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Questions and Answers

What is the purpose of the LSAck message in OSPF?

  • To send routing updates
  • To request router ID information
  • To acknowledge other OSPF messages (correct)
  • To request link-state information

    • What is the length of the OSPF message header?

  • 16 bytes
  • 32 bytes
  • 24 bytes (correct)
  • 48 bytes

    • What is used to compute the OSPF checksum?

  • Only the header is included
  • The entire message is included, except the authentication field (correct)
  • Only the data field is included
  • The entire message is included, including the authentication field

    • What is the purpose of the Router ID in OSPF?

    <p>To uniquely identify a router within an OSPF domain</p> Signup and view all the answers

    How does an OSPF router select its Router ID?

    <p>By selecting the highest IP address on a router's loopback interface, or the highest IP address on a router's running interface if no loopback interface is configured</p> Signup and view all the answers

    What is the purpose of the Authentication field in OSPF?

    <p>To authenticate OSPF messages</p> Signup and view all the answers

    What is the size of the Authentication field in OSPF?

    <p>64 bits</p> Signup and view all the answers

    What is the OSPF message type that is used to request link-state information?

    <p>LSR</p> Signup and view all the answers

    What is the state of a neighbor that has downloaded all required LSAs from another router?

    <p>Full</p> Signup and view all the answers

    What type of LSA is generated by the Designated Router (DR)?

    <p>Type 2 - Network LSA</p> Signup and view all the answers

    What is the purpose of a Type 4 LSA?

    <p>To identify the ASBR</p> Signup and view all the answers

    What type of area does not accept external routes and uses a default route to reach external routing domains?

    <p>Stub Area</p> Signup and view all the answers

    What is the name of the area that has all the properties of a standard area?

    <p>Area 0</p> Signup and view all the answers

    What type of LSA is used to advertise external routes?

    <p>Type 5 - External LSA</p> Signup and view all the answers

    What type of area does not accept external routes or summary routes from other areas?

    <p>Totally Stubby Area</p> Signup and view all the answers

    What is the purpose of a Type 1 LSA?

    <p>To advertise a router's links</p> Signup and view all the answers

    What is the primary purpose of the ExStart state in OSPF?

    <p>To determine the Master and Slave roles for each router</p> Signup and view all the answers

    What is the state of a neighbor when a working adjacency to a neighbor is torn down?

    <p>Down</p> Signup and view all the answers

    What is exchanged between routers in the Exchange state?

    <p>Link-state database elements</p> Signup and view all the answers

    When is a neighbor moved from the Exchange to the Loading state?

    <p>When the neighbor has advertised the complete list of LSAs</p> Signup and view all the answers

    What is the purpose of the 2-Way state in OSPF?

    <p>To confirm a bidirectional visibility between routers</p> Signup and view all the answers

    What is the state of a neighbor when a router can hear the other router but is not certain whether the other router can hear this router?

    <p>Init</p> Signup and view all the answers

    What is the result of a neighbor being in the Down state?

    <p>The router already knows about this neighbor's IP address</p> Signup and view all the answers

    What is the primary purpose of the Loading state in OSPF?

    <p>To download LSAs from the neighbor</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA)?

    <p>It updates its link-state database and forwards the LSA to all neighboring devices</p> Signup and view all the answers

    What is the purpose of a link-state database in OSPF?

    <p>To calculate the best paths through the network</p> Signup and view all the answers

    Why are link-state protocols preferred in larger networks?

    <p>Because they support classless addressing and area hierarchies</p> Signup and view all the answers

    What is the result of an LSA being flooded throughout an area in OSPF?

    <p>All routers within the area recalculate their routing tables</p> Signup and view all the answers

    What is a link in the context of OSPF?

    <p>An interface on a router</p> Signup and view all the answers

    How do link-state routers find the best paths to a destination?

    <p>By applying Dijkstra's SPF algorithm</p> Signup and view all the answers

    What is the purpose of area hierarchies in OSPF?

