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ICND2-Odom > OSPF > Flashcards

OSPF Flashcards

1
Q

How do OSPF-enabled routers establish neighbor adjacencies?

A

Via the exchange of Hello packets

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2
Q

What happens once OSPF-enabled routers have established adjacencies?

A

Routers exchange LSAs

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3
Q

What process occurs once LSAs have been received?

A

Routers build their topology table (LSDB)

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4
Q

Once the LSDB has been built, what process does the router perform?

A

Router runs SPF algorithm against LSDB to create SPF tree. The SPF tree contains the computed best paths to destination networks.

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5
Q

Once the SPF algorithm has been run, what happens next?

A

Computed routes are offered to the routing table

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6
Q

What is contained in the Datalink header of an OSPFv2 message?

A 
Destination MAC (multicast 01-00-5e-00-00-05 or 06)
Source MAC (sending interface)
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7
Q

What is contained in the IP packet header of an an OSPFv2 message?

A

Destination IP address (224.0.0.5 or .6)
Source IP address
Protocol field, with value 89 indicating OSPF message

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8
Q

What is contained within the OSPF packet header of an OSPFv2 message?

A

Type code indicating OSPF packet type
Router ID
Area ID

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9
Q

What is the purpose of a Type 1 OSPF packet?

A

Hello packet, used to establish and maintain adjacencies with other OSPF routers

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10
Q

What is the purpose of a Type 2 OSPF packet?

A

Database Description Packet (DBD, DD), which is an abbreviated list of the sending router’s LSDB.

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11
Q

What is the purpose of a Type 3 OSPF packet?

A

Link-state Request (LSR), used by routers that have received a Type 2 DBD (summary) to request additional information from their neighbor about specific LSDB entries)

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12
Q

What is the purpose of a Type 4 OSPF packet?

A

Link State Update, sent in response to an LSA, and to announce newly-learned information.

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13
Q

What is the purpose of a Type 5 OSPF packet?

A

Link State Acknowledgement (LSAck), sent in response to a received LSA to confirm receipt.

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14
Q

Which packets are involved in the DR/BDR election process?

A

Type 1 (Hello) packets

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15
Q

What fields are included in a Type 1 Hello Packet

A

Type, router ID, Area ID, network mask, hello/dead timer intervals, router priority, DR (if applicable), BDR (if applicable), list of neighbors

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16
Q

What determines whether two OSPF routers form an adjacency?

A

Hello/Dead interval timers must match, both devices must be on same subnet (subnet mask mis-match is a giveaway)

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17
Q

What is the default value of the Hello interval?

A

10 seconds on multiaccess and P2P networks, 30 seconds on NBMA networks (Frame Relay)

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18
Q

OSPFv2 Hello packets are sent to which address?

A

Multicast 224.0.0.5

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19
Q

What is the default value of the Dead Interval?

A

4x hello interval

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20
Q

List the OSPF operational states from Establishment through convergence

A

Down, Init, 2-way, ExStart, Exchange, Loading, Full

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21
Q

What OSPF operational states are expected as adjacencies are established?

A

Down, Init, 2-way

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22
Q

What OSPF operational states are expected routers exchange OSPF databases?

A

ExStart, Exchange, Loading, Full

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23
Q

Identify the OSPF State, and what will happen next

  1. No hello packets yet received
  2. Router sends hello packets
A

Down, transition to Init

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24
Q

Identify the OSPF State, and what will happen next

  1. Hello packets received from neighbors
  2. Sending router’s RID observed
A

Init, transition to 2-way

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25
Q

Identify the OSPF State, and what will happen next

  1. On Ethernet links, elect DR and BDR
A

2-way, transition to ExStart

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26
Q

Identify the OSPF State, and what will happen next

  1. Negotiate Master/Slave relationship
  2. Negotiate DBD sequence number
  3. Initiate DBD packet transmission
A

ExStart, transition to Exchange

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27
Q

Identify the OSPF State, and what will happen next

  1. Exchange of DBD packets
A

Exchange State. If additional router information is required, transition to Loading. Otherwise, transition to Full

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28
Q

Identify the OSPF State, and what will happen next

  1. LSRs and LSUs used to gain additional route information
  2. Routes are processed using SPF
A

Loading. Transition to Full

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29
Q

Identify the OSPF State, and what will happen next

  1. Routers have reached convergence
A

Full State (routers have reached convergence)

30
Q

What determining factor will allow a router too transition from the Init State to the 2-way state?

A

Receiving a hello packet that includes its own router ID

31
Q

Two routers on a common Ethernet network link exchange Hello packets and reach the 2-way state. What process must occur next?

A

DR/BDR election

32
Q

What influences which devices will become DR/BDR?

A

Interface priority (higher is better, default 1), RouterID (higher is better)

33
Q

What is the purpose of the DR?

A

Minimization of adjacencies and LSA flooding. The DR becomes the collection and dissemination point for LSAs. The BDR also collects LSAs in case in ends of becoming the DR.

