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Flashcards in OSPF Deck (96):
1

How many types of packet are there in OSPF?

OSPF has five types of packets.

2

Which of the LSAs has a field called Forwarding Address?

External LSAs have a Forwarding Address field.

3

Which of the LSA(s) are not allowed in a totally stubby area?

External and summary LSAs are not allowed in a totally stubby area.

4

What is the multicast address for AllSPFRouters?

224.0.0.5

5

Which of the OSPF protocol packets is used to elect a master and a slave?

Type 2 DBD packets are used to elect a master and a slave.

6

Which of the OSPF protocol packets implement flooding of the LSA?

Link-state update packets implement flooding of the LSA.

7

What is the time limit in seconds before an LSA is declared as MAXAGED?

The limit is 3600 seconds.

8

How many bytes long is a common LSA header?

A common LSA header is 20 bytes long.

9

What is an OSPF neighbor?

From the perspective of an OSPF router, a neighbor is another OSPF router that is attached to one of the first router’s directly connected links.

10

What is an OSPF adjacency?

An OSPF adjacency is a conceptual link to a neighbor over which LSAs can be sent.

11

What are the five OSPF packet types? What is the purpose of each type?

Hellos, which are used to discover neighbors, and to establish and maintain adjacencies

Updates, which are used to send LSAs between neighbors

Database Description packets, which a router uses to describe its link state database to a neighbor during database synchronization

Link State Requests, which a router uses to request one or more LSAs from a neighbor’s link-state database

Link State Acknowledgments, used to ensure reliable delivery of LSAs

12

What is an LSA? How does an LSA differ from an OSPF Update packet?

A router originates a link-state advertisement to describe one or more destinations. An OSPF Update packet transports LSAs from one neighbor to another. Although LSAs are flooded throughout an area or OSPF domain, Update packets never leave a data link.

13

What are LSA types 1 to 5 and LSA type 7? What is the purpose of each type?

Type 1 (Router LSAs) are originated by every router and describe the originating router, the router’s directly connected links and their states, and the router’s neighbors.

Type 2 (Network LSAs) are originated by Designated Routers on multiaccess links and describe the link and all attached neighbors.

Type 3 (Network Summary LSAs) are originated by Area Border Routers and describe inter-area destinations.

Type 4 LSAs (ASBR Summary LSAs) are originated by Area Border Routers to describe Autonomous System Boundary Routers outside the area.

Type 5 (AS External LSAs) are originated by Autonomous System Boundary Routers to describe destinations external to the OSPF domain.

Type 7 (NSSA External LSAs) are originated by Autonomous System Boundary Routers within not-so-stubby areas.

14

What is a link-state database? What is link-state database synchronization?

The link state database is where a router stores all the OSPF LSAs it knows of, including its own. Database synchronization is the process of ensuring that all routers within an area have identical link-state databases.

15

What is the default HelloInterval?

10 seconds.

16

What is the default RouterDeadInterval?

four times the HelloInterval.

17

What is a Router ID? How is a Router ID determined?

A Router ID is an address by which an OSPF router identifies itself. It is either the numerically highest IP address of all the router’s loopback interfaces, or if no loopback interfaces are configured, it is the numerically highest IP address of all the router’s LAN interfaces. It can also be manually configured.

18

What is an area?

An area is an OSPF sub-domain, within which all routers have an identical link-state database.

19

What is the significance of area 0?

Area 0 is the backbone area. All other areas must send their inter-area traffic through the backbone.

20

What is MaxAge?

MaxAge, one hour, is the age at which an LSA is considered to be obsolete.

21

What are the four OSPF router types?

Internal Routers, whose OSPF interfaces all belong to the same area

Backbone Routers, which are Internal Routers in Area 0

Area Border Routers, which have OSPF interfaces in more than one area

Autonomous System Boundary Routers, which advertise external routes into the OSPF domain

22

What are the four OSPF path types?

Intra-area paths

Inter-area paths

Type 1 external paths

Type 2 external paths

23

What are the five OSPF network types?

Point-to-point networks

Broadcast networks

Non-broadcast multiaccess (NBMA) networks

Point-to-multipoint networks

Virtual links

24

What is a Designated Router?

A Designated Router is a router that represents a multiaccess network, and the routers connected to the network, to the rest of the OSPF domain.

25

How does a Cisco router calculate the outgoing cost of an interface?

Cisco IOS calculates the outgoing cost of an interface as 10^8/BW, where BW is the configured bandwidth of the interface. 108 can be changed with the OSPF command auto-cost reference-bandwidth.

26

What is a partitioned area?

An area is partitioned if one or more of its routers cannot send a packet to the area’s other routers without sending the packet out of the area.

27

What is a virtual link?

A virtual link is a tunnel that extends an OSPF backbone connection through a non-backbone area.

28

What is the difference between a stub area, a totally stubby area, and a not-so-stubby area?

