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OSPF creates adjacencies between neighboring routers for the purpose
of exchanging routing information. Not every two neighboring routers will
become adjacent. |
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The Hello Protocol is responsible for establishing and maintaining
neighbor relationships. It also ensures that communication between neighbors
is bidirectional. Hello packets are sent periodically out all router
interfaces. Bidirectional communication is indicated when the router sees
itself listed in the neighbor's Hello packet. On broadcast and
NBMA networks, the Hello Protocol elects a Designated
Router for the network. |
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On broadcast networks, each router advertises itself by periodically
multicasting Hello packets. This allows neighbors to be discovery
dynamically. On NBMA networks each router that may potentially become
Designated Router has a list of other routers attached to the
network. A router, having DR potential, sends Hello packets to
all other potential DR when its interface to the NBMA network
becomes operational. This is an attempt to find the DR for the
network. If the router itself is elected DR, it begins sending
Hello packets to all other routers attached to the network. On
Point-to-Multipoint (P2MP) networks, a router sends
Hello packets to all neighbors with which it can communicate directly.
These neighbors may be discovered through Inverse ARP or they may be
configured. |
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After a neighbor has been discovered, bidirectional communication ensured
and (on broadcast and NBMA networks) a DR elected, a decision is made
regarding whether or not an adjacency should be formed with the neighbor. If
an adjacency is formed, the first step is to synchronize the neighbor's
link-state databases (LS-databases).. |
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Synchronization of databases |
| In a link-state routing protocol, it is very important for all
router's LS-databases to stay synchronized. Each router describes its
database by sending a sequence of Database Description (DD)
packets to its neighbors. Each DD packet describes a set of LSAs
belonging to the router's database. When the neighbor sees an LSA
that is more recent that its own database copy, it makes a note that this
newer LSA should be requested. |
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This sending and receiving of DD packets is called the "Database
Exchange Process". During this process the two routers form a
master/slave relationship. Each DD packet has a sequence number.
DD packets sent by the master are acked by the slave
through echoing of the sequence number. Each DD packet with the
M-bit set, indicates that there are more packets to follow. The
Database Exchange Process is over when a router has received and sent a
DD packet with the M-bit off. |
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During and after the Database Exchange Process, each router has a
list of those LSAs for which the neighbor has more up-to-date
instances. These LSAs are requested in Link State Request (LSR)
packets. LSR packets that are not satisfied are retransmitted
at fixed intervals of time RxtmInterval. When the Database
Exchange Process has been completed and all LSRs have been
satisfied, the databases are deemed synchronized and the routers are
marked as fully adjacent. At this time the adjacency is
fully-functional and is advertised in the two router's router-LSAs. |
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