OSPF Packet Types

Question 1

What are the different OSPF packet types? (5)

Answer 1

1. Type 1: 0x01 - Hello Packets
2. Type 2: 0x02 - Database Description (DD) Packets
3. Type 3: 0x03 - Link State Request (LSR) Packets
4. Type 4: 0x04 - Link State Update (LSU) Packets
5. Type 5: 0x05 - Link State Acknowledgement (LSA)

Question 2

What is Type 1: 0x01 - Hello Packet used for?

Answer 2

used as a first packet to be sent for establishing and maintaining adjacencies with neighboring OSPF devices

Question 3

What is the role of the Type 1: 0x01 - Hello Packet?

Answer 3

• ensure two-way communication exists
• contain attributes that neighbors must agree on to form adjacency
• serve as a failure detection mechanism

Question 4

What is the TTL value of Type 1: 0x01 - Hello Packet?

Answer 4

TTL value of Hello packet is set to 1

Question 5

What do Type 1: 0x01 - Hello Packets communicate? (9)

Answer 5

• network/subnet mask
• Hello interval
• options
• router priority
• router Dead Interval
• DR/BDR
• neighbor

Question 6

What do the Type 2: 0x02 - Database Description (DD) Packets carry and contain?

Answer 6

carry LSA headers and contain abbreviated link-state database

Question 7

What are the Type 2: 0x02 - Database Description (DD) Packets used for?

Answer 7

for synchronization of link-state databases between OSPF devices

Question 8

What do Type 2: 0x02 - Database Description (DD) Packets communicate?

Answer 8

• link’s MTU
• LSA Headers
• DBD Sequence Number
• DBD bits
• options

Question 9

Do LSA headers contain the entire LSA?

Answer 9

no, they contain just enough information for device that exchange these packets whether they need to send an LSR

Question 10

What are the Type 3: 0x03 - Link State Request (LSR) Packets used for?

Answer 10

to request missing information for LSAs seen during the DBD packet exchange; could be a reaction to a sender of DD packet

Question 11

What do the Type 3: 0x03 - Link State Request (LSR) Packets communicate?

Answer 11

• type of LSA being requested
• link-state ID of the LSA
• Router ID of the router responsible for originating the LSA

Question 12

What are the Type 4: 0x04 - Link State Update (LSU) Packets used for?

Answer 12

• distribute information about the state of OSPF routers within an OSPF area
• convey information about the state of OSPF routers, including their Router IDs and the state of their interfaces

Question 13

What do the Type 4: 0x04 - Link State Update (LSU) Packets contain? (4)

Answer 13

1. Router ID
2. Link-State Advertisement (LSA) Header
3. Link-State ID
4. Advertising Router

Question 14

What is the role of the Type 4 LSU packets when a router changes its state or when there are changes in the network topology, such as an OSPF router going up or down?

Answer 14

Type 4 LSU packets are used to update all OSPF routers in the area about these changes, therefore they play a crucial role in the synchronization process

Question 15

What are the Type 5: 0x05 - Link State Acknowledgement (LSAck) used for?

Answer 15

• confirmation that LSU transfer has been successful
• sent as multicast for broadcast networks

Question 16

What do the Type 5: 0x05 - Link State Acknowledgement (LSA) communicate?

Answer 16

only the LSA header information of the LSAs which are being ACKd

Question 17

What is the L3 destination IP address used by OSPF Hello Packets?

Answer 17

L3 224.0.0.5 multicast address on broadcast networks

Question 18

What is the address type of OSPF Hello Packets on a non-broadcast network?

Answer 18

unicast

Question 19

What is the L2 destination IP address used by OSPF Hello Packets?

Answer 19

either 01-00-5E-00-00-05 | 01-00-5E-00-00-06 multicast address

Question 20

What are the 3 basic functions provided by OSPF Hello packets?

Answer 20

1. ensure two-way communication exists
2. exchange attributes that neighbors must agree on to form adjacency
3. serve as a failure detection mechanism