In this section we're going to talk about the difference between routing versus forwarding. Routing is equivalent to building maps and given directions whereas forwarding is simply moving packets between the interfaces according to those directions when we talk about ib forwarding we mean that the router decides which interface a packet is sent to the forwarding table is usually populated by the routing process boarding decisions are based on the destination address, the class of service and other local requirements such as packet filtering. Forwarding is usually aided by special hardware as you can see from the diagram on the screen different routing tables feed the forwarding table you have several routing tables on the right heart of the screen such as the IPv4 routing table the IS-IS local routing information base the OSPF local routing information base connected routes and static routes the best part from each of these routing tables i used to create the global routing table which is then used to generate the forwarding table the forwarding information base is the forwarding table it contains the destination addresses the interfaces and the nextup addresses to get to those destinations it is built from the router's global routing information base the forwarding information base is used by the router to figure out where the packet is sent to in Cisco IOS you can check the following information base by typing "show ipsef". the global routing information base is the routing table built from the routing tables of all the routing protocols static routes and connected routes of a router please note that different vendors tend to give different priorities to different routing protocols the global routing information base contains all the known destinations and the next hops used to get to those destinations one destination can have a lot of possible next stop addresses but only the best nexup address goes into the global routing information base global routing information base is used to build the forwarding information base with cisco ios you can look at the global writing information base by typing "show ip route".

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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