    <p>To limit the scope of route changes</p> Signup and view all the answers

    What is a characteristic of link-state routing protocols?

    <p>They send updates only when a topology change occurs</p> Signup and view all the answers

    What is the purpose of OSPF Area ID?

    <p>To name an area</p> Signup and view all the answers

    What is the role of the DR in OSPF?

    <p>To act as a central point for exchanging OSPF information</p> Signup and view all the answers

    How do non-DR and non-BDR routers exchange routing information in OSPF?

    <p>They exchange routing information with the DR and BDR only</p> Signup and view all the answers

    What is the default OSPF priority of all routers on their OSPF-enabled interfaces?

    <p>1</p> Signup and view all the answers

    What is the consequence of setting an OSPF router's priority to 0?

    <p>The router does not participate in the DR/BDR election process</p> Signup and view all the answers

    What multicast address do DR routers use to forward link-state information to other routers?

    <p>224.0.0.5</p> Signup and view all the answers

    What is the number of stages before two OSPF routers' LSDB gets synchronized?

    <p>7</p> Signup and view all the answers

    What is the role of the BDR in OSPF?

    <p>To become the DR in case the current DR fails</p> Signup and view all the answers

    What is the role of non-backbone internal routers F, G, and H in OSPF?

    <p>To maintain identical link-state databases about their respective areas</p> Signup and view all the answers

    What type of router is connected to two or more routing domains and exchanges routing information between different routing protocols?

    <p>Autonomous System Boundary Router (ASBR)</p> Signup and view all the answers

    What is the purpose of an ABR router in OSPF?

    <p>To advertise default routes to non-backbone internal routers</p> Signup and view all the answers

    What is the characteristic of link-state routing protocols in OSPF?

    <p>They develop and maintain full knowledge of the network routers and how they interconnect</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA) in OSPF?

    <p>It floods the LSA to all other routers in the area</p> Signup and view all the answers

    What is the purpose of area hierarchies in OSPF?

    <p>To provide a hierarchical structure for routing information exchange</p> Signup and view all the answers

    What type of area does not accept external routes and uses a default route to reach external routing domains?

    <p>Stub area</p> Signup and view all the answers

    What is the primary purpose of the SPF algorithm in OSPF?

    <p>To compute reachability to networked destinations</p> Signup and view all the answers

    What is the state of a neighbor that has downloaded all required LSAs from another router and is fully adjacent?

    <p>Full state</p> Signup and view all the answers

    Which type of LSA is generated by each router for each area it is located?

    <p>Type 2 – Router LSA</p> Signup and view all the answers

    What type of area accepts link updates, summary routes, and external OSPF routes?

    <p>Standard area</p> Signup and view all the answers

    What type of LSA is used to advertise external routes?

    <p>Type 5 – External LSA</p> Signup and view all the answers

    What is the purpose of a Type 3 LSA?

    <p>To summarize routes between areas</p> Signup and view all the answers

    What type of area does not accept external routes or summary routes from other areas?

    <p>Totally stubby area</p> Signup and view all the answers

    Which type of area has all the properties of a standard area?

    <p>Backbone area</p> Signup and view all the answers

    What type of LSA is generated by the ASBR?

    <p>Type 5 – External LSA</p> Signup and view all the answers

    What is the primary characteristic of a Not So Stubby Area (NSSA)?

    <p>It can contain ASBR but does not accept external routes</p> Signup and view all the answers

    What is the purpose of OSPF?

    <p>To find the best path between the source and the destination router</p> Signup and view all the answers

    What is the key difference between a Totally Stubby Not So Stubby Area and a stub area?

    <p>Totally Stubby NSSA can contain ASBR, but stub area cannot</p> Signup and view all the answers

    What type of area does not accept external routes and uses a default route to reach external routing domains?

    <p>Stub area</p> Signup and view all the answers

    What is the characteristic of a link-state routing protocol?

