tailieunhanh - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 74

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 74 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 699 Tuesday May 20 2003 2 53 PM Identifying the Classes of Routing Protocols 699 Z are connected by three link-state routers. Network discovery for link-state routing uses the following processes 1. Routers exchange LSAs with each other. Each router begins with directly connected networks for which it has direct firsthand information. 2. Each router in parallel with the others constructs a topological database consisting of all the LSAs from the internetwork. 3. The SPF algorithm computes network reachability. The router constructs this logical topology as a tree with itself as root consisting of all possible paths to each network in the link-state protocol internetwork. It then sorts these paths by shortest path first SPF . 4. The router lists its best paths and the ports to these destination networks in the routing table. It also maintains other databases of topology elements and status details. Figure 15-21 Link-State Algorithm Shortest Path Calculations Topological Database A Routing Table Topological Database B Routing Table Topological Database C Routing Table When a router becomes aware of a link-state topology change it forwards the information so that all other routers can use it for updates. This involves sending common routing information to all routers in the internetwork. To achieve convergence each router keeps track of its neighbor routers the router name interface status and the cost of the link to the neighbor. The router constructs an LSA packet that lists this information along with new neighbors changes in link costs and links that are no longer valid. The LSA packet then is sent out so that all other routers receive it. Figure 15-22 shows an example of link-state topology changes. Page 700 Tuesday May 20 2003 2 53 PM 700 Chapter 15 Routing and Routing Protocols Figure 15-22 Link-State Topology Changes Process to Update This Routing Table When the router receives an LSA packet the database is updated with the most recent