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Open Shortest Path First (OSPF) > Link-State Advertisement (LSA) and Link-State Database (LSD) > Flashcards

Link-State Advertisement (LSA) and Link-State Database (LSD) Flashcards

1
Q

What does a Link State Advertisement (LSA) contain?

A

fundamental information about topology

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2
Q

What is formed by a collection of LSAs?

A

collection of LSAs forms the Link-State Database

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3
Q

How are the LSAs flooded?

A

flooded to all the routers, so they can build the topology diagram in their Link State Database (LSDb)

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4
Q

What are the first five LSA Types?

A
  1. LSA Type 1: Router LSA
  2. LSA Type 2: Network LSA
  3. LSA Type 3: Summary LSA
  4. LSA Type 4: Autonomous System Boundary Router (ASBR) Summary
  5. LSA Type 5: Autonomous System Boundary Router (ASBR) External LSA
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5
Q

What are the 6th - 11th LSA Types?

A
  1. LSA Type 6: OSPF Group Membership LSA
  2. LSA Type 7: Not-so-stubby area (NSSA) External LSA
  3. LSA Type 8: OSPF External Attributes LSA (OSPFv2) / Link Local LSA (OSPFv3)
  4. LSA Type 9: OSPF Link Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3)
  5. LSA Type 10: OSPF Area Scope Opaque LSA3
  6. LSA Type 11: OSPF AS (Autonomous System) Scope Opaque LSA
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6
Q

What does LSA Type 1: Router LSA describe?

A

the state and configuration of an OSPF router’s interfaces within a single OSPF area

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7
Q

Who generates and floods LSA Type 1: Router LSA?

A

generated and flooded by every OSPF node

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8
Q

How are LSA Type 1: Router LSA flooded and where do these packets stop?

A

flooded within an Area and stops when it reaches the Area Border Router

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9
Q

What is the primary function of LSA Type 1: Router LSA?

A

to help to construct the SPF tree

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10
Q

In which case is the LSA Type 2: Network LSA used?

A
  • when there is a DR/BDR election on a segment
  • network types that don’t support election of DR/BDR have no Type 2 LSAs
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11
Q

What does LSA Type 2: Network LSA describe?

A

all routers connected to an Ethernet subnet

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12
Q

Why is LSA Type 2: Network LSA required? (5)

A
  1. Representation of Multi-Access Networks
  2. DR/BDR Election
  3. Efficient Flooding of LSAs
  4. Reduction of LSDb Size
  5. Synchronization and Consistency
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13
Q

Is it true that anywhere in the topology, in the same area, routers have the same copy of the Link State Database?

A

yes

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14
Q

When are prefixes started to be advertised?

A

only after OSPF builds the SPF structure (routing tables are calculated) - then it extracts best path to given destination and puts it in routing table

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15
Q

What does LSA Header contain?

A

essential information about the LSA itself
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* LSA type
* LSA age
* LSA sequence number
* LSA checksum
* Router ID of the advertising router

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16
Q

What is the Advertising Router?

A

Router ID of the router that originated the LSA

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17
Q

What is the LSA Age?

A
  • field that indicates the age of the LSA, which represents the time since the LSA was originated or last refreshed by the advertising router
  • helps routers determine the freshness of the LSA and decide whether to accept it or not
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18
Q

What is the purpose of the LSA Sequence Number?

A
  • track changes and updates to the LSA
  • routers can determine whether a new LSA is more recent than the one they have in their Link-State Database (LSDB) by comparing sequence numbers
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19
Q

What happens to the SPF tree when a new LSA is received?

A

current SPF tree is destroyed and newly built (might advertise new link in the topology or change related to other links)

20
Q

Why are Database Descriptor (DBD) packets exchanged between neighboring routers?

A

to compare the contents of their respective link-state databases and ensure that they have consistent and up-to-date information about the network topology

21
Q

What are Database Descriptor (DBD) packets used for?

A

to allow routers to exchange summaries of their LSDBs without sending the complete database contents

22
Q

Database Descriptor (DBD) packets are exchanged during which OSPF state?

A

Exchange

23
Q

What is the content of the DBD packet?

A
  1. Interface MTU
  2. Options
  3. Sequence Number
  4. LSA Header(s)
24
Q

What information does LSA Type 1: Router LSA provide?

A
  1. router’s own interfaces
  2. IP addresses
  3. neighboring routers to which it is directly connected within the same OSPF area
25
Q

What is the content of the LSA Type 1: Router LSA? (4)

A
  1. Router ID
  2. Links and Interfaces for each of the router’s interfaces within the OSPF area
  3. Neighbor Information
  4. Optional Information (authentication, OSPF options)
26
Q

Are Router LSAs are specific to the OSPF area in which they are originated?

A

yes - in multi-area OSPF networks, each OSPF area has its own set of Router LSAs that describe the routers and links within that area

27
Q

What is the purpose of the Link State ID (LSA ID)?

A
  • distinguish one LSA from another and help routers determine the specific information contained in each LSA
  • it is a field within an OSPF LSA packet that uniquely identifies the LSA within the scope of the advertising router
28
Q

The format and interpretation of the Link-State ID field depends on what?

A

type of LSA

29
Q

What does the LSA ID contain in case of Router LSA (LSA Type 1)?

A
  • the Router ID of the advertising router
  • allows routers to differentiate the LSAs of different routers
30
Q

What does the LSA ID contain in case of Network LSA (LSA Type 2)?

A

the IP address of the Designated Router (DR) for a multi-access network segment

31
Q

What does the LSA ID contain in case of Summary LSA (LSA Type 3)?

A

the IP address of the destination network

32
Q

What does the LSA ID contain in case of ASBR Summary LSA (LSA Type 4)?

A

the Router ID of the ASBR

33
Q

What is the purpose of the ASBR Summary LSAs

A

to advertise the presence of Autonomous System Boundary Routers (ASBRs) and how to reach them

34
Q

What is the purpose of External LSA (LSA Type 5)

A

advertise routes from external routing domains or other autonomous systems

35
Q

What does the LSA ID contain in case of External LSA (LSA Type 5)?

A

external network’s destination IP address

36
Q

What helps OSPF routers uniquely identify and process LSAs?

A

Link-State ID, in conjunction with the LSA type and the advertising router’s Router ID

37
Q

Who generates the Type 2 Network LSAs?

A

Designated Router

38
Q

Do LSA Type 1 and Type 2 always stay within nthe area they originated from?

A

yes, but the information they contain can be sent to other areas as a different LSA type, depending on the area type (the information they contain is very detailed so it needs to be truncated)

39
Q

How does ABR generate the Type 3 Summary LSA?

A
  • ABR takes information from LSA types 1 and 2 and converts it to a type 3 LSA that it sends to adjoining area
  • the ABR summarizes the Type 1 and Type 2 LSAs into a Type 3 LSA (hence the term Summary LSA)
40
Q

Which router type generates Type 5 AS External LSA?

A

ASBR

41
Q

To which areas are Type 5 AS External LSA flooded?

A

to all normal areas

42
Q

What does Type 5 AS External LSA describe?

A

external prefix and the gateway to reach it

43
Q

Which router type generates Type 4 ASBR Summary

A

generated by an ABR that borders an area containing an ASBR

44
Q

Is the LSDb the same on all OSPF routers within the same area?

A

yes

45
Q

How many LSA types exist for OSPFv2?

A

7

Open Shortest Path First (OSPF) (9 decks)

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