KTI KGS-3120, Руководство по эксплуатации

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• 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 Inter-
val.
• Init - The interface has detected a Hello packet coming from a neighbor but bi-directional com-
munication 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 second-
ary. 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 Any information that looks incomplete or outdated will be put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
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: