19216811.liveD-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
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Authentication
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OSPF allows for the configuration of a password for a specific area.
Two routers on the same segment and belonging to the same area must also have the
same OSPF password before they can become neighbors.
Hello and Dead Intervals
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The Hello interval specifies the length of time, in seconds,
between the hello packets that a router sends on an OSPF interface.
The dead interval
is the number of seconds that a router’s Hello packets have not been seen before its
neighbors declare the OSPF router down.
OSPF routers exchange Hello packets on
each segment in order to acknowledge each other’s existence on a segment and to elect
a Designated Router on multi-access segments.
OSPF requires these intervals to be
exactly the same between any two neighbors. If any of these intervals are different,
these routers will not become neighbors on a particular segment.
Stub Area Flag
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any two routers also have to have the same stub area flag in their Hello
packets in order to become neighbors.
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 router 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 be com 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 can not 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
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No information has been received from any router on the segment.
Attempt
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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
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The interface has detected a Hello packet coming from a neighbor but bi-directional
communication has not yet been established.
Two-way
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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-