Open Shortest Path First

9 Key excerpts on "Open Shortest Path First"

• 

  eBook - ePub

  CCNA Certification Study Guide

  Volume 2 Exam 200-301

  • Todd Lammle(Author)
  • 2020(Publication Date)
  • Sybex

    (Publisher)

  CHAPTER 6 Open Shortest Path First (OSPF)

  The following CCNA exam topics are covered in this chapter:

  3.0 IP Connectivity

  •  3.4 Configure and verify single area OSPFv2
    • 3.4.a Neighbor adjacencies
    • 3.4.b Point-to-point
    • 3.4.c Broadcast (DR/BDR selection)
    • 3.4.d Router ID

   Open Shortest Path First (OSPF) is by far the most popular and important routing protocol in use today—so important, I’m devoting an entire chapter to it! Sticking with the same approach we’ve taken throughout this book, we’ll begin with the basics by completely familiarizing you with key OSPF terminology. Once we’ve covered that thoroughly, I’ll guide you through OSPF’s internal operation and then move on to tell you all about OSPF’s many advantages over RIP.

  This chapter isn’t just going to be chock full of vitally important information, it’s also going to be really exciting because together, we’ll explore some critical factors and issues innate to implementing OSPF. I’ll walk you through exactly how to implement single-area OSPF in a variety of networking environments and then demonstrate some great techniques you’ll need to verify that everything is configured correctly and running smoothly.

   To find your included bonus material, as well as Todd Lammle videos, practice questions & hands-on labs, please see www.lammle.com/ccna

  Open Shortest Path First (OSPF) Basics

  Open Shortest Path First

  is an open standard routing protocol that’s been implemented by a wide variety of network vendors, including Cisco. And it’s that open standard characteristic that’s the key to OSPF’s flexibility and popularity.

  Most people opt for OSPF, which works by using the Dijkstra algorithm to initially construct a shortest path tree and follows that by populating the routing table with the resulting best paths. EIGRP’s convergence time may be blindingly fast, but OSPF isn’t that far behind, and its quick convergence is another reason it’s a favorite. Another two great advantages OSPF offers are that it supports multiple, equal-cost routes to the same destination, and like EIGRP, it also supports both IPv4 and IPv6 routed protocols.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  CompTIA Network+ CertMike: Prepare. Practice. Pass the Test! Get Certified!

  Exam N10-008

  • Mike Chapple, Craig Zacker(Authors)
  • 2023(Publication Date)
  • Sybex

    (Publisher)

  Each hop represents one router that packets must pass through on the way to the destination. Therefore, traffic destined for a network that is 10 hops away must be forwarded by 10 routers to get there. Open Shortest Path First (OSPF) Distance vectoring is not a terribly precise way to measure the proximity of a router. A single hop could be a local area network (LAN) connection running at 1,000 Mbps, or it could be a much slower wide area network (WAN) connection. This is why many of the other routing protocols use alternative measurements to calculate their metric values. Open Shortest Path First (OSPF) is an open standard link state routing protocol. Unlike distance vector protocols, link state protocols gather information about the network by transmitting messages called link state advertisements (LSAs) to other routers in mul- ticast messages. Using the information in the LSAs, the routers create a neighbor table and a topology table, essentially a map of the entire network. The routers then use that information to build their routing tables. Unlike distance vector protocols, link state protocols use various criteria to calculate the efficiency of routes through the network. In addition to hop counts, OSPF factors in the transmission speeds of the various network links, the current levels of traffic congestion on the various routes, and a route cost specified by an administrator. The routers then use the Dijkstra algorithm to calculate the relative efficiency of routes to a given destination. OSPF routing tables support multiple routes to the same destination with the same metric and administrative distance values. When this occurs, the router balances the outgoing traffic among the duplicate routes. Unlike RIP, OSPF does not transmit its entire routing table at regular intervals. Instead, it generates updates containing only the data that has changed, which conserves bandwidth and speeds up convergence. OSPF also does not have a hop count limit, as RIP does.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Implementing and Administering Cisco Solutions: 200-301 CCNA Exam Guide

  • Glen D. Singh(Author)
  • 2020(Publication Date)
  • Packt Publishing

    (Publisher)

  CCNP ) Enterprise certification level. However, it's worth mentioning in this section.

