19 Chapter 3 Single-Area OSPF Link-state routing protocols differ from distance vector protocols. Link-state protocols flood link-state information and thus give every router a complete view of the network topology. With a distance vector protocol, routers do not learn about the complete network topology. Open Shortest Path First (OSPF) is a link-state protocol. In OSPF, the shortest path first (SPF) algorithm, discovered by the computer scientist Dijkstra, is used to determine the best path – the lowest-cost path to a link. The SPF algorithm was conceived as an algorithm for point-to-point network connections. To implement OSPF on the variety of networks that are available today, OSPF must be aware of the network type in which it operates. This allows OSPF to work correctly for these different network types. Before link-state information is exchanged, the OSPF protocol establishes a neighbor relationship between routers. The OSPF Hello protocol is used for this purpose. OSPF is a complex protocol and is described by a set of operational steps. OSPF is a link-state routing protocol based on open standards. It is described in several standards of the Internet Engineering Task Force (IETF); the most recent is RFC 2328. The Open in OSPF means that it is open to the public and is nonproprietary. OSPF is becoming the preferred IGP protocol compared to the Routing Information Protocol (RIP) because it is scalable.
Transcript
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Chapter 3
Single-Area OSPF
Link-state routing protocols differ from distance vector protocols. Link-state protocols flood link-state information and thus give every router a complete view of the network topology. With a distance vector protocol, routers do not learn about the complete network topology. Open Shortest Path First (OSPF) is a link-state protocol.
In OSPF, the shortest path first (SPF) algorithm, discovered by the computer scientist Dijkstra, is used to determine the best path – the lowest-cost path to a link.
The SPF algorithm was conceived as an algorithm for point-to-point network connections. To implement OSPF on the variety of networks that are available today, OSPF must be aware of the network type in which it operates. This allows OSPF to work correctly for these different network types.
Before link-state information is exchanged, the OSPF protocol establishes a neighbor relationship between routers. The OSPF Hello protocol is used for this purpose.
OSPF is a complex protocol and is described by a set of operational steps.
OSPF is a link-state routing protocol based on open standards. It is described in several standards of the Internet Engineering Task Force (IETF); the most recent is RFC 2328.
The Open in OSPF means that it is open to the public and is nonproprietary. OSPF is becoming the preferred IGP protocol compared to the Routing Information Protocol (RIP) because it is scalable.
RIP cannot scale beyond 15 hops, it converges slowly, and it can choose slow routes because it ignores critical factors such as bandwidth in route determination.
OSPF deals with these limitations and has been proven to be a robust, scalable routing protocol suitable for modern networks. OSPF can be used for large networks because it scales to larger networks if hierarchical network design principles are used. OSPF can also be used and configured as a single area for small networks.
OSPF can be used for large networks. OSPF routing scales to large networks if hierarchical network design principles are used.
Large OSPF networks use hierarchical design principles. Multiple areas connect to a distribution area, area 0, also called the backbone. This design approach allows for extensive control of routing updates. Defining areas reduces routing overhead, speeds up convergence, confines network instability to an area, and improves performance.
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Concept Questions
Demonstrate your knowledge of these concepts by answering the following questions in the space provided.
1. As a link-state protocol, how does OSPF operate differently from distance vector routing protocols?
Link-state routers identify neighboring routers and then communicate with these identified neighbors. OSPF comes with a new set of terms. Information gathered from OSPF neighbors is not a complete routing table. Each OSPF router tells about the status of its connections, or links. This information is flooded to all its neighbors.
An OSPF router advertises its link-states and passes on received link-states. The routers process this information and build a link-state database. Every router in the OSPF area, will have the same link-state database. Every router has the same information about the state of the links and every other router’s neighbors.
Each router then runs the Shortest Path First (SPF) algorithm, also known as the Dijkstra algorithm, on its copy of the database. This calculation determines the best route to a destination. OSPF routers record information about their neighbors in the adjacency database. To reduce the number of exchanges of routing information among several neighbors on the same network, OSPF routers elect a Designated Router (DR) and a Backup Designated Router (BDR) that serve as focal points for routing information exchange.
2. OSPF routers establish relationships, or states, with their neighbors to efficiently share link-state information. Fill in the following chart with descriptions of the different OSPF router states given the type of packet that is sent.
