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ag-CCNA > 9 - Implementing OSPF > Flashcards

9 - Implementing OSPF Flashcards

1
Q

How do you enter the OSPF configuration mode for a given OSPF process ID?

A

router ospf

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2
Q

What command is used to configure the router ID from OSPF configuration mode?

A

router-id

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3
Q

What three ways can you configure the router ID for OSPF?

A
  1. OSPF configuration mode
  2. Configure an IP on the loopback interface
  3. Rely on an interface address already configured
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4
Q

How would you configure OSPF using the indirect mode, i.e. mostly in the OSPF configuration mode?

A

router ospf 1
router-id 1.1.1.1
network 10.0.0.0 0.255.255.255 area 0

int s0/0> ip address 10.1.1.1 255.255.255.0

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5
Q

What information does the show ip ospf neighbor command show you?

A
  • Neighbor router IDs
  • Neighbor state
  • Neighbor Address
  • Interface
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6
Q

What does a neighbor state of FULL/- indicate?

A

The link does not use a DR/BDR

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7
Q

What is a DROTHER neighbor?

A

A neighbor that is neither the DR or BDR

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8
Q

What command is used to show the LSDB?

A

show ip ospf database

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9
Q

True/False: DR/BDR election is done per area?

A

False. DR/BDR election is done per subnet

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10
Q

True/False: Point to point links do not have a DR/BDR election process

A

True

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11
Q

What are some commands that are useful for verifying OSPF configuration?

A

show ip ospf interface [brief]
show run
show ip protocols

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12
Q

How do you enter the configuration mode for a loopback interface?

A

interface loopback 0

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13
Q

How do you configure OSPF multi-area from OSPF configuration mode?

A

router ospf 1
network 10.1.1.1 0.0.0.0 area 0
network 10.1.2.1 0.0.0.0 area 2

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14
Q

How do you directly enable OSPF on an interface instead of using the OSPF configuration mode?

A

interface g0/1

ip ospf 3 area 0

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15
Q

How do you remove configured networks from the OSPF configuration?

A

no network 10.0.0.0 area 0

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16
Q

True/False: Wildcards are not used to configure OSPF

A

False

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17
Q

What does “Interfaces configured explicitly” imply in the output of show ip protocols when considering OSPF?

A

OSPF was enabled using the interface subcommand mode (ip ospf 1 area 0)

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18
Q

What is the difference between the following two notes after an interface address in the output of show ip ospf interface

Attached via Interface Enable
&
Attached via Network Statement

A

Attached via Interface Enable implies that OSPF was enabled using the interface configuration mode. While

Attached via Network Statement means it was configured from OSPF configuration mode

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19
Q

What is the impact of making an interface passive in OSPF?

A

It won’t send OSPF Hellos, or process any received Hellos in that interface.

But it will continue to advertise the subnet that is connected to that interface

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20
Q

How do you make an interface passive in OSPF configuration mode per each interface?

A

router ospf 1
passive-interface G0/0.1
passive-interface G0/0.2

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21
Q

How do you configure passive interfaces in OSPF configuration mode by default but exclude certain ones?

A

router ospf 1
passive-interface default
no passive-interface G0/1
no passive-interface G0/2

22
Q

What does the OSPF command default-information originate do?

A

Makes the router advertise a default route using OSPF to remote routers

23
Q

How do you make OSPF always advertise the default route even if its not working?

A

default-information originate always

24
Q

What three ways can you change the OSPF interface cost (used to calculate SPF)?

A
  1. Setting the cost directly with interface subcommand: ip ospf cost x
  2. Alter the ‘bandwidth’ setting for an interface to adjust the default cost calculation
  3. Alter the ‘auto-cost reference-bandwidth’ setting in ospf router config mode to adjust the default cost calculation
25
Q

What command is used to adjust the bandwidth speed for an interface?

A

bandwidth [speed in kbits]

26
Q

What is the formula used to calculate the cost metric?