  A loop-free path is one that does not have a layer 3 routing loop on a network. This is very useful in the event a route is unavailable as EIGRP can almost immediately insert the backup loop-free path within the routing table to ensure connectivity.

  Open Shortest Path First

  One of the most popular link-state routing protocols is Open Shortest Path First version 2 (OSPFv2 ). Defined by RFC 1247, OSPFv2 was introduced to the networking industry back in 1991 and since then, it has been widely adopted and implemented in many organizations.

  The following are the benefits of using OSPF:

  • Open source : Being open source allows an organization with mixed vendor equipment to implement OSPF to exchange routing information between the various manufacturers of routers.
  • Scalability : OSPF can be implemented in a network of any size. Additionally, OSPF can be configured in a hierarchical system where OSPF-enabled routers can be grouped into areas.
  • Secure : The OSPF routing protocol supports both Message Digest 5 (MD5 ) and Secure Hashing Algorithm (SHA ) for authentication. This allows two OSPF-enabled routers to authenticate with each other before exchanging OSPF routing details such as network information.
  • Efficiency : Unlike older dynamic routing protocols, OSPF will only send an update if a change occurs on a network rather than sending periodic updates at specific intervals.
  • Classless : The OSPF routing protocol supports the use of custom subnet masks and VLSM.

  The OSPF routing protocol is made up of various components. These enable the protocol to have a clear idea of the entire network topology when it has to tell the router how to forward a packet. The following are the OSPF components:

  • Adjacency table : Before OSPF exchanges routing information with a neighbor router on the network, they both need to establish an OSPF adjacency with each other. This adjacency is simply like a mutual handshake indicating that both are willing to share network routes. This adjacency table contains a list of all the neighbor routers that have established an adjacency with a local router. This table is sometimes referred to as the neighbor table . The show ip ospf neighbor

  Sign up to read

  Learn more about book
• 

  No longer available |Learn more

  CCNA Routing and Switching 200-125 Certification Guide

  The ultimate solution for passing the CCNA certification and boosting your networking career

  • Lazaro (Laz) Diaz(Author)
  • 2018(Publication Date)
  • Packt Publishing

    (Publisher)

  The World of Open Shortest Path First (OSPF)

  The OSPF interior routing protocol is a very popular protocol in enterprise networks. This chapter will present a brief history of OSPF and provide a comparison with the IS-IS routing protocol, since this is also a link state routing protocol.

  We will be learning the basics of OSPF, its features and configuration, and much more. We will be creating multiple labs, since OSPF has different aspects that we need to learn for the certification and the real world. Not only will we be looking at single-area OSPF, but also multi-area OSPF.

  The following topics will be covered in this chapter:

  • History of OSPF
  • SPF tree calculation
  • OSPF metrics
  • Configuring OSPF
  • Wildcard calculations
  • Configuring OSPF in our network
  • OSPF DR and BDR
  • Verifying OSPF
  • OSPF scalability
  • Categories of multi-area components
  • OSPF router roles
  • Types of LSAs
  • OSPF Hello protocol
  • Neighbor states
  • Basic multi-area configuration

  Passage contains an image

  Brief history

  The origins of OSPF date back to the late 1980s by the IETF. OSPFv1 was published in 1989 and OSPF was published in 1991. OSPFv3 for IPv6 was first published in 1997, and a later revision came out in 1999. For the RFC aficionados, RFC2740 was really the one that introduced OSPFv3 for IPv6 in 1999, but it was replaced by RFC5340 in 2008.

  So, OSPF has been around for a long time and even the IPv6 version of it has been around for ten years, although we still have not fully embraced it.