    <p>It uses shortest path first (SPF) algorithm</p> Signup and view all the answers

    What is the primary advantage of using OSPF in large-scale networks?

    <p>It is a link-state routing protocol</p> Signup and view all the answers

    What is the purpose of area hierarchies in OSPF?

    <p>To improve the scalability and organization of the network</p> Signup and view all the answers

    What is the role of OSPF in a network?

    <p>To find the best path between the source and the destination router</p> Signup and view all the answers

    What is the primary benefit of using link-state routing protocols?

    <p>They respond quickly to network changes and reduce incorrect information</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA)?

    <p>It updates its link-state database and recalculates its best paths</p> Signup and view all the answers

    What is the primary purpose of a link-state database in OSPF?

    <p>To maintain a complex database of topology information</p> Signup and view all the answers

    What is a characteristic of link-state routing protocols?

    <p>They maintain full knowledge of distant routers and how they interconnect</p> Signup and view all the answers

    What is the purpose of area hierarchies in OSPF?

    <p>To improve the scalability of OSPF</p> Signup and view all the answers

    What is the result of an LSA being flooded throughout an area in OSPF?

    <p>All routers in the area recalculate their best paths</p> Signup and view all the answers

    What is the primary purpose of OSPF areas?

    <p>To divide the network into smaller, more manageable parts</p> Signup and view all the answers

    What is the benefit of using OSPF areas with route summarization?

    <p>It reduces the number of LSAs in the network</p> Signup and view all the answers

    What is the purpose of area hierarchies in OSPF?

    <p>To separate the backbone area from non-backbone areas</p> Signup and view all the answers

    Which type of OSPF area does not accept external routes or summary routes from other areas?

    <p>Totally stubby area</p> Signup and view all the answers

    What is the role of an area border router (ABR) in OSPF?

    <p>To route traffic destined for or arriving from other areas</p> Signup and view all the answers

    What is a characteristic of link-state routing protocols?

    <p>They support route summarization</p> Signup and view all the answers

    What is the purpose of a Type 1 LSA in OSPF?

    <p>To advertise router information</p> Signup and view all the answers

    What is the result of an LSA being flooded throughout an area in OSPF?

    <p>Routers update their link-state databases</p> Signup and view all the answers

    What is the purpose of route summarization in OSPF?

    <p>To reduce the link-state database size</p> Signup and view all the answers

    What is an autonomous system (AS) in OSPF?

    <p>A collection of networks under a common administration</p> Signup and view all the answers

    What is the primary function of an Area Border Router (ABR) in OSPF?

    <p>To connect multiple areas and maintain separate link-state databases</p> Signup and view all the answers

    What type of area is configured to reduce the link-state database and routing table size?

    <p>Stub area</p> Signup and view all the answers

    What is the term for the collection of networks under a common administration that share a common routing strategy?

    <p>Autonomous system</p> Signup and view all the answers

    What is the benefit of using route summarization in OSPF?

    <p>Improved network scalability</p> Signup and view all the answers

    What type of LSA is used to advertise external routes?

    <p>Type 5 LSA</p> Signup and view all the answers

    What is the term for the process of logically subdividing an Autonomous System into multiple areas?

    <p>Area hierarchies</p> Signup and view all the answers

    What is the primary function of the backbone area in OSPF?

    <p>To act as a transit area for inter-area traffic</p> Signup and view all the answers

    What is the characteristic of link-state routing protocols that enables route summarization?

    <p>They use a two-layer network hierarchy</p> Signup and view all the answers

    What is the primary purpose of the Summary LSA?

    <p>To summarize routes from other areas</p> Signup and view all the answers

    What type of area does not accept external routes or summary routes from other areas?

    <p>Totally Stubby Area</p> Signup and view all the answers

    What is the purpose of the Type 4 LSA?

    <p>To provide information about the ASBR</p> Signup and view all the answers

    What is the state of a neighbor when it has downloaded all required LSAs from another router?

    <p>Full State</p> Signup and view all the answers

    What type of LSA is generated by the ASBR?

