CCNP Routing Study Guide- P6

CCNP Routing Study Guide- P6:T his book is intended to help you continue on your exciting new path toward obtaining your CCNP and CCIE certification. Before reading this book, it is important to have at least read the Sybex CCNA: Cisco Certified Network Associate Study Guide, Second Edition. You can take the CCNP tests in any order, but you should have passed the CCNA exam before pursuing your CCNP.
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CCNP Routing Study Guide- P6 Chapter OSPF Areas 4 THE CCNP ROUTING TOPICS COVERED IN THIS CHAPTER ARE AS FOLLOWS: Introduction to OSPF terminology Introduction to OSPF functionality Discussion of OSPF areas, routers, and link-state advertisements Discussion of choosing and maintaining routes, in particular in multi-access, PPP, and non-broadcast multi-access networks Configuration and verification of OSPF operation Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com T his chapter is the introduction to Open Shortest Path First (OSPF) areas. It will introduce the term OSPF areas and discuss their role in OSPF routing. It is very important that you take the time to learn the termi- nology used in OSPF. Without this knowledge, the remaining sections of the chapter will be difficult to follow. Open Shortest Path First Open Shortest Path First (OSPF) is an open standards routing proto- col. It is important to recognize that Cisco’s implementation of OSPF is a standards-based version. This means that Cisco based its version of OSPF on the open standards. While doing so, Cisco also has added features to its ver- sion of OSPF that may not be found in other implementations of OSPF. This becomes important when interoperability is needed. John Moy heads up the working group of OSPF. Two RFCs define OSPF: Version 1 is defined by RFC 1131, and Version 2 is defined by RFC 2328. Version 2 is the only version to make it to an operational status. However, many vendors modify OSPF. OSPF is known as a link-state rout- ing protocol (link-state routing protocols were discussed in Chapter 2, “Routing Principles”). The Dijkstra algorithm is used to calculate the short- est path through the network. Within OSPF, links become synonymous with interfaces. OSPF is a robust protocol, and due to the robustness, you must learn many terms in order to understand the operation of OSPF. The next section covers the terminology necessary to enable you to understand the many operations and procedures performed by the OSPF process. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Open Shortest Path First 115 OSPF Terminology The most basic of terms that are related to OSPF are related to many routing protocols. We begin by defining relationships among routers. From there, we will move on to defining terms relating to OSPF operations. Neighbor A neighbor refers to a connected (adjacent) router that is run- ning an OSPF process with the adjacent interface assigned to the same area. Neighbors are found via Hello packets. No routing information is exchanged with neighbors unless adjacencies are formed. Adjacency An adjacency refers to the logical connection between a router and its corresponding designated routers and backup designated routers. The formation of this type of relationship depends heavily on the type of network that connects the OSPF routers. Link In OSPF, a link refers to a network or router interface assigned to any given network. Within OSPF, link is synonymous with interface. Interface An interface is the physical interface on a router. When an interface is added to the OSPF process, it is considered by OSPF as a link. If the interface is up, then the link is up. OSPF uses this association to build its link database. Link State Advertisement Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that is shared among OSPF routers. Designated router A designated router (DR) is used only when the OSPF router is connected to a broadcast (multi-access) network. To min- imize the number of adjacencies formed, a DR is chosen to disseminate/ receive routing information to/from the remaining routers on the broad- cast network or link. Backup designated router A backup designated router (BDR) is a hot standby for the DR on broadcast (multi-access) links. The BDR receives all routing updates from OSPF adjacent routers but does not flood LSA updates. OSPF areas OSPF areas are similar to EIGRP Autonomous Systems. Areas are used to establish a hierarchical network. OSPF uses four types of areas, all of which will be discussed later in this chapter. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 116 Chapter 4 OSPF Areas Area border router An area border router (ABR) is a router that has multiple area assignments. An interface may belong to only one area. If a router has multiple interfaces and if any of these interfaces belong to dif- ferent areas, the router is considered an ABR. Autonomous system boundary router An autonomous system bound- ary router (ASBR) is a router with an interface connected to an external network or a different AS. An external network or autonomous system ...