Internet Routing
How does Internet routing work? IP addresses and packet switching provide the technical infrastructure which routing protocols use to transmit packets across the Internet. The Internet Protocol transfers packets between networks and provides the software bridge that knits the whole thing together.
Robert Kahn and Vinton Cerf invented the basic architecture of Internet routing along with their development of the TCP/IP networking protocol
TYPES OF INTERNET ROUTING PROTOCOLS;
Interior Gateway Protocols (IGP)
Exterior Gateway Protocol (EGP)
INTERIOR GATEWAY PROTOCOLS
Interior Gateway protocols (IGP) are used to route Internet communications within a local area network, such as within an office building. The two main types of IGP protocols are described in the following sections, along with an example proprietary protocol for comparison purposes.
Routing Information Protocol (RIP)
Open Shortest Path First (OSPF)
Interior Gateway Routing Protocol (IGRP).
ROUTING INFORMATION PROTOCOL
The Routing Information Protocol (RIP) provides the standard IGP protocol for local area networks, and provides great network stability, guaranteeing that if one network connection goes down the network can quickly adapt to send packets through another connection. All RIP routing protocols are based on a distance vector algorithm called the Bellman-Ford algorithm, after Bellman’s development of the equation used as the basis of dynamic programming, and Ford’s early work in the area.
What makes RIP work is a routing database that stores information on the fastest route from computer to computer, an update process that enables each router to tell other routers which route is the fastest from its point of view, and an update algorithm that enables each router to update its database with the fastest route communicated from neighboring routers.
OPEN SHORTEST PATH FIRST
Open Shortest Path First (OSPF) is a particularly efficient IGP routing protocol that is faster than RIP, but also more complex.
The OSPF routing algorithm was created to provide an alternative to RIP, based on Shortest Path First algorithms instead of the Bellman-Ford algorithm. It uses a tree that describes the network topology to define the shortest path from each router to each destination address. Since OSPF keeps track of entire paths, it has more overhead than RIP, but provides more options. The main difference between OSPF and RIP is that RIP only keeps track of the closest router for each destination address, while OSPF keeps track of a complete topological database of all connections in the local network..
Interior Gateway Routing Protocol (IGRP)
Enhanced IGRP uses the same distance vector algorithm and distance information as IGRP. However, the convergence properties and the operating efficiency of enhanced IGRP have improved significantly.
The convergence technology is based on research conducted at SRI International and employs an algorithm referred to as the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in re-computations.
The convergence time with DUAL rivals that of any other existing routing protocol. The initial implementation of IGRP operated in Internet Protocol (IP) networks. Enhanced IGRP extends IGRP so that it is independent of the network-layer protocol. In addition to IP, it now also operates in AppleTalk and Novell IPX networks.
Exterior Gateway Protocols (EGP)
While IGP protocols are used within local networks, Exterior Gateway Protocols (EGP) are used for routing between networks, generally on the Internet backbone itself, linking the different networks together. The following sections provide more information on the two common EGP protocols:
Border Gateway Protocol (BGP)
Exterior Gateway Protocol (EGP).
Border Gateway Protocol (BGP)
The most common Exterior Gateway Protocol in use on the Internet is the Border Gateway Protocol (BGP), ensuring that packets get to their destination network regardless of current network conditions.
Like RIP, the BGP algorithm provides great network stability, guaranteeing that if one Internet network line goes down, BGP routers can quickly adapt to send packets through another connection.
When a BGP router first comes up on the Internet, either for the first time or after being turned off, it establishes connections with the other BGP routers with which it directly communicates. The first thing it does is download the entire routing table of each neighboring router. After that it only exchanges much shorter update messages with other routers.
BGP routers send and receive update messages to indicate a change in the preferred path to reach a computer with a given IP address. If the router decides to update its own routing tables because this new path is better, then it will subsequently propagate this information to all of the other neighboring BGP routers to which it is connected, and they will in turn decide whether to update their own tables and propagate the information further.
Exterior Gateway Protocol (EGP)
This protocol is used throughout the 1980’s and into the mid-1990 was also somewhat confusingly named EGP. However, the EGP protocol had several problems, most notably an inability to scale up to support the growth in the size of the Internet Gateway Protocol (EGP).
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