    <p>Type 5 LSA</p> Signup and view all the answers

    What is the purpose of the ABR in OSPF?

    <p>To summarize routes from other areas</p> Signup and view all the answers

    What is the name of the area that has all the properties of a standard area?

    <p>Backbone Area</p> Signup and view all the answers

    What type of LSA is used to advertise external routes?

    <p>Type 5 LSA</p> Signup and view all the answers

    What is the primary advantage of link-state routing protocols over distance vector routing protocols?

    <p>Quick response to network changes</p> Signup and view all the answers

    What is the purpose of the hello mechanism in OSPF?

    <p>To determine the reachability of neighbors</p> Signup and view all the answers

    What type of area does not accept external routes and uses a default route to reach external routing domains?

    <p>Stub Area</p> Signup and view all the answers

    What is the purpose of link-state advertisements (LSAs) in OSPF?

    <p>To advertise routing information to other routers</p> Signup and view all the answers

    How do link-state routers find the best paths to a destination?

    <p>By maintaining full knowledge of distant routers and how they interconnect</p> Signup and view all the answers

    What is a characteristic of link-state routing protocols?

    <p>They are less likely to propagate incorrect information</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA)?

    <p>It recalculates its best paths to all destinations in the network</p> Signup and view all the answers

    Why are link-state protocols preferred in larger networks?

    <p>Because they are more scalable</p> Signup and view all the answers

    What type of area has the properties of a stub area but can contain an Autonomous System Boundary Router (ASBR)?

    <p>Not So Stubby Area (NSSA)</p> Signup and view all the answers

    What type of area has the properties of a totally stub area but can contain an Autonomous System Boundary Router (ASBR)?

    <p>Totally Stubby Not So Stubby Area</p> Signup and view all the answers

    What is the primary purpose of the OSPF protocol?

    <p>To find the best path between the source and the destination router using its own shortest path first (SPF) algorithm</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA)?

    <p>The router stores the LSA in its link-state database</p> Signup and view all the answers

    What is the purpose of OSPF area hierarchies?

    <p>To reduce the number of LSAs flooded throughout the network</p> Signup and view all the answers

    What type of router is connected to two or more routing domains and exchanges routing information between different routing protocols?

    <p>ASBR</p> Signup and view all the answers

    What is the role of the Designated Router (DR) in OSPF?

    <p>To forward link-state information to other routers</p> Signup and view all the answers

    What is the purpose of a Type 4 LSA in OSPF?

    <p>To advertise the ASBR's router ID</p> Signup and view all the answers

    What is the primary purpose of an ABR router in OSPF?

    <p>To advertise a default route to non-backbone internal routers</p> Signup and view all the answers

    What is the characteristic of link-state routing protocols in OSPF?

    <p>They maintain a complex database of the network topology</p> Signup and view all the answers

    What type of router is connected to two or more routing domains and exchanges routing information between different routing protocols?

    <p>ASBR router</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA) in OSPF?

    <p>It updates its routing table</p> Signup and view all the answers

    What type of area does not accept external routes and uses a default route to reach external routing domains?

    <p>Stub area</p> Signup and view all the answers

    What is the purpose of the SPF algorithm in OSPF?

    <p>To compute reachability to networked destinations</p> Signup and view all the answers

    What is the role of non-backbone internal routers F, G, and H in OSPF?

    <p>They are aware of the topology within their respective areas and maintain identical link-state databases</p> Signup and view all the answers

    What is the primary purpose of an Autonomous System Boundary Router (ASBR) in OSPF?

    <p>To exchange routing information between different routing protocols</p> Signup and view all the answers

    What is the format of the OSPF message header?

    <p>24 bytes</p> Signup and view all the answers

    What is the purpose of the Area ID field in OSPF messages?

    <p>To identify the OSPF area</p> Signup and view all the answers

    How does an OSPF router select its Router ID when there are no loopback interfaces?

