Open Shortest Path First

11 Key excerpts on "Open Shortest Path First"

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  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

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  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.

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  eBook - ePub

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

  Exam N10-008

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

    (Publisher)

  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 multicast 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. OSPF has many advantages over RIP, including route efficiency and convergence speed, but it also requires more processing power, memory, and storage from the router.

  Enhanced Interior Gateway Routing Protocol (EIGRP)

  The Enhanced Interior Gateway Routing Protocol (EIGRP) is an updated version of the earlier Interior Gateway Routing Protocol (IGRP). Although ostensibly a distance vector protocol, EIGRP is sometimes referred to as a hybrid routing protocol

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  eBook - ePub

  Network Routing

  Fundamentals, Applications, and Emerging Technologies

  • Sudip Misra, Sumit Goswami(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  Best‐effort routing attempts to provide the best feasible path from source to destination without guarantee of any assured level of service. Although a best‐effort routing does not fulfill QoS requirements, it can be modified to satisfy minimum constraints to cater effectively for the assured level of service. On the Internet, which follows best‐effort routing, a single best path towards the destination is computed by each intermediate routing node [35]. The selected path changes with change in the network topology or congestion in any part of the network. As the path is selected and the packets are delivered on a hop‐by‐hop basis, the packets may reach out of order of delivery and may even get dropped in any of the intermediate routers owing to load condition and unavailability of buffer space. Unlike the single‐best‐path selection strategy used on the Internet, best‐effort QoS in a network requires multiple paths to a destination to support various levels of performance. The routing requires a path selection algorithm that efficiently computes a set of best‐effort paths, and forwards the traffic over multiple paths to the destination. However, routing over multiple paths faces the challenge of loop detection and avoidance. Moreover, the best set of paths have to be recomputed if there are changes in topology, including node or link failures.

  Load‐sharing routing (LSR) [36] is one of the algorithms proposed for best‐effort QoS routing. This algorithm tries to improve the QoS only in those regions of the network where it has deteriorated owing to congestion. The overhead of the routing algorithm is reduced by making only the neighboring nodes of a congested node implement the LSR algorithm. In this routing algorithm, each routing node maintains an active routing table and a passive routing table. The active routing table is created from the node’s awareness of the network topology and gives the shortest path based on Dijkstra’s algorithm from the node to any other node in the network. These paths are referred to as OSPF paths in this protocol. The passive routing table keeps the shortest distance to all other nodes from each of its neighboring nodes as the source node. This is also calculated using Dikstra’a algorithm by keeping the neighbor as the source node. The entry in the routing table comprises the destination node, the next‐hop node, the total cost, the hop count, the OSPF next hop, and the LSR flag. When the LSR flag is false, the node forwards the packet as per the OSPF algorithm and the next‐hop node and the OSPF next‐hop node are the same. When the LSR flag is true, the next‐hop node is calculated on the basis of the LSR algorithm. The algorithm uses a number of different messages to calculate the path. The working of the algorithm can be well understood by referring to these messages.

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  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.

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  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

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  eBook - ePub

  Fundamentals of Data Communication Networks

  • Oliver C. Ibe(Author)
  • 2017(Publication Date)
  • Wiley

    (Publisher)
• RIP cannot handle variable length subnet masks (VLSM). All subnets must have the same subnet mask length.
• Periodic broadcasting of the full routing table consumes a large amount of bandwidth, which is a major problem with large networks especially on slow links and WAN clouds.
• RIP converges slowly. This means that we may not finish running one round of updates before it is time for the next one. Thus, it takes a long time to get a complete update using the algorithm.
• RIP has no concept of network delays and link costs; routing decisions are based on hop counts. The path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and smaller delays.
• RIP networks are flat networks; there is no concept of areas or boundaries.
Some enhancements were introduced in a new version of RIP called RIP2 to address the issues of VLSM, authentication, and multicast routing updates. However, RIP2 is not a big improvement over RIP (now called RIP1) because it still has the limitations of hop counts and slow convergence; these are issues that are not expected in today's large networks.

7.16 Open Shortest Path First Protocol

OSPF is a link-state protocol that distributes routing information between routers in a single AS; thus, it is an interior gateway routing protocol. OSPF chooses a least-cost path as the best path and is suitable for complex networks with a large number of routers. It provides equal cost multipath routing where packets to a single destination can be sent via more than one interface simultaneously.

As a link-state protocol, each router maintains a database describing the entire AS topology, which it builds out of the collected link-state advertisements of all routers. Each participating router distributes its local state (i.e., the router's usable interfaces and reachable neighbors) throughout the AS by flooding. Each network that has at least two attached routers has a designated router and a backup-designated router. The designated router floods a link-state advertisement for the network and has other special responsibilities.