  There is another link state protocol that I will briefly touch upon, which is IS-IS. This protocol has be around since the late 1970s but not until 2008 was RFC5308 ready to go for IPv6. This is still a long time and very little has been spoken about it.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Understanding Operating Systems

  • Ann McHoes, , Ida M. Flynn, , Ann McHoes, Ida M. Flynn(Authors)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  In addition, the tables propagate from one router to another. Thus, in the case of an internetwork with 15 hops, it would take more than seven minutes for a change to be known at the other end of the internetwork. Because not all routers necessarily have the same information about the internetwork, a failure at any one of the hops could create an unstable envi-ronment for all message traffic. Open Shortest Path First In Open Shortest Path First (OSPF) , selection of a transmission path is made only after the state of a network has been determined. This way, if an intermediate hop is mal-functioning, it’s eliminated immediately from consideration until its services have been restored. Routing update messages are sent only when changes in the routing environ-ment occur, thereby reducing the number of messages in the internetwork, and reducing the size of the messages by not sending the entire routing table. However, memory usage is increased because OSPF keeps track of more information than RIP. In addition, the savings in bandwidth consumption are offset by the higher CPU usage needed for the calculation of the shortest path, which is based on Dijkstra’s algorithm, simply stated as finding the shortest paths from a given source to all other destinations by proceeding in stages, and developing the path in increasing path lengths. When a router uses Dijkstra’s algorithm, it computes all the different paths to get to each destination in the internetwork, creating what is known as a topological database. This data structure is maintained by OSPF and is updated whenever failures occur. Therefore, such a router simply checks its topological database to determine whether a path was available, and then uses Dijkstra’s algorithm to generate a shortest-path tree to get around the failed link. Connection Models A communication network isn’t concerned with the content of data being transmitted, but rather with the moving of data from one point to another.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Routing, Flow, and Capacity Design in Communication and Computer Networks

  • Michal Pioro, Deep Medhi(Authors)
  • 2004(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  CHAPTER 7

  Networks With Shortest-Path Routing

  Shortest-path routing algorithms have existed since two independent seminal works by Bellman [Bel58 ] and Ford [FF62 ], and Dijkstra [Dij59 ] in 1950’s. The difference between these two algorithms is the way information needed for computing the shortest-path is used. In the context of packet-switched networks and Internet routing, in particular, Bellman-Ford’s algorithm has enabled the development of distance-vector routing protocols while Dijkstra’s algorithm has paved the way to the introduction of link-state routing protocols [Hui00 ]. In this chapter, we focus on network design problems (NDPs) related to the latter type of routing protocols since they have gained popularity due to deployment of open shortest-path first (OSPF) [Moy98b ]) and intermediate system to intermediate system (IS-IS) [Cal90 ], [ISO90 ], [Ora90 ])—the most common Internet intra-domain routing protocols.

  The shortest-path routing principle in packet networks is attractive because it solves the immanent trade-off between routing implementation complexity and traffic effectiveness. The main advantage of shortest-path routing is that it can be implemented in a distributed way, not losing too much out of the traffic effectiveness achievable with more complicated routing strategies. The idea is to assign a cost metric (weight) to each link in the network, and to route packets incoming to a node using the shortest-path to the packet destination where the path cost (length) is computed using this cost metric. Nodes are aware of the current metrics used for the links, and compute shortest-paths to other destinations, storing in their routing tables the next hop for each destination. When for some reason the metrics in the system are changed (for instance, when a link fails, then it is assigned an infinite weight), nodes are informed about the changes in a distributed manner through a protocol mechanism where the originator (e.g., the node to which a failed link is connected) generates the link metric (and other information about the link), called link-state

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies

  Exam 350-401

  • Ben Piper(Author)
  • 2020(Publication Date)
  • Sybex

    (Publisher)

  Chapter 5 Open Shortest Path First (OSPF)

  THE CCNP ENCOR EXAM OBJECTIVES COVERED IN THIS CHAPTER INCLUDE THE FOLLOWING:

  • Domain 3.0 Infrastructure
    • ✓ 3.2  Layer 3

    OSPF is one of the most popular interior gateway protocols (IGPs) in use today. It was originally designed as a replacement for the simple Router Information Protocol (RIP), but it scales better and converges faster. OSPF is without a doubt the most complex IGP you'll use. In this chapter, we'll start by covering the basics of OSPF, including

  • Areas
  • Link-state advertisements (LSAs)
  • Neighbor operations
  • Network types

  Most importantly, you'll learn how to configure OSPF and several key features, including authentication, passive interfaces, default route injection, inter-area summarization, and route filtering.

