Chapter 9: Open Shortest Path First (OSPF)
B
1. There are three possible routes for a router to reacha a destination network. The first route is from OSPF with a metric of 782. The second route is from RIPv2 with a netric of 4. The third is from EIGRP with a composite metric of 20514560. Which route will be installed by the router in its routing table? A. RIPv2 B. EIGRP C. OSPF D. All three
A
11. Updates addressed to 224.0.0.6 are destined for which type of OSPF Router? A. DR B. ASBR C. ABR D. All OSPF Routers
passive-interface fastEthernet 0/1 (This command will disable OSPF on the specified interface only.)
14. Type the command that will disable OSPF on the Fa0/1 interface under the routing process.
A, B, C
17. What are three reasons for creating OSPF in a hierarchical design? A. To decrease overhead B. To speed up convergence C. To confine network instability to single areas in the network. D. To make configuring OSPF easier
A
19. A(n) ________ is an OSPF data packet containing a link-state and routing information that are shared among OSPF routers. A. LSA B. TSA C. Hello D. SPF
B
20. If routers in a single area are configured with the same priority value, what value does a router use for the OSPF router ID in the absence of a loopback interface? A. The lowest IP address of any physical interface B. The highest IP address of any physical interface C. The lowest IP address of any logical interface D. The highest IP address of any logical interface
A, C
3. Which of the following describe the process identifier that is used to run OSPF on a router? (Choose two.) A. It is locally significant. B. It is globally significant. C. It is needed to identify a unique instance of an OSPF database. D. It is an optional parameter required only if multiple OSPF processes are running on the router. E. All routes in the same OSPF area must have the same process ID if they are to exchange routing information.
B
4. All of the following must match for two OSPF routers to become neighbors except which? A. Area ID B. Router ID C. Stub area flag D. Authentication password if using one
A
6. You get a call from a network administrator who tells you he typed the following into his router: Router(config)#router ospf 1 Router(config-router)#network 10.0.0.0 255.0.0.0 area 0 He tells you he still can't see any routes in the routing table what configuration error did the administrator make? A. The wildcard mask is incorrect B. The OSPF area is wrong. C. The OSPF process IS is incorrect. D. The AS configuration is wrong.
D. (By default OSPF's administrative distance is 110.)
8. What is the administrative distance of OSPF. A. 90 B. 100 C. 110. D. 110
A
9. In OSPF, Hellos are sent to what IP address? A. 255.0.0.5 B. 244.0.0.9 C. 244.0.0.10 D. 224.0.0.1
Link State Advertisement (LSA)
A Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that's shared among OSPF routers. An OSPF router will exchange LSA packets only with routers to which it has established adjacencies.
Link
A ____ is a network or router interface assigned to any given network. When an interface is added to the OSPF process, it's considered to be a____. This ____ or interface, will have up or down state information associated with it as well as one or more IP addresses.
Backup Designated Router (BDR)
A backup designated router (BDR) is a hot standby for the DR on broadcast, or multi-access, links. The BDR receives all routing updates from OSPF adjacent routers but does not disperse LSA updates.
Area border router (ABR)
A router that connects other areas to the backbone area within an AS is called an ________, and even these must have at least one of their interfaces connected to area 0.
Adjacency
An _____ is a relationship between two OSPF routers that permits the direct exchange of route updates. Unlike EIGRP, which directly shares routes with all of its neighbors. OSPF is really picky about sharing routing information and will directly share routes only with neighbors that have also established adjacencies. And not all neighbors will become adjacent—this depends upon both the type of network and the configuration of the routers. In multi-access networks, routers form _____________ with designated and backup designated routers. In point-to-point and point-to-multipoint networks, routers form _________________ with the router on the opposite side of the connection.
OSPF Area
An _______ is a grouping of contiguous networks and routers. All routers in the same area share a common area ID. Because a router can be a member of more than one area at a time, the area ID is associated with specific interfaces on the router. This would allow some interfaces to belong to area 1 while the remaining interfaces can belong to area 0. All of the routers within the same area have the same topology table. When configuring OSPF with multiple ______s, you've got to remember that there must be an area 0 and that this is typically considered the backbone area.
LAS Flooding
Efficient _________ is achieved through the use of a reserved multicast address: 224.0.0.5 (AllSPFRouters). LSA updates, which indicate that something in the topology has changed, are handled a bit differently.
Process ID
Irrelevant number that is only locally significant.
Metrics
OSPF uses _____ referred to as cost. A cost is associated with every outgoing interface included in an SPF tree. The cost of the entire path is the sum of the costs of the outgoing interfaces along the path. Cisco uses a simple equation of 108/bandwidth, where bandwidth is the configured bandwidth for the interface. Using this rule, a 100 Mbps Fast Ethernet interface would have a default OSPF cost of 1 and a 1,000 Mbps Ethernet interface would have a cost of 1.
Designated Router (DR)
RID is not only used to advertise routes, it's also used to elect this. A ____________ is elected whenever OSPF routers are connected to the same broadcast network to minimize the number of adjacencies formed and to publicize received routing information to and from the remaining routers on the broadcast network or link. Elections are won based upon a router's priority level, with the one having the highest priority becoming the winner. If there's a tie, the router ID will be used to break it. All routers on the shared network will establish adjacencies with the DR and the BDR, which ensures that all routers' topology tables are synchronized.
router ospf [number]
Sets the process ID on a router, which is only locally signifcant.
Area 0
Some routers connect to the backbone. OSPF absolutely must have an _____.
