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Interview > OSPF > Flashcards

OSPF Flashcards

1
Q

OSPF Area Types

A
  • Broadcast Area
  • Normal/Standard Area
  • Stub Area
  • Totally Stub Area
  • Not-So-Stubby Area (NSSA)
  • NSSA Totally Stub
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2
Q

Characteristics of Stub Area

A
  • No Type 4, 5 AS-external LSAs allowed
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3
Q

Characteristics of Totally Stub Area

A
  • No Type 3, 4 or 5 LSAs allowed except the default summary route.
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4
Q

Characteristics of Not-So-Stubby Area (NSSA)

A
  • No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse.
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5
Q

Characteristics of NSSA Totally Stub

A
  • No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed.
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6
Q

What are LSA Types?

A
  • LSA Type 1: OSPF Router LSA
  • LSA Type 2: OSPF Network LSA
  • LSA Type 3: OSPF Summary LSA
  • LSA Type 4: OSPF ASBR Summary LSA
  • LSA Type 5: OSPF ASBR External LSA
  • LSA Type 6: OSPF Group Membership LSA
  • LSA Type 7: OSPF Not So Stubby Area (NSSA) External LSA
  • LSA Type 8: OSPF External Attributes LSA (OSPFv2) / Link Local LSA (OSPFv3)
  • LSA Type 9: OSPF Link Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3)
  • LSA Type 10: OSPF Area Scope Opaque LSA
  • LSA Type 11:OSPF AS (Autonomous System) Scope Opaque LSA
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7
Q

LSA Type 1

A

Router LSA

  • Generated by Normal Area Routers
  • Advertising router’s interface and status to neighbors
  • Flooding Map: Intra-Area (Area of origin)
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8
Q

LSA Type 2

A

Network LSA

  • Generated by DR
  • Advertising DRs direct connected neighbors
  • Flooding-Map: Intra-Area (Area of origin)
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9
Q

LSA Type 3

A

Summary LSA

  • Generated by ABR
  • Advertising ABRs areas summary
  • Flooding-Map: Inter-Area (Multiple Areas)

LSA Type 3 packets are generated by Area Border Routers (ABR) to summarize its directly connected area, and advertise inter-area router information to other areas the ABR is connected to, with the use of a summary prefix (e.g 192.168.0.0/22). LSA Type 3 packets are flooded to multiple areas throughout the network and help with OSPF’s scalability with the use of summary prefixes.

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10
Q

LSA Type 4

A

ASBR Summary LSA

  • Generated by ABR
  • Advertising the presence of ASBRs
  • Inter-Area (Multiple Areas)

LSA Type 4 packets are the LSAs that advertise the presence of an Autonomous System Border Router (ASBR) to other areas. In the example below when R2 (ABR) receives the LSA Type 1 packet from R1 it will create a LSA Type 4 (Summary ASBR LSA) packet, which advertises the ASBR route received from Area 1, and inject it into Area 0.

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11
Q

LSA Type 5

A

OSPF ASBR External LSA

  • Generated by ASBR
  • Advertising external routes to internet
  • Inter-Area (Multiple Areas)

LSA Type 5 packets are generated by the ASBR to advertise external redistributed routes into the OSPF’s AS. A typical example of an LSA Type 5 would be an external prefix e.g 192.168.10.0/24 or default route (internet) as shown below:

LSA Type 5

This external route/prefix is redistributed in to the OSPF network by the ASBR (R1) and seen as O E1 or E2 entries in other OSPF routers routing tables.

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12
Q

LSA Type 6

A

OSPF Group Membership LSA
LSA Type 6 packets were designed for Multicast OSPF (MOSPF), a protocol that supports multicast routing through OSPF. MOSPF is not supported by Cisco and is not widely used and is expected to be retired soon.