    <p>The highest IP address of running interfaces</p> Signup and view all the answers

    What is the size of the Authentication field in OSPF messages?

    <p>64 bits</p> Signup and view all the answers

    What is the purpose of the Checksum field in OSPF messages?

    <p>To validate the message integrity</p> Signup and view all the answers

    What is the format of the OSPF message type field?

    <p>A 1-byte field</p> Signup and view all the answers

    What is the purpose of the Data field in OSPF messages?

    <p>To carry different information depending on the OSPF packet type</p> Signup and view all the answers

    What is the purpose of the Version Number field in OSPF messages?

    <p>To identify the OSPF version</p> Signup and view all the answers

    What is the metric used by OSPF protocol?

    <p>COST</p> Signup and view all the answers

    How does OSPF calculate its default cost?

    <p>100Mbps / (bandwidth in Mbps)</p> Signup and view all the answers

    What is the purpose of the Neighbor table in OSPF?

    <p>To store the details of OSPF neighbors</p> Signup and view all the answers

    How often does an OSPF router send HELLO messages to its neighbors?

    <p>Every 10 seconds</p> Signup and view all the answers

    What is the purpose of the Topology table in OSPF?

    <p>To store the Link State Information</p> Signup and view all the answers

    What is the result if an OSPF router does not receive HELLO messages from a neighbor for more than 10 seconds?

    <p>The OSPF router invalidates all the routes learnt from that neighbor</p> Signup and view all the answers

    What is the primary benefit of using a link-state routing protocol in a network?

    <p>It allows for faster convergence of the network.</p> Signup and view all the answers

    What algorithm does each OSPF router use to find the best routes?

    <p>Dijkstra's SPF Algorithm</p> Signup and view all the answers

    What determines the best path to a destination in a link-state routing protocol?

    <p>The metric selected by the protocol.</p> Signup and view all the answers

    What is the purpose of the OSPF metric calculation?

    <p>To find the best path to a destination</p> Signup and view all the answers

    What is the purpose of the table shown in Figure 10?

    <p>To illustrate the routing paths between multiple routers.</p> Signup and view all the answers

    What happens when a link-state router receives a Link-State Advertisement (LSA)?

    <p>The router compares the LSA to its current routing table.</p> Signup and view all the answers

    Why are link-state protocols preferred in larger networks?

    <p>Because they are more scalable.</p> Signup and view all the answers

    What is the role of the Next Hop column in the table shown in Figure 10?

    <p>It specifies the next router in the path to the destination.</p> Signup and view all the answers

    What is the advantage of using a link-state routing protocol in a network with multiple paths to a destination?

    <p>It enables routers to discriminate between multiple paths to a destination.</p> Signup and view all the answers

    What is the purpose of the Cost column in the table shown in Figure 10?

    <p>It shows the cost of the route to the destination.</p> Signup and view all the answers

    What is the primary function of the OSPF Area ID?

    <p>To name an area</p> Signup and view all the answers

    What is the purpose of the DR and BDR in OSPF?

    <p>To reduce the amount of OSPF routing updates</p> Signup and view all the answers

    What happens when a router's OSPF priority is set to 0?

    <p>It does not participate in the DR and BDR election process</p> Signup and view all the answers

    How do non-DR and non-BDR routers exchange routing information in OSPF?

    <p>With the DR and BDR</p> Signup and view all the answers

    What is the default OSPF priority of all routers on their OSPF-enabled interfaces?

    <p>1</p> Signup and view all the answers

    What multicast address do DR routers use to forward link-state information to other routers?

    <p>224.0.0.5</p> Signup and view all the answers

    How many stages are there before two OSPF routers' LSDB gets synchronized?

    <p>7</p> Signup and view all the answers

    What is the role of the BDR in OSPF?

    <p>To take over as DR if the current DR fails</p> Signup and view all the answers

    What type of area has the properties of a stub area but can contain an ASBR?

    <p>Not So Stubby Area</p> Signup and view all the answers

    What is the purpose of the Open Shortest Path First (OSPF) protocol?

