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CCNP-ENCOR > OSPF > Flashcards

OSPF Flashcards

1
Q

What is the backbone area

A

Area 0 or the backbone is the area that all other OSPF areas must connect to. the backbone provides transit connectivity between the non backbone areas.

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2
Q

What is an OSPF area

A

An area provides separation in OSPF. this allows scalability to increase as a routing domain is only within the area and OSPF adjacencies need to be made within the area not with every router running OSPF.

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3
Q

What IP protocol number does OSPF use

A

IP 89

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4
Q

What address does OSPF use to communicate with all OSPF routers

A

224.0.0.5

01:00:5E:00:00:05

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5
Q

What address does OSPF use to communicate with DR routers

A

224.0.0.6

01:00:5E:00:00:06

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6
Q

What are the different OSPF packet types

A

Hello
Database description (DBD)
Link state request
Link state update
Link state Acknowledgement

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7
Q

What is a OSPF Hello packet

A

Used for discovering and maintaining neighbour relationships.

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8
Q

What is a DBD packet

A

Database descriptor: used to describe the contents of the LSDB

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9
Q

What is a LSR packet

A

Link state request: Packets used to to download specific networks from the LSDB

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10
Q

What is a LSU packet

A

Link state update: Sent in response to a LSR

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11
Q

What is a Link state acknowledgement

A

Sent as an acknowledgement of the LSA.

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12
Q

What are the different fields in a OSPF hello packet

A

RID
authentication options
Area ID
Hello & dead timers
DR/BDR IP
Interface address mask
Interface priority
OSPF neighbours

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13
Q

What is an OSPF neighbour

A

IS a router that shares a common OSPF enabled network, OSPF discovers neighbours by Hello packets. All neighbours have a synchronised LSDB

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14
Q

When a OSPF neighbour forms what are the different states of the adjacency

A

Down
Attempt
Init
2way
ExStart
Exchange
Loading
full

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15
Q

What are the requirements must match for OSPF neighbours to form adjacency

A

RID must be unique
Interfaces must share a common subnet
MTU must match
Area ID must match
DR enablement must match
OSPF hello/dead timers must match
Authentication must match
Area type flags must match

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16
Q

What is the purpose of the designated router (DR)

A

To maintain neighbour adjacencies with all OSPF routers within the network segment. All routers can not maintain adjacencies with all routers as this would be impossible with scaling.

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17
Q

What is the process for a OSPF router learning a new route within a broadcast domain.

A
  1. OSPF router sends the route to 224.0.0.6
  2. The DR send a unicast message to the sender acknowledging the route.
  3. DR sends route to 224.0.0.5 to distribute via all OSPF routers on network segment
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18
Q

What are the 2 methods of configuring OSPF

A
  1. the network statement configured under the OSPF process with a wild card mask.
  2. Interface specific configuration enabling OSPF under a specific interface
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19
Q

What is the benefit of interface specific configuration over network statement when enabling OSPF

A

Using the interface configuration provides explicit control over the OSPF process

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20
Q

In a hybrid situation where a network statement and an interface configuration are both enabled on the same OSPF router, which takes precedence

A

Interface specific configuration

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21
Q

By default how is the RID set in OSPF

A

The RID is allocated by:

  1. The highest IP of any UP loopback interface.
  2. The highest IP of an UP physical interface.
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22
Q

How does a router select a new RID

A

OSPF process must be restarted

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23
Q

What is a OSPF passive interface used for

A

To prevent un-authorised routers from been connected to the OSPF domain and advertising false routes to cause traffic problems.

A passive interface prevents hello packets from been sent and therefore adjacencies from being formed on that interface.

Network segments however are still added to the LSDB.

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24
Q

Does OSPF support default route advertisement

A

Yes, command: default-information originate [always][metric]

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25
Q

What are the main optimisations that can be performed on a OSPF router

A

Changing OSPF cost of an interface

Changing OSPF priority of a router to be elected for DR/BDR

Changing Hello/dead timers

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26
Q

What is the reference bandwidth to calculate OSPF link cost

A

100Mbps

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27
Q

What is the default OSPF cost (metric) for E1, ethernet, FA, Gbps, TenGbps

A

E1 - 64
ethernet - 10
FA - 1
Gbps - 1
TeGbps - 1

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28
Q

What is the calculation to calculate OSPF cost

A

Reference bandwidth (in Mbps) divided by interface bandwidth (Mbps)

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29
Q

Why would you change the reference bandwidth and how do you do it

A

To support higher interface speeds. As 1 Gbps link has the same metric as Ten Gbps link with default values.

command: auto cost reference-bandwidth

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30
Q

What is the important thing to do when changing the reference bandwidth

A

change the reference bandwidth on all OSPF devices in the routing domain to ensure the Dijkstra’s algorithm runs correctly.

