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Adjacencies
Adjacent routers go beyond the simple Hello exchange and participate in the link-state database exchange process.OSPF elects one router as the Designated
Router (DR) and a second r
outer as the Backup Designated Router (BDR) on each multi-access segment (the BDR is a backup in case of a DR failure).All other
routers on the segment will then contact the DR for link-state database updates and exchanges.This limits the bandwidth required for link-state database
updates.
Designated Router Election
The election of the DR and BDR is accomplished using the Hello protocol.The router with the highest OSPF priority on a given multi-access segment will
become the DR for that segment.In case of a tie, the router with the highest Router ID wins.The default OSPF priority is 1.A priority of zero indicates a router
that cannot be elected as the DR.
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
vir
tual 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 P
ack
et F
ormats
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 pack
et header
• Hello packet
• Database Description packet
• Link-State Request packet
• Link-State Update packet
• Link-State Acknowledgment packet
OSPF P
ack
et 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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Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch