Basics Router

ROUTERS A router sends information along a route—a path—between two networks. This path may traverse a single router or many routers. Furthermore, in internetworks that have multiple paths to the same destination,modern routers use a set of procedures to determine and use the best route. Should that route become less than optimal or entirely unusable, the router selects the next-best path. The procedures used by the router to determine and select the best route and to share information about network reachability and status with other routers are referred to collectively as a routing protocol. Just as a data link may directly connect two devices, a router also creates a connection between two devices. The difference is that, as Figure 1.8 shows, whereas the communication path between two devices sharing a common data link is a physical path, the communication path provided by routers between two devices on different networks is a higher-level, logical path.

shape 1.8 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Packet Notice that the logical path, or route, between the devices in Figure 1.8 traverses several types of data links: an Ethernet, an FDDI ring, a serial link, and a Token Ring. As noted earlier, to be delivered on the physical path of a data link, data must be encapsulated within a frame, a sort of digital envelope. Likewise, to be delivered across the logical path of a routed internetwork, data must also be encapsulated; the digital envelope used by routers is a packet. -------------------------------As noted earlier, each type of data link has its own unique frame format. The internetwork route depicted in Figure 1.8 crosses several data links, but the packet remains the same from end to end. How is this possible? Figure 1.9 shows how the packet is actually delivered across the route:

Figure 1.9. The frame changes from data link to data link, but the packet remains the same across the entire logical path.

1. The originating host encapsulates the data to be delivered within a packet. The packet must then be delivered across the host's data link to the local router—that host's default gateway—so the host encapsulates the packet within a frame. This operation is the same as placing an envelope inside of a larger envelope, for example, inserting an envelope containing a letter into a Federal Express envelope. The destination data link identifier of the frame is the identifier of the interface of the local router,[10] and the source data link identifier is the host's. 10] Although the purpose of a router is to create pathways between data links (networks), the router must also obey the protocols of the networks to which it is attached. So a router interface connected to an Ethernet will have a MAC identifier and must obey the CSMA/CD rules, a Token Ring interface must obey Token Ring rules, and so forth. In other words, a router is not only a router, but also a station on each of its attached networks.

2. That router (router A in Figure 1.9) removes the packet from the Ethernet frame; router A knows that the next-hop router on the path is router B, out its FDDI interface, so router A encapsulates the packet in an FDDI frame. Now the destination identifier in the frame is the FDDI interface of router B, and the source identifier is the FDDI interface of router A. 3. Router B removes the packet from the FDDI frame, knows that the next-hop router on the path is router C across the serial link, and sends the packet to C encapsulated in the proper frame for the serial link. 4. Router C removes the packet and recognizes that the station for which the packet is destined is on its directly connected Token Ring network; C encapsulates the packet in a Token Ring frame with the destination identifier of the destination station and the source identifier of its Token Ring interface. The packet has been delivered. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------The key to understanding this entire process is to notice that the frames and their related data link identifiers, which have relevance only for each individual network, change for each network the packet traverses. The packet remains the same from end to end. But how did the originating host know that the packet needed to be delivered to its default gateway for routing? And how did the routers know where to send the packet? ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Network Addresses

For devices to correctly communicate on a LAN, they must be uniquely identified by means of a data link identifier. If a routed internetwork—a network of networks—is to be created, then each member network must likewise be uniquely identifiable. Providing this unique identification is the purpose of a network address. Notice also that the point-to-point serial link has an address. A common mistake that beginners make is to forget that serial links are also networks and therefore require their own addresses for routing to work. Figure 1.10. Each network must have a uniquely identifiable address.

Now one of the two questions posed at the end of the last section can be answered: The routers can deliver the packet because the originating host put a destination address in the packet. From the perspective of the router, the destination address is all that is needed. As a rule, all routers really care about is the location of each network. Individual devices are not relevant to the router; the router only needs to deliver the packet to the correct destination network. When the packet arrives at the network, the data link identifier can be used to deliver the data to the individual device on the network. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------The fundamental purpose and function of a router The purpose of a router is to deliver packets to the proper destination networks. As such, the only individual devices routers typically care about are other routers. When a router sees that the destination address of a packet is one of its directly connected networks, it acts as a station on that network and uses the data link identifier of the destination device to deliver the packet (encapsulated in a frame) on the network. With this understanding of the relationship between routers and network addresses, a question arises: When the router sees that the destination address of a packet is one of its directly connected networks,how does the router know where to deliver the packet? After all, Figure 1.10 showed that there is no reference by the originating station to the destination station's data link identifier. A related question was asked at the end of the last section: How did the originating host know that the packet needed to be delivered to its default gateway for routing?

The answer to both of these questions is that the network addresses shown in Figure 1.10 are not sufficient. Each device on a network must be again identified uniquely, this time as a member of that particular network. The network address must have both a network identifier and a host identifier (Figure 1.11). The originating host must be able to recognize its own and others' network addresses, to say in effect: "I need to deliver this packet to device 4.3. My network address is 1.2; therefore, I know that the destination is on a different network than mine, and I'll need to send the packet to my local router for delivery." Each device on a network must be again identified uniquely, this time as a member of that particular network. The network address must have both a network identifier and a host identifier. router C must be able to recognize, "I've received a packet with a destination address of 4.3.Because my Token Ring interface has an address of 4.1, I know that network 4 is one of my directly connected networks. As a member of that network myself, I know that station 4.3 has a MAC identifier of 0000.2354.AC6B; I'll just pop this packet into a Token Ring frame and deliver it. " --------------------------------------------------------------------------------------This chapter has established that a network address must have both a network portion and a host portion and that some mechanism must exist for mapping a network address to a data link identifier.