34
Q

What formula is used to calculate the potential number of adjacencies in a multiaccess broadcast network?

A

n (n-1) / 2

35
Q

Two OSPF routers have reached the ExStart state. Which router will begin the process of exchanging DBD packets?

A

The one with the highest router ID

36
Q

What is contained within DBD packets?

A

Summary information of the sending routers LSDB

37
Q

Two routers in the Exchange state are exchanging DBDs. Router 1 notices that 1 entry has a more current Link-state entry. What happens next?

A

R1 transitions to Loading State, and sends an LSR to R2 asking for complete information regarding that entry. R2 responds with an LSU. R1 sends LSAck to R2 confirming receipt.

38
Q

How often are LSUs sent to neighbors?

A
  1. When a topology change is received

2. Every 30 minutes

39
Q

How does an OSPF router determine its RID?

A
  1. Explicit configuration
  2. Highest IP address of any configured loopbacks
  3. Highest IP address of any configured interfaces
40
Q

What formula is used to calculate OSPF cost

A

Cost=Reference Bandwidth / Interface bandwidth (in bps)

41
Q

What is the default reference value bandwith

A

100,000,000 bps (100Mb)

42
Q

What is the default OSPF cost of a fast ethernet link?

A

1

43
Q

What is the default OSPF cost of an Ethernet (10mb) link

A

10

44
Q

What is the default OSPF cost of a 1.544Mbps serial link

A

64

45
Q

An OSPF router with all interfaces in the same area

A

Internal router

46
Q

All LSDBs on internal routers are…

A

identical

47
Q

All routers in area 0 are considered…

A

Backbone routers

48
Q

An OSPF router with interfaces attached to multiple areas

A

Area Border Routers

49
Q

An OSPF router with at least one interface attached to an external Internetwork

A

Autonomous System Boundary Router

50
Q

Another name for a Type 1 LSA

A

router link entry

51
Q

Another name for a Type 2 LSA

A

network link entry

52
Q

Another name for Type 3 and Type 4 LSAs

A

summary LSA

53
Q

What is the purpose of a type 1 LSA?

A

Advertisement of directly-connected links

54
Q

What is contained in a type 1 LSA

A

directly connected interfaces, link types, neighbors, link states

55
Q

Where are Type 1 LSA flooded?

A

Only within their area of origin (ABRs advertise networks learned via Type 1 LSAs to other areas as Type 3 LSAs)

56
Q

How are Type 1 LSA link IDs identified

A

by the router ID of the originating router

57
Q

What is the purpose of a Type 2 LSA

A

To inform other routers about multiaccess networks in the same area

58
Q

What is contained within a Type 2 LSA

A

Router ID and IP of the DR, and router IDs of all other routers on the multiaccess segment

59
Q

Where are Type 2 LSA flooded?

A

Only within their area of origin

60
Q

What is the link state ID of a Type 2 LSA?

A

Interface IP address of the DR that advertises it

61
Q

What is the purpose of a Type 3 LSA?

A

Used by ABRs to advertise networks from other areas

62
Q

What is the link-state ID of a Type 3 LSA?

A

The advertised network number and mask

63
Q

A router receives a Type 3 LSA from an ABR. Will the SPF algorithm be required?

A

No. SPF calculations are only performed on advertisements within the same area

64
Q

What is the purpose of a Type 4 LSA?

A

Identification and advertisement of ASBR routers

65
Q

An ABR receives a Type 1 LSA with the “e bit” set. What happens next?

A

The ABR notices the “e bit”, which means the sending router has identified itself as an ASBR. The ABR must notify other areas that the ASBR exists. The ABR floods that advertisement to the backbone via a Type 4 LSA. The backbone propagates that Type 4 LSA to other areas, alerting them to existence of that ASBR.

66
Q

What is the link-state ID of a Type 4 LSA?

A

The router ID of the ASBR

67
Q

What is the purpose of a Type 5 LSA?

A

Describes routes to networks outside the OSPF routing domain

68
Q

How are Type 5 LSAs propagated?

A

Type 5 LSAs are originated by the ASBR and flooded to the entire routing domain

69
Q

A routing table entry lists a routing table code of “O” for a specific OSPF route. What LSA Type ultimately produced this entry in the routing table?

A

Type 1 or type 2

70
Q

A routing table entry lists a routing table code of “O IA” for a specific OSPF route. What LSA Type ultimately produced this entry in the routing table?

A

Type 3

71
Q

A routing table entry lists a routing table code of “O E1” or “O E2” for a specific OSPF route. What LSA Type ultimately produced this entry in the routing table?

A

Type 5

72
Q

Each router uses the SPF algorithm to against the LSDB to build the OSPF tree. The SPF tree is used to determine the bas path(s) to add to the routing table. In what order are these best paths calculated?

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

ICND2-Odom (7 decks)

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