OSPF Packet Type DescriptionType 1 – Hello Establishes and maintains
adjacency information with neighbors
Type 2 – Database description packet (DBD) Describes the contents of an OSPF router’s link-state database
Type 3 – Link-state Request Requests specific pieces of a router’s link-state database
Type 4 – Link-state update (LSU) Transports link-state advertisements (LSAs) to neighbor routers
Type 5 – Link-state acknowledgement (LSACK) Acknowledges receipt of a neighbor’s LSA
3. OSPF interfaces can be in one of seven states. OSPF neighbor relationships progress through these states, one at a time, in this order:
Down State - In the Down state, the OSPF process has not exchanged information with any neighbor. OSPF is waiting to enter the next state, which is the Init state.
Init State -OSPF routers send Type 1 (hello) packets at regular intervals (usually 10 seconds) to establish a relationship with neighbor routers. When an interface receives its first hello packet, the router enters the Init state, which means the router knows a neighbor is out there and is waiting to take the relationship to the next step.
There are two kinds of relationships - two-way state and adjacency - although there are many phases in between. A router must receive a hello from a neighbor before it can establish any relationship.
Two-Way State -Using hello packets, every OSPF router tries to establish a Two-way state, or bi-directional communication, with every neighbor router on the same IP network. Among other things, hello packets include a list of the sender's known OSPF neighbors. A router enters the Two-Way state when it sees itself in a neighbor's hello. For example, when RTB learns that RTA knows about RTB, RTB declares a two-way state to exist with RTA.
The Two-Way state is the most basic relationship that OSPF neighbors can have, but routing information is not shared between routers in this relationship. To learn about other routers' link-states and eventually build a routing table, every OSPF router must form at least one adjacency. An adjacency is an advanced relationship between OSPF routers that involves a series of progressive states that rely not just on hellos, but also on the other four types of OSPF packets. Routers that attempt to become adjacent to one another exchange routing information even before the adjacency is fully established. The first step toward full adjacency is the ExStart state, which is described next.
ExStart State - Technically, when a router and its neighbor enter the ExStart state, their conversation is characterized as an adjacency, but the routers have not become fully adjacent yet. ExStart is established using Type 2 database description (DBD) packets, also known as DDPs. The two neighbor routers use hello packets to negotiate who is the "master" and who is the "slave" in their relationship and DBD packets to exchange databases.
The router with the highest OSPF router ID "wins" and becomes master. (The OSPF router ID is discussed later in this chapter.) When the neighbors establish their roles as master and slave, they enter the Exchange state and begin sending routing information.
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Exchange State - In the Exchange state, neighbor routers use Type 2 DBD packets to send each other their link-state information. In other words, the routers describe their link-state databases to each other. The routers compare what they learn with their existing link-state databases. If either of the routers receives information about a link that is not already in its database, the router requests a complete update from its neighbor. Complete routing information is exchanged in the Loading state.
Loading State - After the databases have been described to each router, they may request information that is more complete by using Type 3 packets, link-state requests (LSRs). When a router receives an LSR, it responds with an update by using a Type 4 link-state update (LSU) packet. These Type 4 LSU packets contain the actual link-state advertisements (LSAs), which are the heart of link-state routing protocols. Type 4 LSUs are acknowledged using Type 5 packets, called link-state acknowledgments (LSAcks).
Full Adjacency - With the Loading state complete, the routers are fully adjacent. Each router keeps a list of adjacent neighbors, called the adjacency database. Do not confuse the adjacency database with the link-state database or the forwarding database.
Vocabulary Exercise
Define the following terms as completely as you can. Use the online curriculum or Chapter 3 of the Cisco Networking Academy Program CCNA 3 and 4 Companion Guide, Third Edition, for help.
Backbone - The part of a network that acts as the primary path for traffic that is most often sourced from, and destined for, other networks.
Backup designated router (BDR) – backup to the designated router
Designated router (DR) - OSPF router that generates LSAs for a multiaccess network and has other special responsibilities in running OSPF. Each multiaccess OSPF network that has at least two attached routers has a designated router that is elected by the OSPF Hello protocol. The designated router enables a reduction in the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topological database.
Flooding - Process that sends out information out all ports, with the exception of the port on which the information was received.
Hello protocol - Protocol used by OSPF systems for establishing and maintaining neighbor relationships.
Link-state advertisement (LSA) – Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables. Sometimes called a link-state packet (LSP).