  The exercises at the conclusion of the chapter align with the examples in the chapter. I recommend reading the chapter first, and then performing the exercises on your own, falling back on the chapter text as your guide in case you get stuck. Regardless of how you approach the exercises, be sure to complete them in your own Cisco lab. When it comes to understanding routing protocols, there's no substitute for hands-on configuring and troubleshooting.

  Link-State Advertisements

  OSPF is a link-state protocol, meaning that all routers in an autonomous system (AS) share the states of their local network links—their link-state information—with one another using link-state advertisements (LSAs). An LSA includes but isn't limited to the following key pieces of information:

  • Link state ID—Often contains an IP address or prefix.
  • Advertising router ID (RID)

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  The Internet and Its Protocols

  A Comparative Approach

  • Adrian Farrel(Author)
  • 2004(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  Although RIP responds well to link failures with the routers at the ends of the links able to withdraw routes using immediate updates, RIP relies for this process on the data-link or physical layers detecting the link failures. This means that where the lower layers cannot detect link failures or where the link is up but the neighboring router is down (perhaps a software or component failure) RIP must fall back on its own detection methods. RIP's responsiveness to this sort of failure is poor--it takes 180 seconds for a route to time out and a lot of data can be lost in that time. An increasing concern in modern networks is the fact that RIP includes no support for multicast routing. This function is present in several of the new routing protocols and facilitates the distribution of routes to support multicast traffic (described in Chapter 3). Finally, RIP is not a very secure protocol, although some securiW mechanisms can be applied. 5.5 Open Shortest PathFirst (OSPF) 155 Despite all of these concerns, RIP is still a good starting point for a routing protocol in a small network. 5,5 Open Shortest Path First (OSPF) Open Shortest Path First (OSPF) is a link state, interior gateway protocol developed by the IETF with a good look over the fence at IS-IS (see Section 5.6). We are now at version two of OSPF, version one having lasted only two years from its publication as an RFC to its eclipse by the RFC for OSPFv2. There have been several republications of the OSPFv2 standard over the years, culminating in the current version in RFC 2328, but each change has done little more than fix minor bugs and clarify frequently asked questions or deployment and imple- mentation issues. Further RFCs and drafts have been published to handle specific extensions to OSPF to handle additional requirements such as support for IPv6 and MPLS traffic engineering. OSPFv3 is currently under development within the IETF.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Network Recovery

  Protection and Restoration of Optical, SONET-SDH, IP, and MPLS

  • Jean-Philippe Vasseur, Mario Pickavet, Piet Demeester(Authors)
  • 2004(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  Referring to our previous example depicted in Figure 4.13, if two packets enter node A and have node D as their IP destination, the data packet (marked with a specific DSCP value) will be routed along the SPT1 of MT1 and the voice packet marked with a different DSCP value will follow the SPT2 of MT2. The case of OSPF is slightly different. The support of IP version 6, for instance, requires OSPF version 3 ([OSPFv3]), but the capability to support multiple topol-ogies (with multiple metrics per link) is part of the current protocol specification. When an adjacency is formed between two neighbors, the link can be advertised with more than one metric in the corresponding LSA type 1. Also, when interarea or external routes are advertised in LSAs of type 3 and 5, respectively, multiple metrics can be associated with each route. The protocol can support up to one metric per IP ToS value. Then each router computes an SPF per ToS and packets are routed using the appropriate routing table based on the ToS value of their IP header. This is equivalent to the example 2 presented earlier. Note that OSPF routers can be configured to route all the IP packets on the ToS 0 path only. When routers supporting ToS routing are combined with routers that just support ToS 0 path, then during the SPF computation, the routers that only support ToS 0 routing should be avoided to route non–ToS 0 IP packets. 4.6.2 The Dijkstra Algorithm The famous mathematician Edger Dijkstra, a pioneer in computer science, gave his name to an algorithm allowing the computation of a loop-free shortest path, which has been used for several decades in a wide range of contexts. The algorithm is described in detail in this section, but it is quite interesting to read a quote from Dijkstra related to this invention (from [EWD-1166]): I designed my first nontrivial algorithms.

  Sign up to read

  Learn more about book