Routing/Switching 1.2 OSPF Packets

Which of the following is used with the Hello Packet to uniquely identify the originating router? - Hello Interval - Router ID - Designated Router ID - Network Mask - Dead Interval

- Router ID

Which of the following OSPF packets is responsible for establishing and maintaining adjacency with other OSPF routers? - Type 1: Hello packet - Type 2: DBD packet - Type 3: LSR packet - Type 4: LSU packet - Type 5: LSAck packet

- Type 1: Hello packet

Which of the following OSPF packets contains an abbreviated list of the LSDB of the sending router? - Type 1: Hello packet - Type 2: DBD packet - Type 3: LSR packet - Type 4: LSU packet - Type 5: LSAck packet

- Type 2: DBD packet

Which of the following OSPF packets is used by routers to request more information? - Type 1: Hello packet - Type 2: DBD packet - Type 3: LSR packet - Type 4: LSU packet - Type 5: LSAck packet

- Type 3: LSR packet

Which of the following OSPF packets is used by routers to announce new information? - Type 1: Hello packet - Type 2: DBD packet - Type 3: LSR packet - Type 4: LSU packet - Type 5: LSAck packet

- Type 4: LSU packet

Which of the following OSPF packets is used to confirm receipt of an LSA? - Type 1: Hello packet - Type 2: DBD packet - Type 3: LSR packet - Type 4: LSU packet - Type 5: LSAck packet

- Type 5: LSAck packet

OSPF uses the following link-state packets (LSPs) to establish and maintain neighbor adjacencies and exchange routing updates:

1) Type 1 Hello packet 2) Type 2 Database Description (DBD) packet 3) Type 3 Link-State Request (LSR) packet 4) Type 4 Link-State Update (LSU) packet 5) Type 5 Link-State Acknowledgment (LSAck) packet

LSU vs LSA

LSUs are also used to forward OSPF routing updates, such as link changes. Specifically, an LSU packet can contain 11 different types of OSPFv2 LSAs, with some of the more common ones shown in the figure. OSPFv3 renamed several of these LSAs and also contains two additional LSAs. The difference between the LSU and LSA terms can sometimes be confusing because these terms are often used interchangeably. However, an LSU contains one or more LSAs.

Link-state packets (LSPs)

Link-state packets are the tools used by OSPF to help determine the fastest available route for a packet.

Link-State Request (LSR) packet

Receiving routers can then request more information about any entry in the DBD by sending an LSR. Requests specific link-state records from router to router. A Type 3 LSR packet is used by the receiving routers to request more information about an entry in the DBD.

Hello packet

The OSPF Type 1 packet is the Hello packet. Hello packets are used to do the following: - Discover OSPF neighbors and establish neighbor adjacencies. - Advertise parameters on which two routers must agree to become neighbors. - Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet. Point-to-point links do not require DR or BDR.

Database Description (DBD) packet

This contains an abbreviated list of the LSDB of the sending router and is used by receiving routers to check against the local LSDB. The LSDB must be identical on all link-state routers within an area to construct an accurate SPF tree. Checks for database synchronization between routers. Routers initially exchange Type 2 DBD packets, which is an abbreviated list of the LSDB of the sending router. It is used by receiving routers to check against the local LSDB.

Link-State Update (LSU) packet

This is used to reply to LSRs and to announce new information. LSUs contain several different types of LSAs. Sends specifically requested link-state records. The Type 4 LSU packet is used to reply to an LSR packet.

Link-State Acknowledgment (LSAck) packet

When an LSU is received, the router sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty. A Type 5 packet is used to acknowledge the receipt of a Type 4 LSU.