A stub area is an area into which no type 5 LSAs are flooded. A totally stubby area is an area into which no type 3, 4, or 5 LSAs are flooded, with the exception of type 3 LSAs to advertise a default route. Not-so-stubby areas are areas through which external destinations are advertised into the OSPF domain, but into which no type 5 LSAs are sent by the ABR.

29

What is the difference between OSPF network entries and OSPF router entries?

OSPF network entries are entries in the route table, describing IP destinations. OSPF router entries are entries in a separate route table that record only routes to ABRs and ASBRs.

30

Why is type 2 authentication preferable over type 1 authentication?

Type 2 authentication uses MD5 encryption, whereas type 1 authentication uses clear-text passwords.

31

Which three fields in the LSA header distinguish different LSAs? Which three fields in the LSA header distinguish different instances of the same LSA?

The three fields in the LSA header that distinguish different LSAs are the Type, Advertising Router, and the Link State ID fields. The three fields in the LSA header that distinguish different instances of the same LSA are the Sequence Number, Age, and Checksum fields.

32

Can OSPFv3 support IPv4v?

OSPFv3 cannot support IPv4 at the time of this writing. To route both IPv4 and IPv6 with OSPF, you must run both OSPFv2 and OSPFv3.

33

What is meant, in OSPFv3, that it can support multiple instances per link? What field in the OSPFv3 message header makes this possible?

Multiple instance per link means that there can be separate adjacencies among different routers all connected to the same broadcast link, so that different OSPFv3 routing domains can use the same shared link without interfering with or having knowledge of each other. The Instance ID field in the OSPFv3 header makes this possible.

34

How are OSPFv3 packets authenticated?

OSPFv3 packets are authenticated using the built-in IPv6 authentication (by means of the IPv6 Authentication extension header). OSPFv3 does not have its own authentication mechanism as OSPFv2 does.

35

What is the OSPFv3 Next Header number?

The OSPFv3 Next Header number is the same as the OSPFv2 Protocol Number, 89.

36

What are the two reserved OSPFv3 multicast addresses?

OSPFv3 uses the reserved multicast addresses FF02::5 (AllSPFRouters) and FF02::6 (AllDRouters).

37

Does OSPFv3 use any different message types than OSPFv2?

No. OSPFv3 uses the same five message types as OSPFv2.

38

What is the purpose of the first three bits of the Link State Type field of the OSPFv3 LSA header?

The first bit is the U bit, specifying how the receiving router should treat the LSA if its type is unknown. The second and third bits are the S bits, indicating the flooding scope of the LSA.

39

What flooding scope is supported by OSPFv3 but not by OSPFv2? What LSA uses this flooding scope?

OSPFv3 supports a link-local flooding scope that is not supported by OSPFv2. The Link LSA uses this flooding scope.

40

What is the most significant difference between OSPFv2 Router and Network LSAs, and OSPFv3 Router and Network LSAs?

OSPFv3 Router and Network LSAs do not advertise prefixes, as OSPFv2 Router and Network LSAs do.

41

What is the purpose of the Intra-Area Prefixes LSA?

The Intra-Area Prefix LSA carries IPv6 prefixes connected to the originating router.

42

What is the purpose of the Link LSA?

The Link LSA carries information that is only significant between two directly connected neighbors.

43

Define LSDB

The data structure used by OSPF to hold LSAs.

44

Define Dijkstra

Alternate name for the SPF algorithm, named for its inventor, Edsger W. Dijkstra.

45

Define Link-state routing protocol

Any routing protocol that uses the concept of using the SPF algorithm with an LSDB to compute routes.

46

Define LSA

The OSPF data structure that describes topology information.

47

Define LSU

A type of OSPF packet, used to communicate LSAs to another router.

48

Define DD

A type of OSPF packet used to exchange and acknowledge LSA headers.

49

Define OSPF Hello

A type of OSPF packet used to discover neighbors, check for parameter agreement, and monitor the health of another router.

50

Define LSAck

A type of OSPF packet used to acknowledge LSU packets.

51

Define RID

The 32-bit number used to represent an OSPF router.

52

Define neighbor state (OSPF)

A state variable kept by a router for each known neighbor or potential neighbor.

53

Define neighbor (OSPF)

Any other router, sharing a common data link, with which a router exchanges Hellos, and for which the parameters in the Hello pass the parameter-check process.

54

Define adjacent

Any OSPF neighbor for which the database flooding process has completed.

55

Define Fully adjacent

Any OSPF neighbor for which the database flooding process has completed.

56

Define 2-way

A neighbor state that signifies the other router has reached neighbor status, having passed the parameter check.

57

Define 224.0.0.5

The All OSPF Routers multicast IP address, listened for by all OSPF routers.

58

Define 224.0.0.6

The All OSPF DR Routers multicast IP address, listened for by DR and BDR routers.

59

Define area

A contiguous group of data links that share the same OSPF area number.

60

Define Stub area type

An OSPF area into which external (type 5) LSAs are not introduced by its ABRs; instead, the ABRs originate and inject default routes into the area.

