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CCNP > Ch.8: OSPF Foundations (1) > Flashcards

Ch.8: OSPF Foundations (1) Flashcards

1
Q

What type of protocol is OSPF?

A

A link-state Interior Gateway Protocol (IGP)

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2
Q

What does an OSPF router advertise to show link states and metrics?

A

Link State Advertisements (LSAs)

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3
Q

What does every OSPF router have an identical copy of?

A

Routers in the same area will have an identical copy of LSAs and an identical Link State Database (LSDB).

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4
Q

What algorithm does OSPF use to calculate loop-free paths in a network?

A

Dijkstra’s SPF algorithm (shortest-path first)

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5
Q

Every router will have an identical _____, but a different ____. Why?

A

Each router will have an identical LSDB but the SPF tree (SPT) will differ on each router. This is because each router’s SPT is made from each router’s perspective, even though each router has the same link state / metric information.

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6
Q

What provides scalability with OSPF?

A

The ability to create different areas; logically connected rotuers/subinterfaces that reduce the size and amount of LSAs being flooded to every router

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7
Q

What is the purpose of Area 0? What is it also known as?

A

Area 0, AKA the “backbone,” provids transit connectivity between all areas. All Area Boundary Routers (ABRs) have to connect to the backbone, and type 1 LSAs from one area is advertised as a type 3 (Summary) LSA when being advertised to or from Area 0 by an ABR.

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8
Q

Where does the backbone get its routes from? How do non-backbone areas get routes from other areas?

A

Routes get advertised into the backbone from non-backbone areas. The advertising router is the ABR.

Routes in Area 0 get advertised into non-backbone areas

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9
Q

What IP protocol does OSPF use for inter-router communication?

A

IP Protocol 89

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10
Q

What multicast addresses and MAC addresses do OSPF routers advertise to?

A

AllSPFRouters: 224.0.0.5 or MAC 01:00:5E:00:00:05
AllDRouters: 224.0.0.6 or MAC 01:00:5E:00:00:06

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11
Q

What are the 5 types of OSPF packets?

A

1 - Hello - discovering & maintaining neighbors
2 - Database Description (DBD/DDP) - summarizing DB contents
3 - Link-state request (LSR) - for DB downloads. Requested if part of the LSDB may be stale
4 - Link-state update (LSU) - for DB updates. Normally set as a response to an LSR
5 - Link-state acknowledgement - for flooding acknowledgements, resposne to LSA flooding

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12
Q

What is OSPF packet type 1?

A

1 - Hello - discovering & maintaining neighbors

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13
Q

What is OSPF packet type 2?

A

Database description (DBD/DDP) - summarizing DB contents

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14
Q

What is OSPF packet type 3?

A

3 - Link-state Request (LSR) - for DB downloads. Requested if part of the LSDB may be stale

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15
Q

What is OSPF packet type 4?

A

4 - Link-state update (LSU) - for DB updartes. Normally sent as a response to an LSR

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16
Q

What is OSPF packet type 5?

A

5 - Link-state acknowledgement - for flooding acknowledgements, response to LSA flooding

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17
Q

What is a Router ID?

A

A 32-bit number, unique ID for a router.
Must be unique for each AND between OSPF processes in an OSPF domain

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18
Q

What is an OSPF neighbor?

A

A router that shares a common OSPF-enabled link

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19
Q

What is the difference between a non-adjacent and adjacent OSPF neighbor?

A

Adjacent neighbors are 2 OSPF routers that share a synchronized OSPF DB. Note that this full adjacency does not happen between all routers in an OSPF domain.

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20
Q

In what situation does an OSPF router form a full adjacency with another router?

A

An OSPF router will only form a full adjacency with the Designated Router (DR) and Backup DR (BDR, if applicable).

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21
Q

What is the purpose of the DR/BDR in an OSPF domain?

A

The DR/BDR is responsible for managing adjacency with non-DR/BDR routers in that broadcast segment. This ultimately saves resources on each router.

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22
Q

What does the DR create to manage adjacency with other routers, and allow other routers to form an adjacency with the DR?

A

A “pseudonode” (virtual router)

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23
Q

What happens when a DR fails (re-election general steps):

A

1) DR fails
2) BDR becomes the DR
3) A new BDR is elected

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24
Q

What are the steps of DRs receiving and advertising an LSA?

