tailieunhanh - The Complete IS-IS Routing Protocol- P27

The Complete IS-IS Routing Protocol- P27: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | 248 10. SPF and Route Calculation Then after executing the SPF calculation the router needs to find out if there are dependent routes. The route resolver determines if a change of the IS-IS-supplied topology and routes also results in a change of dependent routes. Routing protocols like BGP rely on a working IGP to map the reachability information to a topology in order to calculate the path cost properly. Finally after the affected dependent routes have been determined the router proceeds to the third stage which is prefix insertion. At the prefix insertion stage the router inserts deletes or changes prefixes of all address families IPv4 IPv6 and their corresponding Next-hops then downloads the new forwarding tables to the line-cards and ASICs of the packet forwarding complex of the router. The next sections explore all three elements both from a theoretical and practical perspective. At the end of each section performance considerations for the network designer are highlighted. The SPF Algorithm The Shortest Path First algorithm was invented and first documented by Edser Dijkstra a Dutch mathematician who was researching the topic of graph theory and looking for an algorithm to determine the shortest spanning distance between two points on a graph. The SPF algorithm is perhaps one of the best-analyzed algorithms in computer science and its scaling properties are well understood. There are many resources available on the Internet that explain and illustrate how the SPF algorithm works. A good tutorial to learn more about the algorithm even running through an animated SPF calculation can be found at http dijkstra Briefly SPF is based on a database of node-to-node costs and using three lists the SPF algorithm can determine the shortest path to all nodes in N steps where N is the number of nodes in the network. Working Principle The SPF algorithm maintains three lists UNKNOWN TENTative PATHs All nodes currently in