Static and Dynamic Routing静态和动态路由.doc

Networking BasicsB. KonkothLesson 13 Network LayerStatic and Dynamic RoutingWhen it comes to routing, there is a huge difference between the two types of routing: static and dynamic. Early routers had to be programmed with exactly which networks they could route between which interfaces, especially if there were many network interfaces. This is called static routing, which consists of adding, maintaining, and deleting routes of the network routing devices by the network administrator. In a small company this may not be much of a chore, but for medium to large networks, this can be nearly impossible. These larger networks almost always employ many logical subnets, which requires you to update the route tables on each routing device. If these remote subnets are connected by routers with static route tables, you have to add the exact static route in order to communicate between the two subnets.Table below gives you an example of what is contained in the routing table.Destination Adjacent RouterHops Network 1Router A1Network 1Router B2Network 2Router B2Network 2Router C3Network 3Router D3An Example of a Routing TableIn this table you can see how you only specify the router to be used to reach your destination, not the actual destination itself. Also notice the column for Hops. This is what determines which route is the most efficient, similar to “Name That Tune.” If a route claims it can reach the destination in one fewer hop than the next router, then let him prove it. If there are two identical routes to the same destination, the route with the fewest hops will be used.One change to a network address means visiting every routing device that employs static routing and updating the entry. What do we do if our network is fairly large and complex? We must then use routing devices capable of dynamically updating the route tables.Dynamic routing does not require the network administrator to edit complex routing tables in order to communicate with other networks or segments. These routers communicate with each other using a powerful routing protocol such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF). They can also query other routers for updated route information, which can create more efficient paths for sending packets or locate an alternative route if the original route fails. The routers can broadcast the routes they have discovered to neighboring routers, and, in turn, accept routes from other neighboring routers. The Internet is comprised of many dynamic routers. Could you imaging having to update a static routing table on thousands of static routers? I dont think so.These dynamic routers, however, cannot update the route tables of static routers or non-dynamic routers. There are a few situations in which integrating static and dynamic routers is acceptable: When you have a router at either end of a slow WAN linkThis router will not increase traffic by broadcasting updated route information to the router on the other end of the link. When you require a packet to travel the same path each time to a remote networkAdd the path you would like the packet to take in order to reach the destination network. You cannot enter the entire path over several routers, only the path to the first router. When you want to configuring a static router to point towards a dynamic router to take advantage of the dynamic router indirectlyThis is the next best thing to using a dynamic router. You can hand off the packet to the dynamic router and let this router determine the most efficient path to the destination based on the paths it has learned from neighboring dynamic routers.Of course, dynamic routers cost much more. Although you can add static paths to a newer dynamic router, you cannot enable dynamic routing on an older static router. Also, dynamic routing generates continuous traffic from the routers with route update information.Comparing Static and Dynamic RoutingWith just two networks, the static routing setup is the more appropriate. If your network has three or four parallel networks, then static routing may still be useful, although dynamic routing would be easier to set up.However, the actual setup is far more complex. You cannot simply have the routers route to the other network by assigning the gateway addresses to the other network interface card. This simply moves the packet to the other side when in actual fact the router needs to send the packet on to the specific gateway from that network to the next network.Default Gateways and SubnetworksWe have learned that packets are routed to their destination through a web of routers. We have discussed how these routers are updated: either statically or dynamically. What gets the whole process rolling when we are routing packets to remote subnetworks, or subnets for short, is the use of the default gateway. The default gateway is specified on each computer, and is what initially sends the packet on its way to the first router. When the packet hits this first router, the router must determine if the destination computer is on the local network, or send the packet to the next router that will get the packet to its destination. You have to be on your toes when you are administering a network with subnets, which is quite common today. Without that default gateway, you are stuck on the local network. The subnet mask, which you will learn about in the next chapter, is also very important. Without a properly configured subnet mask to determine which subnet your computer is on, you arent going to be talking to anyone!The IP address and subnet mask of a Windows 98 computer are shown below . To get to this point, open the Network applet in the Control Panel, double-click on TCP/IP on the Configuration tab (or select the TCP/IP protocol and then click Properties), and select the IP Address tab if it is not already selected.Configuring the IP address and subnet mask on a Windows 98 computerFigure below illustrates configuring a Windows 98 computer with the default gateway. This tab is a