The Semester 2 lab
topology should be thought of as an enterprise WAN for a medium-sized
company with offices around the world. It is not connected to the Internet;
it is the company's private network. Also, the topology, as shown, is not
redundant -- a failure of any router along the chain will break the
network. This network of networks, under a common administration (the
company) is called an autonomous system. -
![](./chapter2_files/image008.gif)
The Internet is a
network of autonomous systems, each of which has routers that typically
play one of four roles. ![](./chapter2_files/image011.gif)
- internal routers -- internal to one area
- area border routers -- connect two or more
areas
- backbone routers -- primary paths for
traffic that is most often sourced from, and destined for, other
networks
- autonomous system (AS)
boundary routers -- communicate with routers in other autonomous systems
While no one entity
controls them, the typical entities are:
- corporations (e.g. MCI Worldcom,
Sprint, AT&T, Qwest, UUNet, France Telecom)
- universities (e.g. University of
Illinois, Stanford University)
- research institutes (e.g. CERN in Switzerland)
- Internet Service Providers
(ISPs)
Although the Semester 2
topology is not a model of the Internet, it is a model of one topology that
might represent an autonomous system. The protocol that is routed almost
universally is IP; the routing protocol Border Gateway Protocol (BGP) is
widely used among the Internet routers.
Router A is in Kuala
Lumpur, Router B in San Francisco, Router C in New York City, and Router D
and E in Paris. Each of the routers connects to an office or campus LAN.
The connections from A-B, B-C, and C-D are leased T1 lines that are
attached to the routers' serial interfaces.
Note that each router
has an Ethernet LAN attached to it. Typical devices on Ethernet LANs, hosts
are shown along with their console cables to allow configuration and
display of the routers' contents. Also note that four of the routers have
wide-area serial connections between them.
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