Data Communications Networking Devices - 4th Ed.pdf

5.2 ROUTERS ____________________________________________________________________ 571Protocol-dependent routersTo understand the characteristics of a protocol-dependent router consider thenetwork previously illustrated in Figure 5.16. If a workstation on network B wishesto transmit data to a second workstation on network F, router R1 must know thatthe second workstation resides on network F and the best path to reach thatnetwork. The method used to determine the network where the destination workstationresides determines the protocol dependency of the router.If the workstation on network B tells router R1 the destination location, it mustsupply a network address in every LAN packet it transmits. This means that allrouters in the network must support the protocol used on network B. Otherwise,workstations on network B could not communicate with workstations residing onother networks and vice versa.NetWare IPX exampleTo illustrate the operation of a protocol-dependent router let us assume networks Band C use Novell's NetWare as their LAN operating system. The routing protocolused at the network layer between a workstation and a server is known as IPX. Thisprotocol can also be used between servers as well as other protocols.In comparison with IP where network addresses are assigned by a centralizedadministrative group InterNIC), IPX network addresses are administratorassigned. Afour-byte destination network and a six-byte destination node whichrepresents the MAC address of the destination host function in a manner similar toan IP network and host address. However, since an IPX header includes thephysical address of the destination network adapter card, there is no need toperform an ARP operation when a packet reaches the destination network. Instead,the router can use the transported destination MAC address to form an appropriateframe at the data link layer that will transport the IPX packet to its destination.Figure 5.18a) illustrates in simpli®ed format the IPX packet composition forworkstation 1 on network B transmitting data to workstation 2 on network C underNovell's NetWare IPX protocol.After router R1 receives and examines the packet it notes that the destinationaddress C requires the routing of the packet to router R2. Thus, it converts the ®rstpacket into a router R1) to router R2) packet as illustrated in Figure 5.18b). Atrouter R2 the packet is again examined. Router R2 notes that the destinationnetwork address C is connected to that router. Thus, router R2 reconverts thepacket for delivery onto network C by converting the destination router address to asource router address and transmitting the packet onto network C. This isillustrated in Figure 5.18c).Addressing differencesIn the preceding example note that each router uses the destination workstation andnetwork addresses to transfer packets. If all protocols used the same format andaddressing structure, routers would be protocol insensitive at the network layer.Unfortunately this is not true. For example, under TCP/IP addressing conventions