Data Communications Networking Devices - 4th Ed.pdf

564 ________________________________________________ LAN INTERNETWORKING DEVICESsequence of frames. The router receives the frame or sequence of frames explicitlyaddressed to it and notes that the packet has the destination address for network 20.The router then searches its routing tables and determines that it should relay thepacket to router 2. When the packet is received at router 2, it recognizes the fact thatnetwork 20 is connected to the router via its connection to the LAN by an adaptercard using MAC address F. Thus, the router will transport the packet to itsdestination using one or more frames at the data link layer with a MAC sourceaddress of F.This represents a simpli®ed description of the routing process. Amore detaileddescription will be presented later in this section.Network layer operationsThe ability to operate at the network layer enables a router to extendinternetworking across multiple data links in an orderly and prede®ned manner.This means that a router can determine the best path through a series of data linksfrom a source network to a destination network. To accomplish this the routeroperates using a network protocol, such as the Internet Protocol IP), DigitalEquipment Corporation's DECnet Phase V, and Novell's IPX. This networkingprotocol must operate on both the source and destination networks when protocoldependentrouters are used. If protocol-independent routers are used you caninterconnect networks using different protocols. The protocol-independent routercan be considered as a sophisticated transparent bridge. Its operation and utilizationare described later in this section. In comparison, since a bridge operates atthe data link layer, it can always be used to transfer information between networksoperating different network protocols. This makes a bridge more ef®cient forlinking networks that have only one or a few paths, while a router is more ef®cientfor interconnecting multiple network links via multiple paths.Network address utilizationUnlike a bridge which must monitor all frames at the MAC layer, a router isspeci®cally addressed at the network layer. This means that a router has to examineonly frames explicitly addressed to that device. In communications terminology,the monitoring of all frames is referred to as a promiscuous mode of operation,while the selective examination of frames is referred to as a non-promiscuous modeof operation.Another difference between the operation of bridges and routers is the structureof the addresses they operate upon. Bridges operate at the data link layer, whichmeans that they typically examine physical addresses that are contained in readonlymemory on adapter cards and used in the generation of frames. In comparison,routers operate at the network layer where addresses are normally assigned by anetwork administrator to a group of stations having a common characteristic, suchas being connected on an Ethernet in one area of a building. This type of address isknown as a logical address and can be assigned and modi®ed by the networkadministrator.