Operation Manual – Routing ProtocolH3C S9500 Series Routing Switches Chapter 1 IP Routing Protocol Overview1-2ARoutesegmentBCAR RRoutesegmentRRRAFigure 1-1 The concept of route segmentAs the networks may have different sizes, the segment lengths connected between twodifferent pairs of routers are also different. The number of route segments multiplies aweighted coefficient can serve as a weighted measurement for the actual length of thesignal transmission path.If a router in a network is regarded as a node and a route segment in the Internet isregarded as a link, message routing in the Internet works in a similar way as themessage routing in a conventional network. Message routed through the shortest routemay not always be the optimal route. For example, routing through three high-speedLAN route segments may be much faster than that through two low-speed WAN routesegments.1.1.2 Route Selection through the Routing TableThe key for a router to forward packets is the routing table. Each router saves a routingtable in its memory, and each entry of this table specifies the physical port of the routerthrough which the packet is sent to a subnet or a host. Therefore, it can reach the nextrouter via a particular path or reach a destination host via a directly connected network.A routing table has the following key entries:z Destination address: It is used to identify the destination IP address or thedestination network of an IP packet.z Network mask: Combined with the destination address, it is used to identify thenetwork address of the destination host or router. If the destination address isANDed with the network mask, you will get the address of the network segmentwhere the destination host or router is located. For example, if the destinationaddress is 129.102.8.10, the address of the network where the host or the routerwith the mask 255.255.0.0 is located will be 129.102.0.0. It is made up of severalconsecutive "1"s, which can also be expressed in the dotted decimal format.
