Adjacencies On Point-To-Point Interfaces; Ospf Packet Formats; The Ospf Packet Header - D-Link DES-6500 User Manual

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
way stage, routers will decide whether to proceed in building an adjacency or not. The
decision is based on whether one of the routers is a DR or a BDR or the link is a point-
to-point or virtual link.
Exstart − (Exchange Start) Routers establish the initial sequence number that is going to
be used in the information exchange packets. The sequence number insures that
routers always get the most recent information. One router will become the primary
and the other will become secondary. The primary router will poll the secondary for
information.
Exchange − Routers will describe their entire link-state database by sending database
description packets.
Loading − The routers are finalizing the information exchange. Routers have link-state
request list and a link-state retransmission list. Any information that looks incomplete
or outdated will be put on the request list. Any update that is sent will be put on the
retransmission list until it gets acknowledged.
Full − The adjacency is now complete. The neighboring routers are fully adjacent.
Adjacent routers will have the same link-state database.

Adjacencies on Point-to-Point Interfaces

OSPF Routers that are linked using point-to-point interfaces (such as serial links) will always
form adjacencies. The concepts of DR and BDR are unnecessary.

OSPF Packet Formats

All OSPF packet types begin with a standard 24 byte header and there are five packet types.
The header is described first, and each packet type is described in a subsequent section.
All OSPF packets (except for Hello packets) forward link-state advertisements. Link-State
Update packets, for example, flood advertisements throughout the OSPF routing domain.
OSPF packet header
Hello packet
Database Description packet
Link-State Request packet
The Link-State Update packet
Link-State Acknowledgment packet

The OSPF Packet Header

Every OSPF packet is preceded by a common 24-byte header. This header contains the
information necessary for a receiving router to determine if the packet should be accepted for
further processing.
The format of the OSPP packet header is shown below:
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