ccna 考試

ccna 考試.Routers use two methods to learn about routes: static and dynamic. The next two subsections discuss each of these two types of routing. Static Routing Routers can learn about static routes in two ways. First, the router looks at its active interfaces and their configured addresses, determines the network numbers corresponding to those addressescorresponding network numbers, and adds this information to the routing table. These routes are often referred to as directly connected routes. The second way in which the router learns about static routes requires you to manually configure the route. For example, you might configure a static route to the destination network, 192.168.2.0/24, with a next-hop address of 192.168 2 (the neighbor to which traffic will be forwarded). There is a special type of static route-the default route-which is often referred to as the gateway of last resort. If the destination network specified by the packet is not included in the routing table, a default route can be used to route it. The IP address of the default route is 0.0.0.0,with a subnet mask of 0.0.0.0,often denoted as 0.0.0.0/0o The default route is commonly used in small networks where the perimeter router is directly connected to the ISP router. The configuration of static and default routes is discussed in Chapter 18.Dynamic RoutingA router learns about dynamic routes by running a routing protocol. The routing protocol learns routes from neighboring routers that are running the same routing protocol. The dynamic routing protocol shares network numbers that the router knows and information about the reachability of those networks. Through this sharing process, the router eventually learns about all the reachable networks and the subnets within those networks.You must know that the terms routing protocol and the protocol being routed mean different things. The routing protocol learns the route for the protocol being routed; the protocol being routed is a Layer 3 protocol, such as Transmission Control Protocol/Internet Protocol (TCP/IP) or Interconnection Packet Exchange (IPX)The protocol being routed carries user traffic, such as email, file transfers, and Web downloads. Table 14.1 lists some common protocols that are routed and the routing protocols they use. This book discusses routing of IP traffic only and includes the basics of the following dynamic IP routing protocols: the Routing Information Protocol (RIP) Version 1 and Version 2 (Chapter 18), Open Shortest Path First (OSPF, Chapter 19), and Enhanced Interior Gateway Routing Protocol (EIGRP, Chapter 20).Table 14.1 Routed Protocols and Routing ProtocolsKeep in mind the difference between the protocol being routed and the routing protocol. Static routes are typically more secure than dynamic routes because with static routes you have complete control over which routes will appear in the routing table, whereas with dynamic routing you have less control over what information will be shared by your routing neighbors. Static routing also places less of a burden on the router than dynamic routing because the router does not need to process routing protocol messages. Static routing is typically used for border routers: it uses a default route to the Internet and a summary static route to the internal network.14.1.2 Autonomous SystemsSome routing protocols support the concept of an autonomous system, while others do not. An Autonomous System (AS) is a group of uniformly managed networks, which can be an entire company, a division of a company, or multiple companies. Internal Gateway Protocol (IGP) refers to routing protocols that handle routing within an autonomous system, including RIP, EIGRP, OSPF, and Intermediate Systems Intermediate Systems (IS-IS); External Gateway Protocol (EGP) handles routing between autonomous systems. One type of EGPH is currently in use: the Border Gateway Protocol (BGP), which is used to route traffic between autonomous systems through the Internet backbone.