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13
Q

LSA Type 7

A

NSSA External LSA

  • Generated by ASBR
  • Advertising external routes to internet to NSSA areas
  • Inter-Area (Multiple Areas)

LSA Type 7 packets are used for some special area types that do not allow external distributed routes to go through and thus block LSA Type 5 packets from flooding through them, LSA Type 7 packets act as a mask for LSA Type 5 packets to allow them to move through these special areas and reach the ABR that is able to translate LSA Type 7 packets back to LSA Type 5 packets.

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14
Q

LSA Type 8 (OSPFv2)

A

OSPF External Attributes LSA (OSPFv2)
LSA Type 8 packets in OSPFv2 (IPv4) are called External Attribute LSAs, and are used to transit BGP attributes through an OSPF network while BGP destinations are conveyed via LSA Type 5 packets, however, this feature isn’t supported by most routers.

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15
Q

LSA Type 8 (OSPFv3)

A

Link Local LSA (OSPFv3)
With OSPFv3 IPv6, LSA Type 8 is redefined to carry IPv6 information through OSPF network.

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16
Q

LSA Type 9, 10 and 11

A

Opaque LSAs
Generally Opaque LSAs are used to extend the capabilities of OSPF allowing the protocol to carry information OSPF doesn’t necessarily care about. Practical application of Opaque LSAs is in MPLS traffic engineering where they are used to communicate interface parameters such as maximum bandwidth, unreserved bandwidth, etc.

17
Q

LSA Type 9

A

Link Scope Opaque LSA
LSA Type 9 in OSPFv2 (IPv4) is defined as a Link Scope Opaque LSA for carrying OSPF information. For OSPFv3 it’s redefined to handle a communication prefix for a special area type called Stub Area.

18
Q

LSA Type 10

A

OSPF Area Scope Opaque LSA
LSA Type 10 packets are used to flood OSPF information through other area routers even if these routers do not process this information in order to extend OSPF functionality, this LSA is used for traffic engineering to advertise MPLS and other protocols.

19
Q

LSA Type 11

A

OSPF AS Scope Opaque LSA
LSA Type 11 packets serve the same purpose as LSA Type 10 packets but are not flooded into special area types (Stub areas)

20
Q

224.0.0.5

A

OSPF - IGP All Routers [RFC1583,JXM1]

21
Q

224.0.0.6

A

OSPF - IGP Designated Routers [RFC1583,JXM1]

22
Q

OSPF States

A
  • Down
  • Init
  • Two-way
  • Exstart
  • Exchange
  • Loading
  • Full
23
Q

OSPF Down

A

The initial state when a router has no information about a neighbor or a network.

24
Q

OSPF Init

A

The state when a router has received a hello packet from a neighbor but has not established bidirectional communication yet.

25
Q

OSPF Two-way

A

The state when a router has established bidirectional communication with a neighbor but has yet to decide whether to exchange routing information or not.

26
Q

OSPF Exstart

A

The state when a router has decided to exchange routing information with a neighbor and has negotiated the master-slave relationship and the initial sequence number.

27
Q

OSPF Exchange

A

The state when a router exchanges DBD packets with a neighbor to synchronize their LSDBs.

28
Q

OSPF Loading

A

The state when a router requests and receives more details about LSAs from a neighbor using LSR, LSU, and LSAck packets.

29
Q

OSPF Full

A

The final state when a router has synchronized its LSDB with a neighbor and is ready to forward packets.

30
Q

Hello

A

The Hello is used to create neighborhood relationships and analyze the proximity of neighbors. It means that “Hello” is necessary to establish a connection between routers.

31
Q

Database description

A

After making a connection, when the neighboring router wants to communicate with the system for the first time. It transmits the information to the database for network topology to the system; through this, the system can update or make changes accordingly.

32
Q

Link State Request

A

The router sends a link-state request to obtain information about the specified route. For example, Router 1 wants information about Router 2, so Router 1 shares a link-state request with Router 2. If Router 2 receives a link-state request, it sends link-state information to Router 1.

33
Q

Link State Update

A

The router uses link-state updates to announce the status of the link. When a route needs to broadcast the state of its link, it uses link-state updates.

Interview (8 decks)

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