Common routing protocol

Problem

Explain any common routing protocol in detail. For example: BGP, OSPF, RIP.

Solution

1. BGP: Border Gateway Protocol
   BGP is the core routing protocol of the Internet . 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.
2. RIP: 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.
   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.
3. OSPF: 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 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. The OSPF algorithm works as described below:
   • Startup. When a router is turned on it sends Hello packets to all of its neighbors, receives their Hello packets in return, and establishes routing connections by synchronizing databases with adjacent routers that agree to synchronize.
   • Update. At regular intervals each router sends an update message called its “link state” describing its routing database to all the other routers, so that all routers have the same description of the topology of the local network.
   • Shortest path tree. Each router then calculates a mathematical data structure called a “shortest path tree” that describes the shortest path to each destination address and therefore indicates the closest router to send to for each communication; in other words — “open shortest path first”.

Reference

• http://tianrunhe.wordpress.com/2012/04/24/common-routing-protocol/