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Cisco CCNP ENCOR > OSPF Theory > Flashcards

OSPF Theory Flashcards

1
Q

What is OSPF?

A

Open Shortest Path First - a non-proprietary link-state routing protocol

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2
Q

What is an LSA?

A

Link State Advertisement - information about an individual routers links aka networks.

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3
Q

What is an ABR?

A

Area Border Router - a router that has interfaces on networks that are in different OSPF areas.

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4
Q

What is an ASBR?

A

Autonomous System Border Router - a router that is running OSPF and also redistributing non-OSPF routes.

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5
Q

Describe a Type 1 LSA

A

Router LSA - Advertises the LSA that originate within an area

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6
Q

Describe a Type 2 LSA

A

Network LSA - Advertises multi-access network segment attached to a DR

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7
Q

Describe a Type 3 LSA

A

Network Summary (OSPFv3: Inter-Area Prefix)- Advertises network prefixes that originated from a different area. (ABR)

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8
Q

Describe a Type 4 LSA

A

ASBR Summary (OSPFv3: Inter-Area Router) - Advertises a summary LSA for a specific ASBR

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9
Q

Describe a Type 5 LSA

A

AS External LSA - Advertises LSAs for routes that have been redistributed.

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10
Q

Describe a Type 7 LSA

A

NSSA External LSA - Advertises redistributed routes in Not So Stubby Areas

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11
Q

Describe a Type 8 LSA

A

OSPFv3 Link - IPv6 only. These are for IPv6 Intra-Area link local addresses.

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12
Q

Describe a Type 9 LSA

A

IPv6 Network Link - IPv6 prefix advertisements that are only flooded within that area.

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13
Q

When do routers exchange their entire Link State DB with each other?

A

When those neighbors are fully adjacent.

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14
Q

What is effectively happening when you issue the ‘network’ command within OSPF config mode?

A

You are telling the router to enable all interfaces within the specified network range for OSPF and to advertise their networks.

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15
Q

What is a stub network?

A

A network where this is only one router. In other words, there are no other networks beyond that network.

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16
Q

What is a stub area?

A

An OSPF area that does not accept redistributed routes from an ASBR.

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17
Q

What is the formula for the number of neighbor adjacencies formed in an OSPF broadcast segment?

A

Where N is the number of OSPF routers: N(N-1)/2

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18
Q

What is a DR in OSPF?

A

Designated Router - The router that is responsible for advertising route updates. Only the DR forms full OSPF adjacencies with the other routers in the segment.

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19
Q

How is a DR elected in a network segment?

A

1.) First to come online
2.) Highest OSPF priority
3.) Highest Router ID (RID)
4.) Highest IP on Loopback
5.) Highest IP on Non-loopback

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20
Q

What is the BDR?

A

Backup Designated Router - takes over if the DR goes offline

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21
Q

Name the OSPF packet types that are sent when OSPF routers are performing neighbor relationships.

A

Hello - type 1
Database Description (DBD) - type 2
Link State Request (LSR) - type 3
Link State Update (LSU) - type 4
Link State Ack - type 5

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22
Q

What is the function of OSPF Hello packets

A

These packets are for discovering and maintaining neighbors.

23
Q

What is the function of OSPF DBD packets

A

These packets are for summarizing database contents. Packets are exchanged when an OSPF adjacency is first being formed. These packets are used to describe the contents of the LSDB.

24
Q

What is the function of OSPF LSR packet

A

These packets are sent from one OSPF router to request full LSA data. This is typically in response to a DBD or when part of the Link State Database is stale.

25
Q

What is the function of OSPF LSU packet

A

These packets are for database updates. This is an explicit LSA for a specific network link and normally is sent in direct response to an LSR.

26
Q

What is the function of OSPF LS Ack packet

A

These packets are acknowledge the receipt of an LSU. These packets can also be flooded across a network segment.

27
Q

What states do routers go through in the process of forming an OSPF full adjacency?

A

Down
Attempt
Init
2-Way
Exstart
Exchange
Loading
Full

28
Q

What happens in the OSPF Down state?

A

This state indicates that no information has been received recently, but the router is sending hello packets

29
Q

What happens in the OSPF Attempt state?