A

Reference bandwidth / interface bandwidth

27
Q

Why should you avoid altering the interface bandwidth to adjust OSPF cost calculation?

A

Because several other IOS features make use of this setting

28
Q

How do you adjust the reference bandwidth?

A

auto-cost reference-bandwidth

29
Q

What does the maximum-paths command do?

A

Configures the maximum number of equal cost OSPF routes allowed in the routing table

30
Q

What is the difference between Point to point and broadcast OSPF network types with respect to DR/BDRs?

A

Point to Point does not utilize a DR/BDR

31
Q

What type of OSPF network type does OSPF default to on Ethernet interfaces?

A

Broadcast

32
Q

What command would you use to show you the OSPF network type?

A

show ip ospf interface

33
Q

What command would you use to configure OSPF to broadcast network type when it is not the default?

A

Interface subcommand:

ip ospf network broadcast

34
Q

True/False: If the ‘best’ router / current DR fails but comes back up, it will immediately become the DR again

A

False. It will not be DR again until the current DR AND BDR fail

35
Q

What are the two things you can use to influence the DR/BDR election in order of precedence?

A
  1. Highest OSPF interface priority

2. Highest OSPF Router ID

36
Q

What are the two key differences between an STP election process and DR/BDR election process?

A
  1. In STP new switches can preempt the current root (overtake it) but this doesn’t happen with DR/BDRs in OSPF
  2. STP uses lowest is best approach (RouterID) whereas OSPF uses highest is best
37
Q

What is the default OSPF priority value?

A

1

38
Q

What kind of OSPF network type would you want for a link that by its nature has just two routers on the link? I.e. Serial or PTP Ethernet WAN

A

Point to point

39
Q

What is an Ethernet Private Wire Service / E-Line?

A

PTP Ethernet WAN.

Only two devices on the link sending / receiving Ethernet frames

40
Q

What does the - mean in the state output for a neighbor when using the show ip ospf neighbor command?

A

The link does not use a DR/BDR

41
Q

What does each field mean in the Nbrs column output of the show ip ospf interface brief command? i.e 2/5

A

Fully adjacent neighbors/total neighbors

42
Q

What are the neighbor requirements that must match for OSPF neighbors?

A
  • Unique Router ID
  • ACLs not filtering routing protocol messages
  • Interfaces same subnet
  • Pass neighbor authentication (if enabled)
  • Same Area
  • Hello/Dead timers must match
  • MTU setting
  • OSPF network type
43
Q

What two neighbor requirement settings for OSPF can be incorrect but still cause the neighbor to show in the neighbor table? What can go wrong if they don’t match?

A
  • MTU setting
  • OSPF network type

Neighbor relationship listed by LSAs will not exchange correctly

44
Q

Whats the easiest command to check if the Hello and dead timers match for OSPF neighbors?

A

show ip ospf interface g0/0

45
Q

How can you easily list the OSPF Router ID?

A

show ip ospf

46
Q

How can you enable / disable and OSPF process?

A

router ospf 1

[no] shutdown

47
Q

What happens when a given OSPF process is shutdown?

A
  • Neighbor relationships down and table cleared
  • LSDB cleared
  • Clear routing table of OSPF routes
48
Q

What does OSPF retain / show in command outputs even when an OSPF process is brought down?

A
  • All OSPF configuration

- Lists all OSPF enabled interfaces but in a down state

49
Q

What is the MTU size?

A

Per interface setting defining the largest network layer packet the router will forward out a given interface

50
Q

What is the default MTU size on many routers?

A

1500 bytes

51
Q

What commands configure the MTU for IPv4 and IPv6?

A

ip mtu

ipv6 mtu

52
Q

What three key things happen when two routers with mismatched OSPF network types connect?

A
  1. They become fully adjacent neighbors
  2. They exchange LSDBs
  3. Routes NOT added to IP routing table (due to difference in expected values in LSAs)

ag-CCNA (21 decks)

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