Background Of OSPF:
The initial development of OSPF began in 1987 by the Internet Engineering Task Force (IETF) OSPF Working Group. At that time the Internet was largely an academic and research network funded by the U.S. government.
In 1989, the specification for OSPFv1 was published in RFC 1131. There were two implementations written: one to run on routers and the other to run on UNIX workstations. The latter implementation later became a widespread UNIX process known as GATED. OSPFv1 was an experimental routing protocol and never deployed.
In 1991, OSPFv2 was introduced in RFC 1247 by John Moy. OSPFv2 offered significant technical improvements over OSPFv1. At the same time, ISO was working on a link-state routing protocol of their own, Intermediate System-to-Intermediate System (IS-IS). Not surprisingly, IETF chose OSPF as their recommended IGP (Interior Gateway Protocol).
In 1998, the OSPFv2 specification was updated in RFC 2328 and is the current RFC for OSPF.
Note: In 1999 OSPFv3 for IPv6 was published in RFC 2740. RFC 2740 was written by John Moy, Rob Coltun, and Dennis Ferguson. OSPFv3 is discussed in CCNP.
"OSPF Version 2," http://www.ietf.org/rfc/rfc2328
OSPF Message Encapsulation:
The data portion of an OSPF message is encapsulated in a packet. This data field can include one of five OSPF packet types. Each packet type is briefly discussed in the next topic.
The OSPF packet header is included with every OSPF packet, regardless of its type. The OSPF packet header and packet type-specific data are then encapsulated in an IP packet. In the IP packet header, the protocol field is set to 89 to indicate OSPF, and the destination address is set to one of two multicast addresses: or If the OSPF packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address: 01-00-5E-00-00-05 or 01-00-5E-00-00-06.
OSPF Packet Types:
In the previous chapter, we introduced Link-State Packets (LSPs). The figure shows the five different types of OSPF LSPs. Each packet serves a specific purpose in the OSPF routing process:
1. Hello - Hello packets are used to establish and maintain adjacency with other OSPF routers. The hello protocol is discussed in detail in the next topic.
2. DBD - The Database Description (DBD) packet contains an abbreviated list of the sending router's link-state database and is used by receiving routers to check against the local link-state database.
3. LSR - Receiving routers can then request more information about any entry in the DBD by sending a Link-State Request (LSR).
4. LSU - Link-State Update (LSU) packets are used to reply to LSRs as well as to announce new information. LSUs contain seven different types of Link-State Advertisements (LSAs). LSUs and LSAs are briefly discussed in a later topic.
5. LSAck - When an LSU is received, the router sends a Link-State Acknowledgement (LSAck) to confirm receipt of the LSU.
Hello Protocol:
OSPF packet Type 1 is the OSPF Hello packet. Hello packets are used to:
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 and Frame Relay.
Type: OSPF Packet Type: Hello (1), DD (2), LS Request (3), LS Update (4), LS ACK (5)
Router ID: ID of the originating router
Area ID: area from which the packet originated
Network Mask: Subnet mask associated with the sending interface
Hello Interval: number of seconds between the sending router's hellos
Router Priority: Used in DR/BDR election (discussed later)
Designated Router (DR): Router ID of the DR, if any
Backup Designated Router (BDR): Router ID of the BDR, if any
List of Neighbors: lists the OSPF Router ID of the neighboring router(s)


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