OSPF

Question 1

What are LSA used for?

Answer 1

Build a LSDB

Question 2

Who will OSPF routers form a FULL adjacency with?

Answer 2

Designated Router and Backup Designated Router.

Question 3

Who does a OSPF router send its LSA to?

Answer 3

Designated Router only.

DR distributes to all other routers.

Question 4

What tpye of network would you have DR/BDR and what network would you not have a DR/BDR?

Answer 4

Multi-access Network requiers a DR/BDR. Point-to-Point does not requier a DR/BDR.

Question 5

What is used to calculate the best path of the LSDB?

Answer 5

Shortest-path first Algorithm SPF

Question 6

• ID ABR
• ID ASBR
• Why is it a ABR?
• Why is it a ASBR?

Answer 6

ABR are between two area’s and can summarize routes from one area to another. ASBR are routers that run OSPF and connect to another network that runs another routing protocol.

Question 7

What is in an OSPF Hello Packet?

Answer 7

• Router ID
• Hello/Dead Interval *
• Neighbors
• Area ID *
• Router Priority
• DR IP
• BDR IP
• Authentication
• Password*
• Sub Area Flag

Question 8

What metric does OSPF use?

Answer 8

Cost=Reference BW/Interface BW

Question 9

How to change the Reference B/W?

Answer 9

(config)#router ospf 1

(config-router)# auto-cost reference-bandwidth 1000

Question 10

What is the default Reference Bandwith for OSPF

Answer 10

100 Mbits

Question 11

What does OSPF do if it has 2 path with equal cost?

Answer 11

Load-Balance

Question 12

What are the Max Load Balance paths of IOS 12

Answer 12

• 16 Paths

(32 for IOS 15)

Question 13

What does the network command do while under the router ospf 1 process.

(conf-router)# network 10.10.10.0 0.0.0.255 area 0

Answer 13

• Advertise the networks that fall within this range in OSPF.
• Activate OSPF on the interface(s) that fall within this range. This means that OSPF will send hello packets on the interface.

Question 14

What is used as the router ID in OSPF?

Answer 14

1. Loopback interface
2. highest active IP

or set manualy.

(conf-router)#router-id x.x.x.x

Question 15

How do you change the COST of a OSFP link?

Answer 15

ip ospf cost 50

within the interface use the command.

Question 16

How would you make all routers send there unknow traffic to you.

Answer 16

• Advertise a default router.
• (config-router)# default-information originate [always]

Question 17

How would you configure clear text Authentication in OSPF?

Answer 17

R1(config)#interface fastEthernet 1/0

R1(config-if)#ip ospf authentication

R1(config-if)#ip ospf authentication-key password

Question 18

How would you configure MD5 Authentication in OSPF?

Answer 18

R3(config)#interface fastEthernet 1/0

R3(config-if)#ip ospf authentication message-digest

R3(config-if)#ip ospf message-digest-key 1 md5 0 password

Question 19

What does this output of the show ip route mean.

O IA 4.4.4.4 [110/3] via 192.168.12.2, 00:00:54, GigabitEthernet0/1

Question 20

How many States does OFPF go through to become Neighbors?

Answer 20

7

Question 21

What are the 7 States that OSPF goes through to become neighbors?

Answer 21

1. Down: no OSPF neighbors detected at this moment.
2. Init: Hello packet received.
3. Two-way: own router ID found in received hello packet.
4. Exstart: master and slave roles determined.
5. Exchange: database description packets (DBD) are sent.
6. Loading: exchange of LSRs (Link state request) and LSUs (Link state update) packets.
7. Full: OSPF routers now have an adjacency.

Question 22

What are the OSPF Packet Types?

Answer 22

Question 23

What Two Multicast Address does OSPF Send to?

Answer 23

224. 0.0.5 - goes to All routers
225. 0.0.6 - goes to DR & BDR

Question 24

When a Router in OSPF is not a DR or BDR, what is it called?

Answer 24

DROTHER

Question 25

True or False

DR/BDR election is per multi-access segment…not per area!

Answer 25

True

Question 26

Explain in the picture why you would not want to advertise OSPF out of interface FA0/1?

Answer 26

It is a security risk. anyone could become neighbors and inject routes.

Question 27

What is a LSA Type 1 ?

Answer 27

Question 28

What is a LSA Type 2?

Answer 28

The network LSA or type 2 is created for each multi-access network. Remember the OSPF network types? The broadcast and non-broadcast network types require a DR/BDR. If this is the case you will see these network LSAs being generated by the DR. In this LSA we will find all the routers that are connected to the multi-access network, the DR and of course the prefix and subnet mask.

Question 29

What is a type 3 LSA?

Answer 29

R2 will take the Type 1 message and create a type 3 summary LSA and flood it to Area 0

Question 30

What is a type 4 LSA?

Answer 30

In this example we have R1 that is redistributing information from the RIP router into OSPF. This makes R1 an ASBR (Autonomous System Border Router). What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2 who is an ABR receives this router LSA it will create a type 4 summary ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so all OSPF routers know where to find the ASBR.

Question 31

What is a Type 5 LSA?

Answer 31

Same topology but I’ve added a prefix (5.5.5.0 /24) at our RIP router. This prefix will be redistributed into OSPF. R1 (our ASBR) will take care of this and create a type 5 external LSA for this. Don’t forget we still need type 4 summary ASBR LSA to locate R1. If you ever tried redistribution with OSPF you might have seen O E1 or E2 entries. Those are the external prefixes and our type 5 LSAs.

Question 32

What are type 7 LSA?

Answer 32

NSSA areas do not allow type 5 external LSAs. In my picture R1 is still our ASBR redistributing information from RIP into OSPF.

Since type 5 is not allowed we have to think of something else. That’s why we have a type 7 external LSA that carries the exact same information but is not blocked within the NSSA area. R2 will translate this type 7 into a type 5 and flood it into the other areas.