31 IP ROUTING PROTOCOL OVERVIEWn When running a routing protocol, the Ethernet switch also functions as a router.The word “router” and the router icons covered in the following text representrouters in common sense and Ethernet switches running a routing protocol. Toimprove readability, this will not be mentioned again in this manual.Introduction to IPRoute and RoutingTableIP Route Routers are used for route selection on the Internet. As a router receives a packet,it selects an appropriate route (through a network) according to the destinationaddress of the packet and forwards the packet to the next router. The last routeron the route is responsible for delivering the packet to the destination host.Routing Table The key for a router to forward packets is the routing table. Each router maintainsa routing table. Each entry in this table contains an IP address that represents ahost/subnet and specifies which physical port on the router should be used toforward the packets destined for the host/subnet. And the router forwards thosepackets through this port to the next router or directly to the destination host ifthe host is on a network directly connected to the router.Each entry in a routing table contains:■ Destination address: It identifies the address of the destination host or networkof an IP packet.■ Network mask: Along with the destination address, it identifies the address ofthe network segment where the destination host or router resides. Byperforming “logical AND” between destination address and network mask,you can get the address of the network segment where the destination host orrouter resides. For example, if the destination address is 129.102.8.10 and themask is 255.255.0.0, the address of the network segment where thedestination host or router resides is 129.102.0.0.A mask consists of someconsecutive 1s, represented either in dotted decimal notation or by the numberof the consecutive 1s in the mask.■ Output interface: It indicates through which interface IP packets should beforwarded to reach the destination.■ Next hop address: It indicates the next router that IP packets will pass throughto reach the destination.■ Preference of the route added to the IP routing table: There may be multipleroutes with different next hops to the same destination. These routes may bediscovered by different routing protocols, or be manually configured static