    <p>To find the best path between the source and the destination router</p> Signup and view all the answers

    What type of area has the properties of a totally stub area but can contain an ASBR?

    <p>Totally Stubby Not So Stubby Area</p> Signup and view all the answers

    What is a characteristic of OSPF?

    <p>It is a link-state routing protocol</p> Signup and view all the answers

    Why is OSPF preferred in larger networks?

    <p>Because it is a link-state protocol</p> Signup and view all the answers

    What is the primary purpose of the SPF algorithm in OSPF?

    <p>To find the best path between the source and the destination router</p> Signup and view all the answers

    What type of router is connected to two or more routing domains and exchanges routing information between different routing protocols?

    <p>ASBR</p> Signup and view all the answers

    What is the purpose of area hierarchies in OSPF?

    <p>To improve the scalability of the network</p> Signup and view all the answers

    Study Notes

    OSPF Message Format

    • OSPF message format consists of various fields, including:
      • Version Number (set to 2 for OSPF version 2)
      • Type (Hello, Database Description, LSR, LSU, LSAck)
      • Packet Length (in bytes, including the 24-byte header)
      • Router ID (the ID of the router that generated the message)
      • Area ID (an identification of the OSPF area to which the message belongs)
      • Checksum (a 16-bit checksum computed in a manner similar to a standard IP checksum)
      • Authentication Type (plaintext, no authentication, or MD5 type)
      • Authentication (a 64-bit field used for authentication of the message, as needed)
      • Data (contains different information depending on the OSPF packet type)

    OSPF Router ID

    • OSPF router ID is selected by the router itself when the OSPF process initializes
      • The highest IP address on a loopback interface is selected as the router ID
      • If no loopback interfaces are configured, the highest IP address of running interfaces is selected
      • The router ID uniquely identifies a router within an OSPF domain

    OSPF LSA Types

    • There are several types of LSAs, including:
      • Type 1 – Router LSA (generated by each router for each area it is located in)
      • Type 2 – Network LSA (generated by the DR)
      • Type 3 – Summary LSA (created by the ABR and flooded into other areas)
      • Type 4 – Summary ASBR LSA (generated by the ABR to include the router ID of the ASBR)
      • Type 5 – External LSA (also known as autonomous system external LSA, generated by the ASBR)
      • Type 6 – Multicast OSPF LSA
      • Type 7 – External LSA (also known as not-so-stubby-area (NSSA) LSA)
      • Type 8 – Used to internetwork OSPF and BGP
      • Type 9, 10, 11 – Designated for future use, for application-specific purposes, used along with MPLS

    OSPF Area Types

    • There are several types of OSPF areas, including:
      • Standard Area (accepts link updates, summary routes, and external OSPF routes)
      • Backbone Area (also known as transit area, named as Area 0, has all properties of standard area)
      • Stub Area (does not accept external routes, uses default route to reach external routing domains)
      • Totally Stubby Area (does not accept external routes or summary routes from other areas, uses default route to reach non-local area networks)

    OSPF Neighbor States

    • OSPF neighbor states include:
      • Down (initial state for a neighbor, seen when a working adjacency to a neighbor is torn down or when a manually configured neighbor does not respond to initial Hello packets)
      • Init (this router can hear the other router, but it is not certain whether the other router can hear this router)
      • 2-Way (confirms a bidirectional visibility between the two routers)
      • ExStart (establishes the Master/Slave relationship, determines the Master and Slave roles for each router, and agrees on a common starting sequence number)
      • Exchange (Database Description packets are exchanged between routers, carrying the list of link-state database elements)
      • Loading (neighbor is moved from the Exchange to Loading state after it has advertised the complete list of LSAs and this router needs to download some of the LSAs from the neighbor)
      • Full (stable state between routers that have become fully adjacent, all required LSAs have been downloaded from the neighbor)

    OSPF LSDB Exchange Process

    • The OSPF LSDB exchange process involves seven stages before two OSPF routers' LSDB gets synchronized
      • Down
      • Init
      • 2-Way
      • ExStart
      • Exchange
      • Loading
      • Full

    OSPF Overview

    • OSPF (Open Shortest Path First) is a link-state routing protocol used to find the best path between the source and destination router using its own SPF algorithm.
    • It is a reliable Interior Gateway protocol for large-scale networks.

