Kamal Hasan

OSPF Q & A Part -II

1: What are the five OSPF packet types? What is the purpose of each type?

*** The five OSPF packet types, and their purposes, are:

Hellos, which are used to discover neighbors, and to establish and maintain adjacencies

Updates, which are used to send LSAs between neighbors

Database Description packets, which a router uses to describe its link state database to a neighbor during database synchronization

Link State Requests, which a router uses to request one or more LSAs from a neighbor's link state database

Link State Acknowledgments, used to ensure reliable delivery of LSAs


2: What are LSA types 1 to 5 and LSA type 7? What is the purpose of each type?

*** The most common LSA types and their purposes are:

Type 1 (Router LSAs) are originated by every router and describe the originating router, the router's directly connected links and their states, and the router\xd5 s neighbors.

Type 2 (Network LSAs) are originated by Designated Routers on multiaccess links and describe the link and all attached neighbors.

Type 3 (Network Summary LSAs) are originated by Area Border Routers and describe inter-area destinations.

Type 4 LSAs (ASBR Summary LSAs) are originated by Area Border Routers to describe Autonomous System Boundary Routers outside the area.

Type 5 (AS External LSAs) are originated by Autonomous System Boundary Routers to describe destinations external to the OSPF domain.

Type 7 (NSSA External LSAs) are originated by Autonomous System Boundary Routers within not-so-stubby areas.

3: What are the four OSPF router types?

***The four OSPF router types are:

# Internal Routers, whose OSPF interfaces all belong to the same area

# Backbone Routers, which are Internal Routers in Area 0

# Area Border Routers, which have OSPF interfaces in more than one area

# Autonomous System Boundary Routers, which advertise external routes into the OSPF domain


4: What are the four OSPF path types?

***The four OSPF path types are:

Intra-area paths

Inter-area paths

Type 1 external paths

Type 2 external paths


5: What are the five OSPF network types?

*** The five OSPF network types are:

i)Point-to-point networks

ii) Broadcast networks

iii) Non-broadcast multi-access (NBMA) networks

iv) Point-to-multipoint networks

v) Virtual links


6: What is a Designated Router?

***A Designated Router is a router that represents a multiaccess network, and the routers connected to the network, to the rest of the OSFP domain.


7: How does a Cisco router calculate the outgoing cost of an interface?

***Cisco IOS calculates the outgoing cost of an interface as 108/BW, where BW is the configured bandwidth of the interface.


8: What is a partitioned area?

***An area is partitioned if one or more of its routers cannot send a packet to the area's other routers without sending the packet out of the area.


9: What is a virtual link?

*** A virtual link is a tunnel that extends an OSPF backbone connection through a non-backbone area.


10: What is the difference between a stub area, a totally stubby area, and a not-so-stubby area?

***A stub area is an area into which no type 5 LSAs are flooded. A totally stubby area is an area into which no type 3, 4, or 5 LSAs are flooded, with the exception of type 3 LSAs to advertise a default route. Not-so-stubby areas are areas through which external destinations are advertised into the OSPF domain, but into which no type 5 LSAs are sent by the ABR.


11: What is the difference between OSPF network entries and OSPF router entries?

*** OSPF network entries are entries in the route table, describing IP destinations. OSPF router entries are entries in a separate route table that record only routes to ABRs and ASBRs.


12: Why is type 2 authentication preferable over type 1 authentication?

***Type 2 authentication uses MD5 encryption, whereas type 1 authentication uses clear-text passwords.


13: Which three fields in the LSA header distinguish different LSAs? Which three fields in the LSA header distinguish different instances of the same LSA?

***The three fields in the LSA header that distinguish different LSAs are the Type, Advertising Router, and the Link State ID fields. The three fields in the LSA header that distinguish different instances of the same LSA are the Sequence Number, Age, and Checksum fields.