OSPF Neighbor Adjacencies

Question 1

When does an adjacency occur?

Answer 1

when connected neighbors use Hello packets to agree on unique and common attributes - the exchanged parameters in the Hello packets need to match

Question 2

What parameters must match in OSPF Hello to form an adjacency? (8)

Answer 2

1. Area ID
2. Hello interval
3. Dead interval
4. Interface MTU
5. Network Type
6. Authentication (if used)
7. Stub Flags
8. Subnet/mask

Question 3

What parameters in Hello packets do not need to match?

Answer 3

• OSPF router ID (has to be different on both routers)
• list of neighbors reachable on the interface
• router priority
• DR IP address
• Backup DR IP address

Question 4

What are the two different neighborship that can exist?

Answer 4

• 2-Way Neighbors
  
  • neighbors recognize each other and send Hello to each other
  • neighbors didn’t exchange LSAs with each other - no topology information was exchanged
• Fully Adjacent Neighbors
  
  • LSAs were exchanged, link state databases match

Question 5

What is usually the default value for the Hello packet interval?

Answer 5

10 seconds

Question 6

How is the Dead Interval calculated?

Answer 6

as 4 times of the Hello Interval

Question 7

What are the 8 different adjacency states in OSPF?

Answer 7

1. Down
2. Attempt
3. Init
4. 2-Way State
5. ExStart
6. Exchange
7. Loading
8. Full

Question 8

Describe the Down adjacency state

Answer 8

no Hellos have been received from neighbor

Question 9

Describe the Attempt adjacency state

Answer 9

unicast Hello packet has been sent to neighbor, but no Hello packet has been received back

Question 10

Describe the Init adjacency state

Answer 10

• an OSPF router has sent an OSPF Hello packet to its neighbor and is waiting for a response
• the router has detected a potential neighbor on the link but has not yet confirmed the neighborship

Question 11

Describe the 2-Way State adjacency state

Answer 11

• router received a hello packet from neighbor and the neighbor acknowledged hello packet
• router ID of the initializing router is sent back from the neighbor (acknowledges that two routers receive each other packets)

Question 12

Describe the ExStart adjacency state

Answer 12

• first step of actual adjacency
• master and slave relationship is formed where master has higher router-id
• master chooses the starting sequence number for the database descriptor packets that are used for actual LSA exchange

Question 13

Describe the Exchange adjacency state

Answer 13

• local link state database is sent through DBD packets
• DBD sequence number is used for reliable acknowledgement/retransmission

Question 14

Describe the Loading adjacency state

Answer 14

Link State Request packets are sent to ask for more information about a particular LSA

Question 15

Describe the Full adjacency state

Answer 15

neighbours are fully adjacent and databases synchronized