OSPF

Question 1

OSPF Area Types

Answer 1

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

Question 2

Characteristics of Broadcast Area

Question 3

Characteristics of Normal/Standard Area

Question 4

Characteristics of Stub Area

Answer 4

• No Type 4, 5 AS-external LSAs allowed

Question 5

Characteristics of Totally Stub Area

Answer 5

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

Question 6

Characteristics of Not-So-Stubby Area (NSSA)

Answer 6

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

Question 7

Characteristics of NSSA Totally Stub

Answer 7

• 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.

Question 8

What are LSA Types?

Answer 8

• 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

Question 9

LSA Type 1

Answer 9

Router LSA

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

Question 10

LSA Type 2

Answer 10

Network LSA

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

Question 11

LSA Type 3

Answer 11

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.

Question 12

LSA Type 4

Answer 12

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.

Question 13

LSA Type 5

Answer 13

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.

Question 14

LSA Type 6

Answer 14

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.

Question 15

LSA Type 7

Answer 15

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.

Question 16

LSA Type 8 (OSPFv2)

Answer 16

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.

Question 17

LSA Type 8 (OSPFv3)

Answer 17

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

Question 18

LSA Type 9, 10 and 11

Answer 18

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.

Question 19

LSA Type 9

Answer 19

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.

Question 20

LSA Type 10

Answer 20

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.

Question 21

LSA Type 11

Answer 21

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)

Question 22

224.0.0.5

Answer 22

OSPF - IGP All Routers [RFC1583,JXM1]

Question 23

224.0.0.6

Answer 23

OSPF - IGP Designated Routers [RFC1583,JXM1]

Question 24

OSPF States

Answer 24

• Down
• Init
• Two-way
• Exstart
• Exchange
• Loading
• Full

Question 25

OSPF Down

Answer 25

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

Question 26

OSPF Init

Answer 26

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

Question 27

OSPF Two-way

Answer 27

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

Question 28

OSPF Exstart

Answer 28

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.

Question 29

OSPF Exchange

Answer 29

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

Question 30

OSPF Loading

Answer 30

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

Question 31

OSPF Full

Answer 31

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

Question 32

Hello

Answer 32

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.

Question 33

Database description

Answer 33

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.

Question 34

Link State Request

Answer 34

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.

Question 35

Link State Update

Answer 35

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.