A

This state only occurs when neighbors are manually configured on a non-broadcast multi-access network (multicast disabled) link and hellos are sent via unicast.

30
Q

What happens in the OSPF Init state?

A

This state indicates that a hello packet has been received from another router and the routers in a segment are aware of each other (hello packets have seen each router in the segment).

31
Q

What happens in the OSPF 2-way state?

A
  • The router has received a hello packet with its RID inside of it, indicating that the neighbor now knows that it exists.
  • Bidirectional communication has been established.
  • If a DR or BDR is needed, the election occurs during this state.
32
Q

What happens in the OSPF Exstart state?

A

This is the first state in forming an adjacency. Database Descriptor Packets (DBD) are sent from both sides and the routers negotiate Master/Slave.

33
Q

What happens in the OSPF Exchange state?

A

During this state, the Master/Slave election is complete and DBD packets are sent with Link State DB summaries.

34
Q

What happens in the OSPF Loading state?

A

LSAs are exchanged via the LSR, LSU, LS Ack process.

35
Q

What happens in the OSPF Full state?

A

Neighboring routers are fully adjacent.

36
Q

What effect does the ‘passive-interface’ command have?

A

It tells OSPF to advertise the network for the interface, but the interface will not send or receive OSPF hello packets. This is to prevent a rogue device from making an OSPF adjacency and inserting routes into the network.

37
Q

What are the types of OSPF Networks?

A

Broadcast
Non-broadcast
Point-to-Point
Point-to-Multipoint
Point-to-Multipoint Non-Broadcast

38
Q

What are the OSPF network types that use a DR?

A

Broadcast
Non-broadcast

39
Q

What are the default OSPF timers?

A

It depends on the network type…
Type; Hello; Wait; Dead
Broadcast & Point-to-Point; 10; 40; 40
Non-Broadcast & Point-to-Multipoint; 30; 120; 120

40
Q

What is the default OSPF network type for Ethernet?

A

Broadcast

41
Q

What types of network connections use Point-to-Point OSPF network types by default?

A

Serial (HDLC, PPP), GRE Tunnels, P2P Frame Relay.

NOTE: This is because only 2 endpoints can exist and OSPF does not want to waste CPU cycles.

42
Q

When dealing with point-to-point or point-to-multipoint networks, what needs to be configured properly to ensure adjacency?

A

Timers need to match on both ends (Hello, Dead, Wait, ReTx)

43
Q

When can neighboring OSPF interfaces have different types and still be adjacent?

A

When the timers match on both ends

44
Q

What are the different types of OSPF authentication?

A

Type 0 = None
Type 1 = Plain text
Type 2 = MD5

45
Q

What are the default OSPF link costs for T1, Ethernet, FastEth, GigEth,10-GigEth?

A

T1: 64
Eth: 10
Fast, Gig, & 10-Gig: 1

46
Q

In OSPF, what is area 0 called?

A

The backbone area where all other areas must connect. This could also be referred to as the transit area.

47
Q

For Inter-Router communication, OSPF uses what two multicast addresses (IP and MAC) for AllSPFRouters?

A

224.0.0.5

01:00:5e:00:00:05

48
Q

For Inter-Router communication, OSPF uses what two multicast addresses (IP and MAC) for AllDRouters?

A

224.0.0.6

01:00:5e:00:00:06

49
Q

OSPF does not use TCP or UDP as a transport protocol. Instead it uses its own called what?

A

Protocol 89

50
Q

What does it mean to say that OSPF is a two-tier hierarchy?

A

All areas must connect to the backbone area (area 0) via an ABR.

51
Q

What is a passive interface? Why might you need it?

A

A passive interface does not try to form OSPF adjacencies. This may be needed for security reasons, so a rogue device cannot be plugged in and introduce false routes.

52
Q

What is the IPv6 multicast address (group) for OSPFv6?

A

FF02::5

53
Q

In OSPFv3, what is the new name for the LSA Type 3?

A

Inter-Area Prefix

54
Q

In OSPFv3, what is the new name for the LSA Type 4?

A

Inter-Area Router

Cisco CCNP ENCOR (27 decks)

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