Manual

J-series™ Services Router User Guide
(destination prefix). Subnetworks have routing gateways and share routing
information in exactly the same way as large networks.

Autonomous Systems

A large network or collection of routers under a single administrative authority
is termed an autonomous system (AS). Autonomous systems are identified
by a unique numeric identifier that is assigned by the Internet Assigned
Numbers Authority (IANA). Typically, the hosts within an AS are treated as
internal peers, and hosts in a peer AS are treated as external peers. The
status of the relationship between hosts—internal or external—governs
the protocol used to exchange routing information.

Interior and Exterior Gateway Protocols

Routing information that is shared within an AS is transmitted by an interior
gateway protocol (IGP). Of the different IGPs, the most common are RIP, OSPF,
and IS-IS. IGPs are designed to be fast acting and light duty. They typically
incorporate only a moderate security system, because trusted internal peers
do not require the stringent security measures that untrusted peers require.
As a result, you can usually begin routing within an AS by enabling the IGP
on all internal interfaces and performing minimal additional configuration.
You do not need to establish individual adjacencies.
Routing information that is shared with a peer AS is transmitted by an
exterior gateway protocol (EGP). The primary EGP in use in almost all
networks is the Border Gateway Protocol (BGP). BGP is designed to be very
secure. Individual connections must be explicitly configured on each side
of the link. As a result, although large numbers of connections are difficult
to configure and maintain, each connection is secure.

Routing Tables

To route traffic from a source host to a destination host, the routers through which
the traffic will pass must learn the path that the packet is to take. Once learned, the
information is stored in routing tables. The routing table maintains a list of all the
possible paths from point A to point B. Figure 55 shows a simple network of routers.
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Routing Overview