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The Illustrated Network- P41

The Illustrated Network- P41:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 14 IGPs RIP OSPF and IS-IS 369 FIGURE 14.5 OSPF area types showing the various ways that areas can be given numbers decimal IP address or other . Note that ABRs connect areas and ASBRs have links outside the AS or to other routing protocols. OSPF routing domain and external routes are often very numerous in an OSPF routing domain attached to the global Internet. If a router is not an ABR or ASBR it is either an internal router and has all of its interfaces within the same area or a backbone router with at least one link to the backbone. However these terms are not as critical to OSPF configurations as to ABRs or ASBRs. That is not all backbone routers are ABRs or ASBRs backbone routers can also be internal routers and so on. Non-backbone Non-stub Areas These areas are really smaller versions of the backbone area. There can be links to other routing domains ASBRs and the only real restriction on a non-backbone non-stub area is that it cannot be Area 0. Area 11 in Figure 14.5 is a non-backbone non-stub area. Stub Area Stub areas cannot have links outside the AS. So there can be no ASBRs in a stub area. This minimizes the amount of external routing information that needs to be distributed into the link-state databases of the stub area routers. Because an AS might be an ISP on the 370 PART III Routing and Routing Protocols Internet the number of external routes required in an OSPF routing domain is usually many times larger than the internal routes of the AS itself. Stub area routers only obtain information on routes external to the AS from the ABR. Area 1.17 in Figure 14.5 is a stub area. Total Stub Area This is also called a totally stubby area. Recall that stub areas cannot have ASBRs within them by definition. But stub areas can only reach other ASBRs which have the links leading to and from other ASs through an ABR. So why include detailed external route information in the stub area router s link-state database All that is really needed is the proper .