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Overview Of Key Routing Protocol Concepts: Architectures, Protocol Types, Algorithms and Metrics
(Page 2 of 3)
Modern Protocol Types: Interior and Exterior Routing Protocols
The different nature of routing within
an AS and between ASes can be seen in the fact that distinct sets of
TCP/IP routing protocols are used for each type:
- Interior Routing Protocols: These protocols
are used to exchange routing information between routers within an autonomous
system. Interior routing protocols are not used between ASes.
- Exterior Routing Protocols: These protocols
are used to exchange routing information between autonomous systems.
They may in some cases be used between routers within an AS, but primarily
deal with exchanging information between autonomous systems.
Key Concept: Interior routing protocols are used to share routing information within an autonomous system; each AS may use a different interior routing protocol because the system is, as the name says, autonomous. Exterior routing protocols convey routing data between autonomous systems; each AS must use the same exterior protocol to ensure that they can communicate.
Since autonomous systems are just
sets of routers, this means that ASes are connected by linking a router
in one AS to a router in another AS. Architecturally, an AS consists
of a set of routers with two different types of connectivity:
- Internal Routers: Some routers in an AS
connect only to other routers in the same AS. These run interior routing
- Border Routers: Some routers in an AS
connect both to routers within the AS and to routers in one or more
other ASes. These devices are responsible for passing traffic between
the AS and the rest of the internetwork. They run both interior and
exterior routing protocols.
Due to its advantages, the autonomous
system architecture, an example of which can be seen in Figure 171,
has become the standard for TCP/IP networks, most notably the Internet.
The division of routing protocols into the interior and exterior classifications
has thus also become standard, and all modern TCP/IP routing protocols
are first subdivided by type in this manner. You can see this reflected
in the subsection titles in the rest of this section on routing protocols.
Figure 171: TCP/IP Autonomous System (AS) Routing Architecture
This diagram shows a simplified internet organized into three autonomous systems (ASes), each of which is managed independently of the others. Communication within each AS is done using an interior routing protocol chosen by that ASs administrators (blue links); communication between ASes must be done using a common exterior routing protocol (red links). Internal routers are shown in blue and border routers in red.
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Version 3.0 - Version Date: September 20, 2005
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