61

Define Network type

A characteristic of OSPF interfaces that determines whether a DR election is attempted, whether or not neighbors must be statically configured, and the default Hello and Dead timer settings.

62

Define External route

From the perspective of one routing protocol, a route that was learned by using route redistribution.

63

Define E1 route

An OSPF external route for which internal OSPF cost is added to the cost of the route as it was redistributed into OSPF.

64

Define E2 route

An OSPF external route for which internal OSPF cost is not added to the cost of the route as it was redistributed into OSPF.

65

Define Hello timer (OSPF)

10 seconds (bcast/p2p), 30 seconds (other)

66

Define dead time/interval

With OSPF, the timer used to determine when a neighboring router has failed, based on a router not receiving any OSPF messages, including Hellos, in this timer period. (30/120)

67

Define Sequence number

In OSPF, a number assigned to each LSA, ranging from 0x80000001 and wrapping back around to 0x7FFFFFFF, which is used to determine which LSA is most recent.

68

Define DR

the OSPF router that wins an election amongst all current neighbors. The DR is responsible for flooding on the subnet, and for creating and flooding the type 2 LSA for the subnet.

69

Define BDR

In OSPF, a router that is prepared to take over the designated router.

70

Define DROther

The term to describe a router that is neither the DR nor the BDR on a subnet that elects a DR and BDR.

71

Define Priority

An administrative setting, included in Hellos, that is the first criteria for electing a DR. The highest priority wins, with values from 1–255, with priority 0 meaning a router cannot become DR or BDR.

72

Define LSA Flooding

The process of successive neighboring routers exchanging LSAs such that all routers have an identical LSDB for each area to which they are attached.

73

Define DR election

The process by which neighboring OSPF routers examine their Hello messages and elect the DR. The decision is based on priority (highest), or RID (highest) if priority is a tie.

74

Define SPF calculation

The process of running the SPF algorithm against the LSDB, with the result being the determination of the current best route(s) to each subnet.

75

Define Partial SPF calculation

An SPF calculation for which a router does not need to run SPF for any LSAs inside its area, but instead runs a very simple algorithm for changes to LSAs outside its own area.

76

Define Full SPF calculation

An SPF calculation as a result of changes inside the same area as a router, for which the SPF run must examine the full LSDB.

77

Define LSRefresh

Link-State Refresh. A timer that determines how often the originating router should reflood an LSA, even if no changes have occurred to the LSA.

78

Define Maxage

An OSPF timer that determines how long an LSA can remain in the LSDB without having heard a reflooded copy of the LSA.

79

Define ABR

An OSPF router that connects to the backbone area and to one or more non-backbone area.

80

Define ASBR

Autonomous System Boundary Router. An OSPF router that redistributes routes from some other source into OSPF.

81

Define Internal router

router that is not an ABR or ASBR in that all of its interfaces connect to only a single OSPF area.

82

Define backbone area

Area 0; the area to which all other OSPF areas much connect in order for OSPF to work.

83

Define Transit network

A network/subnet over which two or more OSPF routers have become neighbors, thereby being able to forward packets from one router to another across that network.

84

Define Stub network

A network/subnet to which only one OSPF router is connected.

85

Define LSA type

A definition that determines the data structure and information implied by a particular LSA.

86

Define stub area

An OSPF area into which external (type 5) LSAs are not introduced by its ABRs; instead, the ABRs originate and inject default routes into the area.

87

Define NSSA

A type of OSPF stub area that, unlike stub areas, can inject external routes into the NSSA area.

88

Define totally stubby area

A type of OSPF stub area for which neither external (type 5) LSAs are introduced, nor type 3 summary LSAs; instead, the ABRs originate and inject default routes into the area. External routes cannot be injected into a totally stubby area.

89

Define Totally NSSA area

A type of OSPF NSSA area for which neither external (type 5) LSAs are introduced, nor type 3 summary LSAs; instead, the ABRs originate and inject default routes into the area. External routes can be injected into a totally NSSA area.

90

Define Virtual link

With OSPF, the encapsulation of OSPF messages inside IP, to a router with which no common subnet is shared, for the purpose of either mending partitioned areas or providing a connection from some remote area to the backbone area.

91

Define Stub router

A router that should either permanently or temporarily not be used as a transit router. Can wait a certain time after OSPF process start, or after BGP notifies OSPF that BGP has converged, before ceasing to be a stub router.

92

Define Transit router

A router that is allowed to receive a packet from an OSPF router and then forward the packet to another OSPF router.

93

Define SPF algorithm

The algorithm used by OSPF and IS-IS to compute routes based on the LSDB.

94

Define All OSPF DR Routers

The multicast IP address 224.0.0.6, listened for by DR and BDR routers.

95

Define All OSPF Routers

The multicast IP address 224.0.0.5, listened for by all OSPF routers.

96

Define Graceful restart

allows for uninterrupted forwarding in the event that an OSPF router’s OSPF routing process must restart. The router does this by first notifying the neighbor routers that the restart is about to occur; the neighbors must be RFC 3623–compliant, and the restart must occur within the defined grace period.