ccna 課程

ccna 課程.Not every routing protocol understands the concept of AS, which provides clear boundaries to routing protocols and therefore has some advantages. For example, routers can be controlled how far to propagate network numbers; in addition, they can control which routes will be advertised to other autonomous systems and which routes from other autonomous systems will be accepted.There are public and private IP addresses (private IP addresses are defined in RFC 1918), and usually there are public and private AS numbers as well. If you want to connect to the Internet backbone, are running BGP, and want to accept BGP routes from the Intemet, you must have a public AS number. However, if you are simply dividing your internal network into multiple autonomous systems, you can use private AS numbers. Routing protocols that support the AS concept include EIGRP, OSPF, IS-IS, and BGPoRIP does not support autonomous systems, OSPF does but requires you to configure AS numbers, and all other protocols (such as EIGRP) do. Cisco CCNP certification training spends a lot of time discussing autonomous systems and inter-autonomous system routing.The CCNA exam covers only IGP basics and intra-AS routing.Exam Essentials An autonomous system (AS) is a group of networks that are unified and managed. Each AS is assigned a unique number that is used to distinguish it from other autonomous systems.14.1.3 Managing DistanceAs stated at the beginning of this chapter, each router needs to choose the best path to the destination network. If a router receives routing updates for the same network from multiple sources (e.g., direct routes, static routes, IGP routing protocols), the process of selecting the best path will be somewhat complicated because the router must select the best from it and place it in the routing table. As you will see in the next few subsections, there are two factors that routers consider when selecting the best path: administrative distance and routing metric values. The first factor that the router considers is the administrative distance of the routing source. Administrative distance is a Cisco-specific mechanism that is used to rank IP routing protocols. For example, if a router is running both IGP-RIP and EIGRP-and learns of a route to network 10.0.0.0/8 through both routing protocols, which route should the router choose and add to its routing table? Which routing protocol should the router trust more? In fact, the term administrative distance is a bit misleading because it has nothing to do with distance half the time. It is more appropriate to use the term trustworthiness.Administrative distance ranks IP routing protocols, assigning a value (weight) to each protocol. Administrative distance has a value range of 0.255, with smaller values being more trustworthy for the router, so administrative distance 0 is best and 255 is worst. Table 14.2 lists the administrative distances that Cisco assigns to its IP routing protocols. Returning to the previous example, the router is informed of the route to network 10.0.0.0/8 via both RIP and EIGRP. 120 is the administrative distance for RIP, while 90 is the administrative distance for EIGRP, which has a better (smaller) administrative distance, so the router will use the route provided by EIGRP.Here are some important protocols and their administrative distances that you must know: direct route (0), static route (0 or 1), EIGRP (90),OSPF ( 110), and RIP (120). Protocols with small administrative distances are more trusted than those with large administrative distances, and the most trusted routes are directconnected routes.Table 14.2 Administrative Distance Values