Chapter 3-5: Adjust and Troubleshoot Single-Area OSPF

Question 1

What are five [5] network types of OSPF Network.

Answer 1

1. Point-to-point – WAN links
2. Broadcast Multiaccess – Ethernet network
3. NBMA (non-broadcast multiple access network ) – Frame Relay, ATM or x.25
4. Point-to-multipoint – hub-and-spoke topology over NBMA
5. Virtual links – connect distant OSPF areas to the backbone

Question 2

What are two [2] challengers for multiaccess Network?

Answer 2

1. Multiple adjacencies

2. Extensive LSA Flooding - every LSA must be ACKed

Question 3

What are two [2] solutions for LSA flooding issue

Answer 3

1. Designated router (DR)

2. Backup DR (BDR)

Question 4

What are the IPv4 and IPv6 address for sending and receiving LSA

Answer 4

Designated router (DR) other to:
IPv4 224.0.0.6
IPv6 FF02::05

Question 5

What are five [5] reasons that a OSPF Designated Router is used?

Answer 5

1. The DR helps manage adj and flooding on multi-access nets
2. The BDR is elected in case the DR fails
3. All other routers become DROTHERs; form adj with the DR/BDR
4. DR/BDR elections take place only on multi-access networks
5. They DO NOT occur in point to point networks

Question 6

What are the four [4] states of neighbours in multi-access networks

Answer 6

1. FULL/DROTHER
2. FULL/DR
3. FULL/BDR
4. 2-WAY/DROTHER

Question 7

What are the three [3] Default DR/BDR Election Process

Answer 7

1. The router with the highest interface priority is elected as the DR
2. The router with the 2nd highest interface priority → BDR
3. Priority range: 0-255 (0 → cannot become the DR, 255 → usually only two “best” routers set to this value )

Question 8

DR remains the DR until one of the following three [3] issues occurs

Answer 8

1. The DR fails
2. The OSPF process on the DR fails or is stopped
3. The multi-access interface on the DR fails or is shutdown

Question 9

What two [2] things happens with the DR (Designated router) fails?

Answer 9

1. The BDR (backup Designated Router ) is automatically promoted to DR (Designated Router)
2. There is then a new BDR election and the DROTHER with the higher priority or router ID is elected as the new BDR

Question 10

What are three [3] types of authentication of OSPF secure routing?

Answer 10

1. Null – no authentication
2. Simple password authentication – plain text (outdated)
3. MD5 – Most secure and recommended method

Question 11

What are seven [7] reasons why adjacencies will not form?

Answer 11

1. Interfaces are not on same network
2. Network types no identical
3. Hello or Dead timers don’t match
4. Passive interfaces are set
5. There are missing/incorrect network commands
6. Authentication is mis-configured
7. RouterID is missing or duplicated

Question 12

What are the Seven [7] Transitioning via OSPF States

Answer 12

1. Down – no Hellos received
2. Init – Hello’s received, but not with my RouterID
3. 2-Way → Adjacency ← DR Elections happen
4. Exstart – Master/Slave relationship established
5. Exchange – LSDBDs exchanged
6. Loading – LSUs exchanged
7. Full – convergence

Question 13

Command to configure MD5 on a per-interface basis

Answer 13

ip ospf message-digest-key md5

ip ospf authentication message-digest

Question 14

Describe the down state

Answer 14

No Hellos received

Question 15

Describe the init state

Answer 15

Hello received but not with my RouterID

Question 16

Describe the 2-way state

Answer 16

DR Elections take place

Question 17

Describe the Exstart state

Answer 17

Master/slave relationship established

Question 18

Describe the Exchange state

Answer 18

LS DBD exchanged

Question 19

Describe the Loading state

Answer 19

LSU exchanged

Question 20

Describe the Full state

Answer 20

Fully converged

Question 21

Command to show adjacencies

Answer 21

show ip ospf neighbour

Question 22

Command to show process ID and router ID

Answer 22

show ip ospf

Question 23

Command to show OSPF learned routes

Answer 23

show ip route ospf

Question 24

Command to show process ID, timers, DR/BDR info

Answer 24

Show ip ospf interface

Question 25

Command to rest adjacency

Answer 25

clear ip ospf process