A

1) All routers in broadcast segment form full adjacency with the DR/BDR
2) When a router learns of a new route, it sends an updated LSA to the AllDRouters multicast address
3) DR sends a unicast ack message to advertising router
4) DR floods the LSA to the AllSPFRouters multicast address

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25
Q

How do you enable an OSPF process, and use a network statement?

A

> enable
configure terminal
router ospf 1 —- go to ospf process 1 on router
network [Network IP Address] [Subnet Wildcard Mask] area [Area #]

ex. network 192.168.0.0 255.255.255.0 area 0 – any up interface that matches 192.168.0.0-192.168.255.0 will have OSPF enabled in area 0

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26
Q

What command(s) would you use to enable OSPF on a per-interface basis?

A

> enable
configure terminal
int [interface ID] —- go to interface-level configuration
ip ospf [process-id] area [area-id] [secondaries none]

If [secondaries none] is not specified, secondary connected networks will be added

27
Q

What does the router use for the RID?

A

The router-id (RID) can be set under the OSPF interface. Otherwise, the router will normally use the highest “up” loopback interface IP, or highest active & up physical interface.

28
Q

How do you restart an OSPF process?

A

> enable
clear ip ospf process

29
Q

What is an OSPF passive interface, and how do you set it?

A

Passive interfaces will not form adjacencies (process or send hello packets). Can be used to stop rogue OSPF routers

> passive [interface-id] — per interface
passive interface default — all interfaces

30
Q

What are the neighbor adjacency requirements?

A

1) RIDs must be unique betw. 2 devices forming an adjacency. The RID must also be unique across the OSPF domain
2) Interfaces must share common subnet
The following should match on both routers:
3) MTUs on each interface
4) Area IDs
5) DR enablement setting
6) OSPF hello & dead timers
7) Auth type and any credenetials
8) Area type flags

31
Q

What is a simple link cost formula for OSPF? What is the default reference bandwidth (BW)?

A

Cost = Reference BW / Interface BW

Default is 100 Mbps

32
Q

How do you change the reference BW for all interfaces?

A

On an OSPF process:
> auto-cost reference-bandwidth [bw in mbps]

Per-interface:
> ip ospf cost [1-65535]

33
Q

What timers does OSPF use for failure detection?

A

1) Hello timer (10 by default on Ethernet or broadcast multi-access)
2) Dead interval timer (4x hello timer by default)

Neighbor considered down when dead interval timer reaches 0. Both timers can be set from 1-65535

34
Q

How do you set the hello and dead timer for OSPF?

A

On an interface level:
>ip ospf hello-interval [1-65535]
>ip ospf dead-interval [1-65535]

35
Q

Routers with _________ may encounter issues if placed as the DR.

A

low resources

36
Q

What are DR elections based off of?

A

Highest OSPF priority, then highest RID. During the last phase of 2-Way state

37
Q

How do you set the OSPF priority?

A

Per-interface:
>ip ospf priority [0-255]

38
Q

After changing the OSPF priority for an interface, what might you need to do?

A

Might need to restart the OSPF process if it is changed after DR election.

39
Q

What are the different OSPF network types (relevant to the CCNP)?

A

1) Broadcast - normal multi-access ethernet links (this is default)
- Hello 10, Wait 40, Dead 40
2) Point-to-point - advertised as a /32, even if interface prefixes are different.
- Hello 10, Wait 40, Dead 40
- NO DR election. Faster adjacency.

40
Q

What is the OSPF wait timer?

A

of seconds a router waits before a router participates in DR election.

A router will elect itself if hello packets it receives still don’t have a DR or BDR present. Once there is a DR/BDR it uses those RIDs in its hello packets for DR/BDR.

41
Q

After OSPF adjacency, when does a router send LSA packets?

A

Immediately after it detects a state change, and also every 30 minutes by default

42
Q

What is the hello and dead intervals, and the default for each?

A

Hello interval – frequency that a router sends hello packets, 10 seconds by default
Dead interval – amount of time router waits to hear from a neighbor before declaring them out of service. 4 times the hello interval by default, so 40 seconds

43
Q

How do you manually set a RID for a router?

A

(Under the OSPF process)
> router ospf [process]
> router-id [router-id]

44
Q

When two OSPF routers establish an adjacency, what are four OSPF packet types exchanged between them?