    OSPF States

    • A neighbor is moved to the Full state when all required LSAs have been downloaded from the neighbor, and all missing or outdated LSAs have been acquired.
    • The Full state is a stable state between routers that have become fully adjacent.

    OSPF LSA Types

    • Type 1: Router LSA - generated by each router for each area it is located in.
    • Type 2: Network LSA - generated by the DR (Designated Router).
    • Type 3: Summary LSA - created by the ABR (Area Border Router) and flooded into other areas.
    • Type 4: Summary ASBR LSA - generated by the ABR to inform other routers where to find the ASBR (Autonomous System Boundary Router).
    • Type 5: External LSA (Autonomous System External LSA) - generated by the ASBR.
    • Type 6: Multicast OSPF LSA
    • Type 7: External LSA (Not-So-Stubby Area (NSSA) LSA)
    • Type 8: Used to internetwork OSPF and BGP
    • Type 9, 10, 11: Designated for future use, for application-specific purposes, used along with MPLS.

    OSPF Area Types

    • Standard Area: accepts link updates, summary routes, and external OSPF routes.
    • Backbone Area (Transit Area): named as Area 0, has all properties of standard area.
    • Stub Area: does not accept external routes, uses default route to reach external routing domains.
    • Totally Stubby Area: does not accept external routes or summary routes from other areas, uses default route to reach non-local area networks.
    • Not So Stubby Area (NSSA): has the properties of stub area, but can contain ASBR.
    • Totally Stubby Not So Stubby Area (NSSA): has the properties of totally stub area, but can contain ASBR.
    • Link-state routing algorithms maintain a complex database of network topology.
    • Each router constructs a topological database using all received LSPs (Link-State Packets).
    • SPF algorithm is used to compute reachability to networked destinations, and updates the routing table.
    • Link-state routing protocols collect routing information from all other routers in the network or within a defined area of the internetwork.
    • Each router independently calculates its best paths to all destinations in the network.

    OSPF Router Nomenclature

    • Backbone Routers: routers A, B are called backbone routers, all their interfaces participate in Area 0.
    • Area Border Routers (ABR): routers C, D, E are called ABRs, attach to multiple areas, maintain separate link-state databases for each area.
    • Non-Backbone Internal Routers: routers F, G, and H are called non-backbone internal routers, aware of the topology within their respective areas.
    • Autonomous System Boundary Router (ASBR): a router that is connected to two or more routing domains, exchanges routing information between different routing protocols.

    OSPF Overview

    • OSPF (Open Shortest Path First) is a link-state routing protocol used to find the best path between the source and destination router using its own SPF algorithm.
    • It is a reliable Interior Gateway protocol for large-scale networks.

    OSPF States

    • A neighbor is moved to the Full state when all required LSAs have been downloaded from the neighbor, and all missing or outdated LSAs have been acquired.
    • The Full state is a stable state between routers that have become fully adjacent.

    OSPF LSA Types

    • Type 1: Router LSA - generated by each router for each area it is located in.
    • Type 2: Network LSA - generated by the DR (Designated Router).
    • Type 3: Summary LSA - created by the ABR (Area Border Router) and flooded into other areas.
    • Type 4: Summary ASBR LSA - generated by the ABR to inform other routers where to find the ASBR (Autonomous System Boundary Router).
    • Type 5: External LSA (Autonomous System External LSA) - generated by the ASBR.
    • Type 6: Multicast OSPF LSA
    • Type 7: External LSA (Not-So-Stubby Area (NSSA) LSA)
    • Type 8: Used to internetwork OSPF and BGP
    • Type 9, 10, 11: Designated for future use, for application-specific purposes, used along with MPLS.

