In an autonomous system of two routers inside of each other routers. For example, because the core form an autonomous system, the two Internet routers, core routers inside each other. The two university campuses within the router, the router also each other, because all the machines in the campus are part of the same autonomous system.
Autonomous system of routers to obtain information on the internal network, the system information? In the small, slow changes in the interconnection network, administrators can use the manual way to establish and modify the route. Managers keep a form on the network, and new networks to the autonomous system or from the autonomous system to remove a network, update the table. For such interconnection network routing cost will be negligible because only one route between any two points. Way managers can manually configure all hosts and routers routing. Interconnection network to change state (such as adding a network), the manager re-configure all the machines on the route.
Obvious shortcomings of artificial systems, it can not meet the network's rapid growth or rapid change. In the large, rapidly changing environment, such as Internet network, people slow response to changing circumstances, no time to deal with the problem; must use the automatic mechanism. Automatic mechanism will also help improve reliability, and some routed interconnection networks of small variable failure to take response measures.
After an increase of R5 enables network routers 2 and 3, more than a backup path between the original route when the fault, the routing software to quickly switch to the alternate route
Have more than one physical path for the interconnection network architecture, management is usually one of the basic path. If the basic path of the routers fail, we must change the routing so that communication traffic through the standby router to transmit. Artificial means to change the routing of a wrong time-consuming and easy. Thus, even a small Internet, the moving mechanism should also be of use to quickly and reliably change the route.
In order to automatically maintain accurate up information networks, to communicate between the internal router, that router and can get another router to exchange network reachability, data or network routing information. The entire autonomous system can be brought together to reach the information, the system, a router on the use of EGP them a notice to the other autonomous systems.
Internal communications and external routers routers communicate the difference is: EGP provides for the external router widely used communications standard, but the internal routers do not have a single communications standard. One of the reasons for this situation is the autonomous system topology and the diversity of specific technologies. Another reason is simple and powerful compromise between, that is easy to install and configure the protocols often fail to provide powerful functionality. Therefore, application of popular communication protocol in the internal router there are many, but most choose only one of the autonomous system in-house to disseminate routing information.
Since there is no single standard, we use an internal gateway protocol IGP (Interior Gateway Protocol) as collectively to describe all the internal routers used to exchange information and network routing information up to the algorithm. For example, the core router Butterfly pose a particular autonomous system, which uses an internal gateway protocol SPREAD as its IGP. Some autonomous systems for use EGP to IGP, but it has a broadcast function to those from the local area network composed of small autonomous systems do not have much significance.
IGP1 and IGP2 autonomous systems, respectively, 1 and 2 using the internal gateway protocol.
Important concept:
A single router can use two kinds of routing protocols, one for communication outside the autonomous system to another autonomous system for internal communication.
Specifically, the circular running EGP reachability router, usually need to run a kind of IGP, in order to obtain information about its autonomy within the system.
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