A

1) DBD - includes LSA header information that helps build a router’s LSDB
2) LSAck - acknowledgement receipt of DBD
3) LSR - request for an updated link state advertisement (LSA)
4) LSU - response to LSR with updated LSA information

45
Q

What are the steps OSPF routers take to build a link-state database during adjacency establishment?

A

1) Routers exchange one or more DBD packets (OSPF type 2). This will include LSA headers in the router’s LSDB.
2) The router(s) receiving the DBD will send out an LSAck (acknowledgement).
3) Routers will compare the DBD to their own LSDB. If the DBD has a more up-to-date link-state entry, then a LSR will be sent to the other router to request a more updated link-state entry
4) Other router responds with the complete link-state entry in an LSU, which may contain one or more LSAs.
5) The receiving router adds the new entries to the LSDB and sends an LSAck.

46
Q

What OSPF neighbor states include steps to establish neighbor adjacencies?

A

1) Down
2) Init
3) 2-Way

47
Q

What OSPF neighbor states include the synchronization of OSPF databases?

A

1) ExStart
2) Exchange
3) Loading

48
Q

What is the last OSPF neighbor state?

A

Full

49
Q

What happens during the OSPF neighbor states involving neighbor adjacency establishment?

A

1) Down - no information has been received on a segment
2) Init - router has detected a hello packet. Bidirectional communication has not been established
3) 2-Way - Bidirectional communication is established. Neighbors decide whether or not to proceed in building an adjacency, and perform DR and BDR election if necessary.

50
Q

What happens during the OSPF neighbor states that involve sycnhronizing OSPF DBs?

A

1) ExStart - Routers will estalish an initial SEQ number to use in information exchange, to ensure they get the most recent info. One router becomes master, and the other slave; master router polls the slave for info.
2) Exchange - routers send DBD to describe their LSDB
3) Loading - Routers finalize information exchange with a link-state request and link-state retransmission list. Outdated info is put on the request list–any update sent is put on the retransmission list until an LSAck is received for it.

51
Q

What does OSPF NOT use for reliable transport? What does it use for this instead?

A

OSPF does not use TCP. Uses packet type 5 (LSAck) instead to ensure reliability.

52
Q

What is OSPF LSA type 1?

A

Router LSA

Generated by every OSPF router for each area it belongs to, and only flooded in that area. Describes state of router links in the area

53
Q

What is OSPF LSA type 2?

A

Network LSA

Generated for multiaccess networks, and describes the set of routers attached to a particular MA network. Flooded in the area that contains the network, and uses the DR’s IP interface address for the link-state ID.

54
Q

What is OSPF LSA type 3?

A

Summary LSA

A summarization of LSAs in one area, for another area. The link-state ID is the destination network number.

Commonly generated by ABRs to describe networks from non-backbone areas into the backbone.

55
Q

What is a type 4 OSPF LSA?

A

Summary ASBR LSA

Tells the rest of the OSPF domain how to get to the ASBR. Link-state ID includes the RID of the ASBR.

56
Q

What is a type 5 OSPF LSA?

A

External autonomous system LSA

Describe routes to destinations external to the AS. Flooded everywhere ecept in special areas. Link-state ID is the external network number.

57
Q

Does the BDR perform any DR functions in an OSPF area?

A

It does not, but it receives all information that would go to the DR. That way, it can failover to be the DR quickly.

58
Q

What are two fields used in an OSPF hello packet for DR/BDR election?

A

1) Router priority (highest value is elected as DR, second-highest as BDR)
2) Highest router ID when there is a tie between router priority

Set at the interface config level:
> ip ospf priority [0-255]

59
Q

What is a router called that is neither a DR or BDR?

A

DROTHER

60
Q

On network types that are not multiaccess broadcast or point-to-point, what are the default Hello and Dead timers?

A

30 seconds hello timer
120 seconds dead timer

61
Q

How do you configure the hello/dead timers?

A

(At the interface configuration)
> ip ospf hello-interval
> ip ospf dead-interval

62
Q

How would you see OSPF information for an interface, including the area, network type, DR, BDR, and hello/dead timers?

A

> show ip ospf interface [brief]

63
Q

How would you see OSPF neighbor information, including the DR/BDR/DROTHER and state information?

A

> show ip ospf neighbor

64
Q

How would you show route information for OSPF specifically?

A

> show ip route ospf

CCNP (8 decks)

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