    OSPF Area Types

    • Standard Area: accepts link updates, summary routes, and external OSPF routes.
    • Backbone Area (Transit Area): named as Area 0, has all properties of standard area.
    • Stub Area: does not accept external routes, uses default route to reach external routing domains.
    • Totally Stubby Area: does not accept external routes or summary routes from other areas, uses default route to reach non-local area networks.
    • Not So Stubby Area (NSSA): has the properties of stub area, but can contain ASBR.
    • Totally Stubby Not So Stubby Area (NSSA): has the properties of totally stub area, but can contain ASBR.
    • Link-state routing algorithms maintain a complex database of network topology.
    • Each router constructs a topological database using all received LSPs (Link-State Packets).
    • SPF algorithm is used to compute reachability to networked destinations, and updates the routing table.
    • Link-state routing protocols collect routing information from all other routers in the network or within a defined area of the internetwork.
    • Each router independently calculates its best paths to all destinations in the network.

    OSPF Router Nomenclature

    • Backbone Routers: routers A, B are called backbone routers, all their interfaces participate in Area 0.
    • Area Border Routers (ABR): routers C, D, E are called ABRs, attach to multiple areas, maintain separate link-state databases for each area.
    • Non-Backbone Internal Routers: routers F, G, and H are called non-backbone internal routers, aware of the topology within their respective areas.
    • Autonomous System Boundary Router (ASBR): a router that is connected to two or more routing domains, exchanges routing information between different routing protocols.
    • Convergence process can be accelerated with link-state routing protocol, as wait time for timers to expire is eliminated
    • In networks with link-state protocols, connectivity between remote areas is not a concern
    • The protocol can discriminate between two paths to the same destination and chose the best one

    OSPF (Open Shortest Path First)

    • OSPF is a link-state routing protocol
    • It maintains three tables for efficient routing: Neighbor table, Topology table, and Routing table
    • Each OSPF-speaking router is identified by a router ID

    OSPF Metric Calculation

    • OSPF uses cost as its metric
    • Cost is calculated using the formula: 100Mbps / (bandwidth in Mbps)
    • The reference bandwidth can be altered by the network administrator

    OSPF Tables

    • Neighbor table contains details of OSPF neighbors with which the router has built adjacency
    • Topology table contains link-state information flooded by all OSPF routers in an area
    • Each OSPF router builds an SPF tree and finds the best route to remote networks independently

    OSPF Messages

    • OSPF message format includes fields such as version number, type, packet length, router ID, area ID, checksum, and authentication
    • OSPF message types include Hello, Database description, LSR, LSU, and LSAck

    OSPF Router ID

    • OSPF router ID is selected as the highest IP address on a router's loopback interface
    • If no loopback interfaces are configured, the highest IP address of running interfaces is selected as the router ID
    • OSPF router ID can also be configured manually by the network administrator

    OSPF Area ID

    • OSPF Area ID is a number used to name an area
    • It is used to identify an area in an OSPF domain

    OSPF DR and BDR Election Process

    • OSPF routers connected via a broadcast network elect two routers as DR (Designated Router) and BDR (Backup Designated Router)
    • DR and BDR act as a central point for exchanging OSPF information between multiple routers on the same segment
    • Election is based on OSPF priority, with the highest priority winning; in case of a tie, the highest router ID wins

    OSPF LSDB Exchange Process

    • There are seven stages before two OSPF routers' LSDB gets synchronized
    • LSDB exchange process involves the exchange of link-state information between OSPF routers

    OSPF Area Types

    • Not So Stubby Area (NSSA): has the properties of a stub area, but can contain ASBR (Autonomous System Boundary Router)
    • Totally Stubby Not So Stubby Area: has the properties of a totally stub area, but can contain ASBR

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    Description

    This quiz covers the different fields in an OSPF message format, including version number, type, packet length, and router ID.

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