Open Shortest Path First (OSPF)

Question 1

What is OSPF based on?

Answer 1

An early version of ISO IS-IS.

Question 2

What is OSPF designed to replace?

Answer 2

RIP

Question 3

What does OSPF provide and what does it send?

Answer 3

Provides Link State Routing with all routers and sends changes in tables

Question 4

How is OSPF transmitted?

Answer 4

IP datagram, multicast addressing

Question 5

Name 4 things OSPF routing decisions are based on.

Answer 5

• Link speed
• congestion level
• hop count
• cost

Question 6

What is an LSA packet? Why would you flood it?

Answer 6

Link State Advertisement. Flooded LSA packets provide information about the status of a router’s direct links and neighbours.

Question 7

What 2 things do LSA packets contain?

Answer 7

• list of names (IP addresses)

- cost (hops) to reach each neighbour

Question 8

What is faster to converge: link state protocols or distance vector protocols?

Answer 8

Link State! Link State! Link State!

Question 9

T/F: Routers must only know n+15 routers for a link state protocol to operate efficiently.

Answer 9

False. I don’t know what n+15 is. Routers must know the entire network topology.

Question 10

What are the 4 steps of link state operation?

Answer 10

1. LSAs are flooded to other routers
2. most recent received LSAs are stored
3. routes to each destination are computed
4. shortest paths are recalculated when an LSA is received

Question 11

What 4 types of routers are in an OSPF hierarchy?

Answer 11

• Internal Router (IR)
• Area Border Router (ABR)
• Backbone Router (BBR)
• Autonomous System Boundary Router (ASBR)

Question 12

What is an IR?

Answer 12

Internal Router. directly connects networks in the same area

Question 13

What is an ABR?

Answer 13

Area Border Router. Connects to multiple areas, one of which is the backbone 0.0.0.0

Question 14

What is a BBR?

Answer 14

Backbone Router. Connects to the OSPF backbone area 0.0.0.0

Question 15

What is an ASBR?

Answer 15

Autonomous System Boundary Router. Exchanges info with routers belonging to other ASs, typically using BGP4 or EGP2.

Question 16

Pick one:
A. All ABRs must be BBRs.
B. All BBRs must be ABRs.

Answer 16

A. BBRs may have all interfaces in the backbone area.

Question 17

What are OSPF specific requriements? (Four)

Answer 17

• a network can only belong to one area
• backbone is 0.0.0.0
• area 0.0.0.0 is contiguous
• all non-backbone areas must be connected to the backbone when more than one area exists

Question 18

T/F: OSPF areas are a group of contiguous networks where the topology of an area is visible outside the area.

Answer 18

False. OSPF areas are a group of contiguous networks where the topology is invisible outside the area.