Section 1.2 - Multiarea OSPF Overview

Question 1

What are limitations to single area OSPF?

Answer 1

• Additional computational complexity as the networks scale
• Computational complexity is proportional to the square of the number of nodes (routers)

Question 2

What are some benefits of multiarea OSPF?

Answer 2

• Minimizes routing table entries
• Localizes impact of a topology change (link flapping) within an area
• Detailed link-state advertisement (LSA) flooding stops at area boundary

Question 3

What are the characteristics of the “backbone area”?

Answer 3

• must connect to all other non-backbone areas
• must always be contiguous (cannot be split)
• end users are generally not connected to routers in the backbone area

Question 4

What are the characteristics of a “non-backbone” area?

Answer 4

• connect end users and resources
  usually set up according to functional and/or geographical groupings
• traffic between different non-backbone areas must always pass through the backbone area

Question 5

What is a backbone router?

Answer 5

a router that has at least one interface that is connected to the backbone area

Question 6

What is an area border router (ABR)?

Answer 6

a router that has interfaces connected to at least two different OSPF areas, including the backbone area; it contains LSDB info for each area

Question 7

What is an internal router?

Answer 7

a router that has all its interfaces connected to only one OSPF area

Question 8

What is an autonomous system boundary router (ASBR)?

Answer 8

a router that has at least one interface that is connected to an OSPF area and at least one interface connected to an external non-OSPF routing domain

Question 9

How to configure and enable OSPF for interfaces in IOS-XR (commands)?

Answer 9

conf t
router ospf 1
area 0
interface GigabitEthernet0/1/0/1
area 1
interface GigabitEthernet0/1/0/2

Question 10

How to configure and enable OSPF for interfaces in IOS-XE (commands)?

Answer 10

conf t
router ospf 1
network 172.16.12.0 0.0.0.3 area 0
network 192.168.1.0 0.0.0.255 area 1

Question 11

Design guidelines - single area OSPF?

Answer 11

• provides optimal routing because of a fully visible topology
• provides simplicity for implementation and maintenance
• best suited for MPLS TE

Question 12

Design guidelines - multiarea OSPF?

Answer 12

• only use in large environments (many hundreds of routers)
• use regular areas if possible
• use stub and NSSA in case of old, CPU-hungry and/or memory-hungry routers
• ensure that all areas have redundant ABRs (two)
• focus on the addressing plan to enable effective summarization

Question 13

Which two statements about multiarea OSPF are true? (choose two)
A) A router can be in only one area
B) All nonbackbone areas must be connected to the backbone area
C) ASBRs can be in only nonbackbone areas
D) Area 0 is always the backbone area
E) All areas have full topology visibility of other areas
F) ABRs cannot be ASBRs at the same time

Answer 13

B, D