ccna/p

ccna/p.Keep in mind the advantages of distance vectoring protocols. Distance vectoring protocols use broadcasts to propagate routing information and do not care whether neighbors listen for theirrouting updates. Distance vectoring protocols make periodic announcements, even if no changes have occurred. The routing table is based on what the router learns from its neighborsinformation that the router learns from its neighbors.14.2.3 Link State ProtocolThe link state protocol uses the shortest path first (SPF) algorithm invented by Edsger W. Dijkstra to find the best path to the destination network. While distance vectoring protocols rely on anecdotal information provided by neighbors about remote routes, link-state protocols are informed about the complete network topology: which routers are connected to which networks. This can present scalability problems when the network is large. Therefore, link-state protocols often include features that limit the scope of learning, allowing routers to learn about only a small number of routers and routes in the network topology.Link-state protocols include OSPF and IS-IS oOSPF is covered in more depth in Chapter 19 oIS-IS is an ISO link-state protocol originally developed by DEC as the DECnet PhaseV routing protocol.IS.IS is capable of routing TCP/IP traffic, and can also route Connectionless Network Protocol (CLNP) and Connectionless Network Service ( CLNS) traffic; IS-IS is more scalable than OSPF but more complex to configure. Some ISPs use IS.IS as a routing protocol in their own networks.Notification UpdatesDistance vectoring protocols use local broadcasts to disseminate routing information, while link-state protocols use multicasts. Distance vectoring protocols send out their routing tables periodically, regardless of whether changes have occurred. Link-state protocols are smarter; they send link-state announcements (LSAs) as multicasts, and this routing information contains the sender and network number of the announcement.Typically, LSAs are generated only when the network has changed, which reduces the amount of network device resources consumed. In other words, periodic updates are rare. The distance vector protocol uses local broadcasts, which every machine in the network segment must handle, while the link state protocol uses multicast, which is only handled by the device running that link state protocol. In addition, the link state protocol sends updates in a reliable manner. After the destination router receives an LSA update, it will use an acknowledgement to respond to the source router. This differs from the distance vectoring protocol, which does not verify that neighboring routers have received routing updates.By learning routes from LSAs sent by other routers in the network, the router can outline the complete network topology - which routers are connected to other routers and what the network numbers are. This information is stored in a local topology database. Distance vectoring protocols are known as anecdotal routing, while link-state protocols are known as propaganda-based routing because link-state routers are informed of which routers are connected to the network. The link-state router collects LSAs,and stores them in a local database, which is sometimes called a topology table. Whenever this database changes, the router runs the SPF algorithm, which creates an upside-down tree with the current router at the top, followed by the other routers and their connected segments. This algorithm is somewhat like the STP algorithm used by Layer 2 devices to eliminate loops. Depending on the tree structure and the metric used, the link-state router populates the routing table using the best path to the network in the SPF tree.