Networks use different routing protocols to select the best routing path to reac

Question

Networks use different routing protocols to select the best routing path to reach other networks. A distance-vector routing uses a set of protocols (IPX, SPX, IP, or DECnet) and a distance calculation through an outgoing network interface to select the most efficient path to a network destination.

Link-state routing protocols track the connection status and type to each link that comes from a network. The criteria for selection are links with greater hops and faster mediums. Those are considered the most efficient links that will be used by the routing protocols.

Explanation / Answer

Before exploring the issues surrounding the selection and configuration of dynamic IP routing protocols, it is appropriate to discuss static routing as an alternative. In Chapter 1, The Basics of IP Networking, we saw that each machine in an IP network makes decisions about how to reach a destination by consulting its own private routing table. Rather than computing the entire path to a destination, it merely selects the next hop leading to that destination, and relies on the next hop machine to select a further hop that gets the packet closer to its destination. Independent hop-by-hop routing requires that all machines have a consistent view of how to reach all destinations in the network. If consistency is lost, two or more machines (presumably routers) can form a routing loop, and the packet never makes it to its destination.

To achieve consistency, a network administrator can either manually configure each machine with a precomputed set of routes that he or she knows to be consistent, or the machines can communicate routing information to each other through some kind of protocol. The first approach is known as static routing, and the second as dynamic routing.

Static routing has some enormous advantages over dynamic routing. Chief among these advantages is predictability. Because the network administrator computes the routing table in advance, the path a packet takes between two destinations is always known precisely, and can be controlled exactly. With dynamic routing, the path taken depends on which devices and links are functioning, and how the routers have interpreted the updates from other routers.

Additionally, because no dynamic routing protocol is needed, static routing doesn't impose any overhead on the routers or the network links. While this overhead may be minimal on an FDDI ring, or even on an Ethernet segment, it could be a significant portion of network bandwidth on a low-speed dial-up link. Consider a network with 200 network segments. Every 30 seconds, as required by the RIP specification, the routers all send an update containing reachability information for all 200 of these segments. With each route taking 16 octets of space, plus a small amount of overhead, the minimum size for an update in this network is over three kilobytes. Each router must therefore send a 3 Kb update on each of its interfaces every 30 seconds. As you can see, for a large network, the bandwidth devoted to routing updates can add up quickly.

Finally, static routing is easy to configure on a small network. The network administrator simply tells each router how to reach every network segment to which it is not directly attached. Consider the network shown in Figure 5-1. This network has three routers connecting five network segments together. Clearly, the only path from Router1 to a host on 172.16.3.0/24 goes through Router2. Likewise, the only path to hosts on subnet 172.16.4.0/24 goes through Router3.

Related Questions

Navigate

• Browse (All)
• Browse N
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.