Hello Protocol
The OSPF _________- provides dynamic neighbor discovery and maintains neighbor relationships. Hello packets and Link State Advertisements (LSAs) build and maintain the topological database. Hello packets are addressed to multicast address 224.0.0.5.
Router ID (RID)
The ________ is an IP address used to identify the router. Cisco chooses the _________ by using the highest IP address of all configured loopback interfaces. If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address out of all active physical interfaces. To OSPF, this is basically the "name" of each router.
Neighborship Database
The ___________ is a list of all OSPF routers for which Hello packets have been seen. A variety of details, including the router ID and state, are maintained on each router in the neighborship database.
Topological Database
The ____________- contains information from all of the Link State Advertisement packets that have been received for an area. The router uses the information from the topology database as input into the Dijkstra algorithm that computes the shortest path to every network.
Autonomous system boundary router (ASBR)
The router that connects these many autonomous systems is called an ___________.
Loopback interface
These are logical interfaces, which means they're virtual, software-only interfaces, not actual, physical router interfaces. A big reason we use loopback interfaces with OSPF configurations is because they ensure that an interface is always active and available for OSPF processes. Logical, software interface, the ip of which will become the router ID (RID).
ip ospf cost
This command changes the cost is manipulated by changing the value to a number within the range of 1 to 65,535. Because the cost is assigned to each link, the value must be changed on the specific interface you want to change the cost on.
show ip protocol
This command is also highly useful, whether you're running OSPF, EIGRP, RIP, BGP, IS-IS, or any other routing protocol that can be configured on your router. It provides an excellent overview of the actual operation of all currently running protocols!
show ip ospf neighbor
This command is super-useful because it summarizes the pertinent OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that information will also be displayed.
show ip ospf
This command is what you'll need to display OSPF information for one or all OSPF processes running on the router. Information contained therein includes the router ID, area information, SPF statistics, and LSA timer information.
show ip ospf interface
This command reveals all interface-related OSPF information. Data is displayed about OSPF information for all OSPF-enabled interfaces or for specified interfaces.
show ip opsf database
This command will give you information about the number of routers in the internetwork (AS) plus the neighboring router's ID—the topology database I mentioned earlier.
show ip eigrp topology
This command, this command reveals the OSPF routers, but not each and every link in the AS like EIGRP does. The output is broken down by area.
Point-to-point (PtP)
Uses 240.0.0.5 This refers to a type of network topology made up of a direct connection between two routers that provides a single communication path. The _________ connection can be physical—for example, a serial cable that directly connects two routers—or logical, where two routers thousands of miles apart are connected by a circuit in a Frame Relay network. Either way, ___________ configurations eliminate the need for DRs or BDRs.
Broadcast (multi access) networks
Uses 240.0.0.6 ____________________ such as Ethernet allow multiple devices to connect to or access the same network, enabling a broadcast ability in which a single packet is delivered to all nodes on the network. In OSPF, a DR and BDR must be elected for each ____________.
A
What are OSPF's best features? a. Allows for the creation of areas and autonomous systems, Minimizes routing update traffic, Is highly flexible, versatile, and scalable, Supports VLSM/CIDR, Offers an unlimited hop count, Is open standard and supports multi-vendor deployment b. Provides for open source products c. Easy to use and understand, easy to deploy on routers, and much more d. Robust, reliable, and helpful
Wildcard
When configuring these, the block size is always one less than this number. A 0 octet in the ________ mask indicates that the corresponding octet in the network must match exactly. On the other hand, a 255 indicates that you don't care what the corresponding octet is in the network number.
SPF Tree Calculation
Within an area, each router calculates the best/shortest path to every network in that same area. This calculation is based upon the information collected in the topology database and an algorithm called shortest path first (SPF).
Cost
_____ is associated with every outgoing interface included in an SPF tree. The cost of the entire path is the sum of the costs of the outgoing interfaces along the path.
Neighbors
_______ are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link. OSPF neighbors must have a number of common configuration options to be able to successfully establish a neighbor relationship, and all of these options must be configured exactly the same way: - Area ID - Stub area flag - Authentication password (if using one) - Hello and Dead intervals\
Point-to-multipoint (PtMP)
_____________ refers to a type of network topology made up of a series of connections between a single interface on one router and multiple destination routers. All interfaces on all routers share the __________________ connection and belong to the same network. ______________ networks can be further classified according to whether they support broadcasts or not. This is important because it defines the kind of OSPF configurations you can deploy.
Open Shortest Path First (OSPF)
______________ is an open standard routing protocol that's been implemented by a wide variety of network vendors, including Cisco. This is a link-state routing protocol. It works by constructing a shortest path tree and following that by populating the routing table with the resulting best paths. It also supports IP and IPv6 routed protocols.
Nonbroadcast multi-access
______________ networks are networks such as Frame Relay, X.25, and Asynchronous Transfer Mode (ATM). These types of networks allow for multi-access without broadcast ability like Ethernet. NBMA networks require special OSPF configuration to function properly.
Dijkstra's algorithm
_______________ is an algorithm for finding the shortest paths between nodes in a graph, which may represent, for example, road networks. It was conceived by computer scientist Edsger W. ______ in 1956 and published in 1959. The algorithm exists in many variants; ________'s original variant found the shortest path between two nodes, but a more common variant fixes a single node as the "source" node and finds shortest paths from the source to all other nodes in the graph, producing a shortest path tree.
Show ip ospf
command to show the ip of the router and it's accompanying process ID.