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31
Q

What is the default priority for DR elections on a OSPF router

A

1

32
Q

If there is a match the OSPF priority how is the winner identified

A

the highest RID will become the DR

33
Q

how do you remove a router from the election process

A

set the priority to 0

34
Q

what is the maximum OSPF priority value

A

255

35
Q

Does OSPF DR priority update support pre-emption

A

No

36
Q

What are the different OSPF network types

A

Broadcast
Non-Broadcast
P2P
Loopback

37
Q

What are the default timers in OSPF Broadcast network

A

Hello: 10
Wait: 40
Dead: 40

38
Q

What are the default timers in a OSPF non broadcast network

A

Hello: 30
Wait: 120
Dead: 120

39
Q

What are the default timers in a OSPF P2P network

A

Hello: 10
Wait: 40
Dead: 40

40
Q

What are the default timers in a OSPF P2MP network

A

Hello: 30
Wait: 120
Dead: 120

41
Q

What are the default timers in a OSPF Loopback network

A

No timers for loopbacks

42
Q

What is the OSPF wait timer used for

A

To allow newly booted routers to Identify the DR/BDR on a segment instead of immediately declaring themselves a DR

43
Q

Which OSPF network types have a DR/BDR field in the ‘hello packet’

A

Broadcast and non broadcast OSPF networks.

44
Q

How does OSPF determine which network type to use

A

Media plugged in.

45
Q

What are the drawbacks to using a single area in OSPF

A

Does not scale well due to:
Neighbour adjacencies required with all router within the area.
LSDB becomes unmanageable.
No summarisation

46
Q

What are the advantages of multi area OSPF model

A

LSDBs are smaller
Quicker SPF calculations
Decreases LSA flooding when a link flaps

47
Q

All areas transmit routes to all other areas injecting them into each other.

True or False

A

False. Only area 0 can inject routes into non-backbone areas.

48
Q

What is a ABR

A

Area border router, a router that has multiple areas directly connected. One of the areas must be area 0.

49
Q

What is a intra-area route

A

Route learnt from an OSPF router within the same area.

50
Q

What is a Inter-area route

A

A route learnt from a OSPF router in another area.

51
Q

What is the code in the RIB for a intra-area route

A

O

52
Q

What is the code in the RIB for a inter-area route

A

O IA

53
Q

What is the purpose of LSAs

A

to build the LSDB and the OSPF tree

54
Q

How many LSAs are used for IPv4 routing

A

6

55
Q

What is a type 1 LSA

A

Router LSA - advertises LSA the originate within the area

56
Q

What is a type 2 LSA

A

Multi access network segment LSA: Used when a DR is used.

57
Q

What is a type 3 LSA

A

Summary, used to advertise networks to OSPF routers outside of the originating router.

58
Q

What is a type 4,5,7 LSA

A

Type 4: ASBR Summary LSA
Type 5: AS external LSA
Type 7: NSSA external LSA

59
Q

Every OSPF enabled link advertises one type 1 LSA.

True or False

A

True

60
Q

Only some routers advertise Type 1 LSAs

True or False

A

False. All routers advertise type 1 LSA

61
Q

On what type of router is a Type 2 LSA created

A

DR

62
Q

Does a type 1 or 2 LSA leave the area

A

No both are only intra-area

63
Q

How are type 3 LSAs generated

A

When a ABR receives a Type 1 LSA it generates a type 3 LSA and sets itself as the advertising router and sends it to another area.

64
Q

What happens if a ABR receives 2 type 3 LSAs for the same network segment

A

the ABR only advertises the type 3 LSA with the lowest metric.

65
Q

What is a discontiguous network

A

when inter-area traffic is trying to cross a non-backbone area.

e.g. area 12 > area 40

Not possible as it MUST transmit through area 0.

66
Q

What is the priority for OSPF path selection when running Dijkstra’s algorithm

A
  1. Intra-area routes
  2. Inter-area routes
  3. External routes
67
Q

What happens if there is a match of metrics with 2 routes to the same network and prefix length

A

ECMP is used.

68
Q

What is OSPF route summarisation

A

Summarising the network prefix’s into a shorter subnet mask that are advertised in the type 3 LSA.

69
Q

What is the benefit of OSPF summarisation

A

Reduces number of type 3 LSAs generated on the ABR and the amount advertised into other areas.

70
Q

How is the metric calculated for a summarised network

A

Default is the lowest metric associated with the LSA, can be manually configured

71
Q

What is OSPF route filtering

A

Method of selecting which identified routes are advertised or received from or to neighbours

72
Q

What are the advantages of route filtering

A

Manipulate traffic flow
reduce memory utilisation
improve security

73
Q

What are the different methods of route filtering

A

Summarisation filtering
Area filtering
Local OSPF router filtering

74
Q

What is Summarisation filtering

A

Preventing type 3 LSAs from been generated on ABR of a specific summarised network.

This prevents the route to that network from been advertised outside of the area.

75
Q

What is Area filtering

A

Allows ABR to filter routes as they leave or enter the area.

76
Q

What is Local OSPF filtering

A

To remove routes from specific routers within a OSPF area, this is done by still installing LSA into LSDB as they must be synchronised across the area.

However the route is not installed into the RIB.

77
Q
A

CCNP-ENCOR (17 decks)

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