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Module 1: Single-Area OSPFv2 Concepts, image, image - Coggle Diagram
Module 1: Single-Area OSPFv2 Concepts
1.1 OSPF Features and Characteristics
1.1.1 Introduction to OSPF
1.1.2 Components of OSPF
1.1.3 Link-State Operation
1.1.4 Single-Area and Multiarea OSPF
1.1.5 Multiarea OSPF
1.1.6 OSPFv3
1.1.7 Check Your Understanding - OSPF Features and Characteristics
1.2 OSPF Packets
1.2.2 Types of OSPF Packets
Type 1: Hello packet
Type 2: Database Description (DBD) packet
Type 3: Link-State Request (LSR) packet
Type 4: Link-State Update (LSU) packet
Type 5: Link-State Acknowledgment (LSAck) packet
1.2.4 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.
1.2.3 Link-State Updates
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.
A Type 3 LSR packet is used by the receiving routers to request more information about an entry in the DBD.
The Type 4 LSU packet is used to reply to an LSR packet.
A Type 5 packet is used to acknowledge the receipt of a Type 4 LSU.
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.
1.3 OSPF Operation
Operational States (1.3.2)
Form neighbor adjacencies
Exchange routing information
Calculate best routes
Reach convergence
LSA Flooding con DR (1.3.6)
Without DR: chaos from multiple LSAs + ACKs.
With DR:
DR → central point for LSA distribution.
BDR → backup if DR fails.
Others = DROTHER.
Note: DR only manages LSAs; packet forwarding still follows best next-hop.
Neighbor Adjacency Establishment (1.3.3)
Hello Packets (multicast 224.0.0.5, unique Router ID)
State transitions:
Down → Init: sends Hello packets to discover neighbors.
Init → Two-Way: routers recognize each other’s IDs.
Two-Way → ExStart: on point-to-point links.
Two-Way → DR/BDR election: on multiaccess networks (Ethernet).
DR/BDR Election:
DR (Designated Router) and BDR (Backup) are elected in multiaccess networks.
They keep communication efficient.
Need for DR/BDR (1.3.5)
Problems in multiaccess networks:
Too many unnecessary adjacencies.
Excessive flooding of LSAs.
Adjacency formula: n(n-1)/2
Example: 5 routers = 10 adjacencies
20 routers = 190 adjacencies
OSPF Database Synchronizations (1.3.4)
Steps:
ExStart: decide which router sends first (highest Router ID).
Exchange: exchange DBDs (Database Description).
Loading: request and load missing LSAs (LSR/LSU).
Full: databases are synchronized.
Updates:
Incremental (upon changes).
Periodic (every 30 minutes).
