دانلود کتاب پروتکل های مسیریابی IP

An Interior Gateway Protocol (IGP) is a type of routing protocol used by routers within an Autonomous System for exchanging routing information between themselves. Based on the method by which IGP routers within the Autonomous System exchange routing information and compute best paths to network destinations, IGPs are divided into two categories: distance-vector routing protocols and link-state routing protocols. Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS) are the two main link-state routing protocols in use today. These protocols were developed to meet the inability of the Routing Information Protocol (RIP) to scale beyond a maximum hop count of 15 routers. OSPF and IS-IS have fast convergence, support Variable-Length Subnet Mask (VLSM), make efficient use of network bandwidth (by propagating routing changes using triggered routing updates instead of periodic updates), and support large network sizes. Link-state routing protocols send periodic link-state refresh, but only at longer intervals (such as every 30 minutes for OSPF LSA Refresh interval) to conserve network bandwidth. RIP and Enhanced Interior Gateway Routing Protocol (EIGRP) are classified as distance-vector routing protocols while Border Gateway Protocol (BGP) is a path-vector routing protocol as discussed in Volume 1 of this two-part book. Each router in a link-state routing protocol network discovers and establishes an adjacency or neighbor relationship with each of its link-state neighbor routers. The adjacency information for each neighbor is maintained in an adjacency database (also called a neighbor table). This database on any given router keeps a record of the parameters and attributes of the router’s immediate neighbors. Each router then begins sending link-state messages (called Link-State Advertisements [LSAs] in OSPF, and Link-State Packets [LSPs] in IS-IS) to its neighbors. Each neighbor in turn, receives link-state messages and forwards them to every neighbor except the one that sent the message (a process referred to as flooding). When a network topology change occurs, the link-state router experiencing the change creates a link-state message advertising that event. Routers receiving the link-state message will immediately forward it to their neighbors. Each neighbor router receives copies of these link-state messages and stores them in a topological table called the Link-State Database (LSDB). The LSDB contains a list of all network destinations and the costs to reach them. If the state of the overall network converges and is stable, the LSDB in all routers should be identical. The LSDB describes a complete view or graph of the entire network allowing all routers in the link-state network to have the same complete picture of the network. This allows each link-state router in the network to independently make a best path routing decision based on the same accurate picture of the network topology. The linkstate routers use link-state refresh (LSAs) to confirm the network topology information before the information in the LSDB ages out. Using the Dijkstra algorithm and the information in the LSDB, each link-state router then computes the shortest path to each known network destination, and enters these best paths in its IP Routing Table (also referred to the IP Routing Information Base [RIB]). The information in the IP Routing Table is the final routing information used for forwarding IP packets. Typically, the information in the IP Routing Table is distilled into a more compact database called the IP Forwarding Table (also referred to the IP Forwarding Information Base [FIB]). This is because the information in the Routing Table is very extensive and contains information that is not directly relevant for IP packet forwarding. The IP Forwarding Table weeds out non-essential packet forwarding information and allows for faster routing information lookups and packet forwarding. Link-state routers must know the detailed topology of their network in order to be able to compute best routes to network destinations. The link-state message (LSA or LSP) is the basic messaging unit for communicating routing information among routers in a link-state network. LSAs/LSPs are used to describe the topology details of a network and also form the building blocks of the LSDB a link-state router maintains. Each LSA/LSP serves as a database record that contains pieces of network topology information (i.e., link-state information) about a portion of the network that is described in the LSDB. The LSAs/LSPs in the LSDB, collectively, describe the entire topology of the link-state network.