Process identifier – A numerical number assigned during the OSPF configuration process that identifies each OSPF process running on a router.
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Router identifier (Router ID) – IP address that identifies a router participating in OSPF. The router ID is taken from the active interface with the highest ip address or from a configured loopback interface.
Shortest Path First (SPF)Algorithm - Routing algorithm that iterates on length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra's algorithm.
Focus Questions
1. Link-state routers maintain a common picture of the network and exchange link information upon initial ________ or ________. Link-state routers do not broadcast their routing tables ________, like distance vector routing protocols do.
2. Whereas ________ is appropriate for small networks, ________ was written to address the needs of large ______ internetworks.
3. An issue OSPF addresses is speed of ________. With OSPF, ________ is faster because only the routing changes (not the entire routing table) are flooded rapidly to other routers in the OSPF network.
4. RIP broadcasts full routing tables to all neighbors every ______ seconds. However, this is especially problematic over slow WAN links, because these updates consume large amounts of bandwidth. Alternatively, OSPF ________ minimally sized link-state updates and sends the updates only when a ________ is in the network.
5. In contrast, OSPF selects optimal routes using ________ as a factor. (This is a metric based on bandwidth.)
6. List the OSPF network type that matches each topology shown in Figure 3-1.
Figure 3-1 List the OSPF Network Type
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7. OSPF operations follow these steps:
Establish router ________.
Elect a ________ ________ and a ________ ________ ________.
________ routes.
________ appropriate routes to use.
________ routing information
8. Because OSPF routing is dependent on the status of a link between two routers, neighbor routers must “________” each other on the network before they can share information.
9. Another noteworthy point about OSPF is that it does not perform ________ and ________; these functions are performed at the IP layer.
10. Every OSPF packet shares a common ____-byte protocol header.
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11. Fill in Figure 3-2, which shows the different parts of the OSPF header.
Figure 3-2 List the OSPF Header Fields
12. ________ packets are sent periodically out of each interface using IP multicast addresses.
13. The Hello process provides quicker detection of failed routers because Hellos are exchanged every ____ seconds. If a router is silent for ____ seconds, its neighbors believe it is down.
14. The ________ IP address on an active physical or loopback interface is the Router ID.
15. Routers from the same ________ each have the same link-state database information.
16. In a typical broadcast LAN environment such as Ethernet or Token Ring, the OSPF routers communicate with the DR using multicast address ________.
17. The following are used to determine which routers are elected as DR and BDR:
The router with the highest set ________ value is the ____.
The router with the second-highest set priority value is the _____.
The default value for OSPF interface priority is ___. In case of a tie, the router ___ is used as a tiebreaker.
The only time a DR or BDR changes is if one of them ________ ________.
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18. Fill in the circles pointing out the two DRs and one BDR in Figure 3-3.
Figure 3-3 List the DRs and BDR
19. The process used to discover the network routes is called the ________ process. It is performed to advance the routers to a ________ state of communication.
20. The routers must be in the ________ state before they can route traffic.
21. To calculate the lowest cost to a destination, link-state protocols such as OSPF use the ________ algorithm.
22. Sometimes, a link goes up and down rapidly. This is called ________.
23. The ___ ________ command was added to the Cisco IOS software to prevent routers from computing a new routing table until ____ seconds after a route change.
24. When there is a change in a link state, the router uses a ________ process to notify the other routers in the network of the change.
25. When a router notices a change in a link state, it ________ an LSU packet that includes the updated LSA entry to ________, the “all DR routers” (and BDR) address.
26. Write the configuration entry to enable OSPF on a router:
router(config)#router ospf process-id
27. ________ -____ is a user-defined number that identifies multiple OSPF processes running in a single router.
28. To debug a variety of OSPF operations, enter the follow command:
p2r2#debug ip ospf ?
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Focus Questions Answers
1. Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or changes. Link-state routers do not broadcast their routing tables periodically, like distance vector routing protocols do.
2. While RIP is appropriate for small networks, OSPF was written to address the needs of large, scaleable internetworks.
3. An issue OSPF addresses speed of convergence --- With OSPF, convergence is faster because only the routing changes (not the entire routing table) are flooded rapidly to other routers in the OSPF network.
4. RIP broadcasts full routing tables to all neighbors every 30 seconds. However, this is especially problematical over slow WAN links because these updates consume large amounts of bandwidth. Alternately, OSPF multicasts minimally sized link-state updates and sends the updates only when there is a change in the network.
