cisco book value

cisco book value.For example, area 2 does not need to be informed of all subnets within network number 172.18.0.0 assigned to area 1 because there are only two paths to the backbone from area 2. However, area 2 must know all the subnets within it so that it can create an optimal routing table containing the best paths to each subnet within area 2. Thus, in a properly designed OSPF network, each area will contain specific routes within it as well as aggregated routes to other areas. By performing this aggregation, routers have smaller topology databases (they know only the links within the area they belong to and the aggregated routes to other areas) and smaller routing tables (they know only the routes within the area they belong to and the aggregated routes to other areas). By using the correct hierarchical design, OSPF can be made to support very large networks. Routing aggregation is discussed in Chapter 7.Note that the CCNA exam covers only single-area designs, so the remainder of this chapter covers only single-area concepts. However, a significant amount of space is devoted to single-area and multi-area designs in the CCNP material. Multi-area OSPF networks are very complex to design and require extensive network knowledge and skills.The OSPFDesignGuide on the Cisco Web site provides a wealth of OSPF material that discusses single-area and multi-area design in depth at www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf77039-l .htmL19.1.2 Metric Value StructureUnlike RIP, which uses hop count as a metric, OSPF uses overhead. The overhead is actually the inverse of the link bandwidth: the faster the connection, the lower the overhead. The path with the lowest cumulative overhead is the best path.By using overhead as a metric, OSPF is smarter than RIP in choosing paths (metrics are discussed in more depth in the "OSPF Metrics" section later in this section).As mentioned earlier in this book, the default bandwidth on a synchronous serial link is 1544 kbit/s regardless of the clock frequency, so you need to set the bandwidth correctly using the interface sub-configuration mode command bandwidth discussed in Chapter 15. This is important when there are multiple synchronous serial paths to the same destination network, especially if they are clocked at different frequencies. OSPF supports load balancing between up to 16 equal-overhead paths to the same destination network, but only 4 equal-overhead paths are used by default. Don't forget that routers may treat paths with different clock frequencies as equal overhead paths if the bandwidth is not configured correctly on the serial interface, which can cause problems for load balancing.Exam Essentials OSPF uses overhead as a metric that is the inverse of the link bandwidth.For example, suppose there are two serial connections with clock frequencies of 1544 kbit/s and 256 kbit/s; if you do not modify the bandwidth value, OSPF will assume that both connections have a bandwidth of 1544 kbit/s and use them to travel to the same destination network. This is because the default bandwidth of a serial link is 1544 regardless of its clock frequencyO This can present a throughput problem when the router performs load balancingI useChapter 19 OSPF Routing 499