Chap 9 - Advanced OSPF

Question 1

What is an Area Border Router (ABR)

Answer 1

• It has interfaces in area 0 and another area.
• ABRs take routes from one area and advertise them into another area
• Every ABR needs to participate in Area 0.

Question 2

When advertising an area ID what format does OSPF use?

Answer 2

Dotted decimal format (Looks like an IP address)

Question 3

What are routes to subnets in the same area?

Answer 3

Intra-Area Routes

Question 4

What are routes to subnets in a different area?

Answer 4

Inter-Area Routes

Question 5

How are Intra-Area routes displayed in a routing table?

Answer 5

O

Question 6

How are Inter-Area routes displayed in a routing table?

Answer 6

O IA

Question 7

What are routes called that are a different OSPF domain or routes learned from a different routing protocol?

Answer 7

External Routes

Question 8

What are the 6 LSA types called?

Answer 8

• Type 1 - Router LSA
• Type 2 - Network LSA
• Type 3 - Summary LSA
• Type 4 - ASBR Summary LSA
• Type 5 - AS External LSA
• Type 7 - NSSA External LSA

Question 9

Which LSA types are used for building the SPF Tree for Inter and Intra-area routes?

Answer 9

Types 1, 2, and 3

Question 10

When does a router send out a new LSA?

Answer 10

When the last LSA was sent 30 minutes ago.

Question 11

When does a router purge an LSA?

Answer 11

When its age exceeds 60 minutes.

Question 12

What is a Type 1 LSA?

Answer 12

• Router LSA
• Sent by every router
• Not advertised outside its area
• exists for every interface and attached network

Question 13

Why is the underlying topology of an area invisible to other areas?

Answer 13

Because Type 1 Router LSAs are not advertised outside its area.

Question 14

What is a Type 2 LSA?

Answer 14

• Network LSA
• Sent by the DR
• Represents a multi-access network
• Identifies all routers attached to the network segment
• Not forwarded outside the area

Question 15

If a DR hasn’t been elected yet on a multi-access network will there be a Type 2 LSA present?

Answer 15

No

Question 16

What is a pseudo-node?

Answer 16

Represents the DR on a mult-access network.

Question 17

What is a Type 3 LSA?

Answer 17

• Summary LSA
• Sent by ABR to Area 0
• Represents networks from other areas

Question 18

When does an ABR create a Type 3 LSA?

Answer 18

• When it receives Type 1 LSAs from Non-Backbone Area to send to Backbone Area
• When it receives a Type 1 LSA from the Backbone Area to send to Non-Backbone Area
• When it receives a Type 3 LSA from Area 0 it creates a new Type 3 LSA to send to the Non-Backbone area

Question 19

What does an ABR use a received Type 2 LSA for?

Answer 19

It uses the Type 2 LSA to obtain the network mask of the multi-access network.

Question 20

What does an ABR do when it receives a Type 3 LSA from the backbone area?

Answer 20

• Creates a New Type 3 LSA for the Non-Backbone Area
• Lists itself as the Sender
• Adds the additional Cost metric

Question 21

What Metric is advertised by an ABR when it creates a Type 3 LSA from a Type 1 LSA?

Answer 21

It advertises the Total Path Metric to reach the originating router.

Question 22

What Metric is advertised by an ABR when it creates a Type 3 LSA from a Type 3 LSA?

Answer 22

It advertises the Total Path Metric to reach the Advertising ABR plus the metric contained in the original Type 3 LSA.

Question 23

What does a Type 3 LSA contain?

Answer 23

• Link-state ID (IP address of interface advertising the subnet)
• subnet mask
• IP address of Advertising ABR
• Metric for the network prefix

Question 24

If an ABR is aware of multiple paths to a network how many Type 3 LSAs are created?

Answer 24

Only 1 Type 3 LSA per prefix using the best metric out of all the paths.

Question 25

What are 3 fundamental rules used by ABRs when creating Type 3 LSAs?

Answer 25

• Type 1 LSAs create Type 3 LSAs into the backbone and non-backbone areas
• Type 3 LSAs from the backbone create Type 3 LSAs for the non-backbone area
• Type 3 LSAs from a non-backbone area only insert into the LSDB of the Source Area.

Question 26

In order of priority how are routes selected?

Answer 26

• Choose the Intra-area route first
• Second choose the Inter-area route
• Choose an External route last

Question 27

What happens if there is are two paths to a destination - one is inter-area and the other is intra-area?

Answer 27

Intra-area routes from a Type 1 LSA are always preferred over routes from a Type 3 LSA even if the Inter-area route has a better metric.

Question 28

What happens if there is a tie in metrics between 2 Inter-Area routes?

Answer 28

They both go into the RIB.

Question 29

What is ECMP?

Answer 29

Equal Cost Multi-pathing.

Question 30

What is the default for maximum paths?

Answer 30

4

Question 31

If summarization is configured by default what does the ABR assign as a metric to a Type 3 LSA?

Answer 31

The best metric out of all the Type 1 LSAs routes being summarized.

Question 32

When summarization is configured what is the default behavior - to Advertise or not to Advertise?

Answer 32

Default is to Advertise.

Question 33

When summarization is configured what does the ABR do?

Answer 33

Creates a Discard Route to Null0 for the summary.

Question 34

What is the Administrative Distance for a summary route that shows up as an External Route?

Answer 34

254

Question 35

What are 3 methods of route filtering?

Answer 35

• Filtering with Summarization
• Area Filtering
• Local OSPF Filtering

Question 36

What happens when an LSA Age reaches 30 minutes (1800 seconds)?

Answer 36

The originating router sends out a new one.

Question 37

What happens when an LSA Age reaches 60 minutes (3600 seconds)?

Answer 37

The receiving router deems the LSA to be invalid and purges it from its LSDB

Question 38

What happens when each router forwards an LSA?

Answer 38

The LSA Age is incremented with a calculated minimal delay that reflects that of the link.

Question 39

Display the Type 3 Summary LSA.

Answer 39

show ip ospf database summary