5. In contrast, OSPF selects optimal routes using cost as a factor (This is a metric based on bandwidth).
6. List the OSPF network type that matches each topology:
7. OSPF operations follow these steps: Establish router adjacencies.
Elect a Designated Router and a Backup Designated Router.
Discover routes.
Select appropriate routes to use.
Maintain routing information
8. Because OSPF routing is dependent on the status of a link between two routers, neighbor routers must "recognize" each other on the network before they can
Broadcast Multi-access Point to Point NBMA
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share information.
9. Another noteworthy point about OSPF is that it does not perform fragmentation and reassembly; these functions are performed at the IP layer.
10. Every OSPF packet shares a common 24-byte protocol header
11.Fill in the graphic explaining the different parts of the OSPF header:
12.Hello packets are sent periodically out of each interface using IP multicast addresses.
13.The Hello process also provides quicker detection of failed routers because Hellos are exchanged every 10 seconds. Routers expect to hear from their neighbors every 10 seconds. If a router is silent for 40 seconds, its neighbors believe it is down.
14.The highest IP address on an active physical or loopback interface is the Router ID
15. Routers from the same area will each have the same link-state database information.
16. In a typical broadcast LAN environment such as Ethernet or Token Ring, the OSPF routers communicate with the DR by using multicast address 224.0.0.6
17.The following are used to determine which routers are elected as DR and BDR:
-The router with the highest set priority value is the DR-The router with the second highest set priority value is the BDR-The default value for OSPF interface priority is 1. In case of a tie, the router's Router ID is used as a tie breaker.
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- The only time a DR or BDR will change is if one of them goes down.
18.Fill in the circles pointing out the locations of the two DRs and one BDR below:
19.The process used to discover the network routes is called the exchange process, and is performed to advance the routers to a Full state of communication.
20.The routers must be in the Full state before they can route traffic.
21.To calculate the lowest cost to a destination, link-state protocols such as OSPF use the Dijkstra algorithm.
22.Sometimes a link will go up and down rapidly. This is called flapping.
23.The spf holdtime command was added to the Cisco IOS software to prevent routers from computing a new routing table until 10 seconds (default) after a route change.
24.When there is a change in a link-state, the router uses a flooding process to notify the other routers in the network of the change.
25.When a router notices a change in a link-state it multicasts an LSU packet that includes the updated LSA entry to 224.0.0.6, the "all DR Routers" (and BDR) address.
27. process-id is a user defined number to identify multiple OSPF processes running within a single router.
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CCNA Exam Review Questions
The following questions help you review for the CCNA exam. The answers appear in Appendix A, “Answers to CCNA Exam Review Questions.”
1. What state are the routers in an OSPF network in after the DR and BDR are elected?
A. Exstart
B. Full
C. Loading
D. Exchange
2. What OSPF packet type establishes and maintains neighbor adjacencies?
A. Link-state request
B. Link-state acknowledgment
C. Hello
D. Database description packet
3. What is the default cost metric for OSPF based on?
A. Delay
B. Media bandwidth
C. Efficiency
D. Network traffic
4. What multicast address represents all OSPF routers?
A. 224.0.0.6
B. 224.0.0.1
C. 224.0.0.4
D. 224.0.0.5
5. What command can be used to change OSPF priority on an interface?
A. ip priority number ospf
B. ip ospf priority number
C. ospf priority number
D. set priority ospf number
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6. What multicast address is used to send LSUs to all DR/BDR routers?
A. 224.0.0.6
B. 224.0.0.1
C. 224.0.0.4
D. 224.0.0.5
7. What is a common feature of NBMA networks?
A. Support for only two routers
B. Support for more than two routers
C. No election of DRs
D. Full support for broadcast and multicast packets
8. What command allows OSPF routers to exchange routing updates without multicasts?
A. ip ospf neighbor
B. ospf neighbor
C. neighbor
D. ip neighbor
9. What command displays the routes known to a router and how they were learned?
A. show ip protocol
B. show ip route
C. show ip ospf
D. show ip ospf neighbor detail
10. Which of the following are two basic types of dynamic routing?
A. Static and default
B. TCP and UDP exchange
C. Distance vector and link-state
D. None of the above
11. __________ routing protocols determine the direction and distance to any link in the internetwork; __________ routing protocols are also called shortest path first.
