Ospf Packet Formats - D-Link DES-3326SR Manual

Building Adjacency
Two routers undergo a multi-step process in building the adjacency relationship. The following is a simplified
description of the steps required:
• Down − No information has been received from any router on the segment.
• Attempt − On non-broadcast multi-access networks (such as Frame Relay or X.25), this state indicates
that no recent information has been received from the neighbor. An effort should be made to contact the
neighbor by sending Hello packets at the reduced rate set by the Poll Interval.
• Init − The interface has detected a Hello packet coming from a neighbor but bi-directional
communication has not yet been established.
• Two-way − Bi-directional communication with a neighbor has been established. The router has seen its
address in the Hello packets coming from a neighbor. At the end of this stage the DR and BDR election
would have been done. At the end of the Two-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 Any information that looks incomplete or outdated will be put on the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:
D-Link DES-3326SR Layer 3 Switch
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