Understanding the network.pdf - Back to Home

o Hello—Messages are used to create adjacent neighbor
relationships. Depending upon the type of network,
different hello protocol rules are followed. Hello messages
are exchanged between routers that have become
adjacent. Once adjacency is established, routers will
exchange routing information with one another. In
situations where there is a designated (and backup)
router, hello messages will be exchanged between the
designated router and the other routers on the network.
o Database description—This message is sent in response
to a hello request. Database description messages are
used to exchange topological database information. The
topological or link state database is the result of the link
state (Dijkstra) algorithm calculation performed by the
router. The result of the calculation lists every possible
route to every network in the routing domain. The
database is used to identify possible routing loops,
redundant paths, and so on. Adjacent routers exchange
topological database information until both have the
same information. Once they are identical, the routers
will exchange link state requests and link state updates to
learn about networks they did not have knowledge of
before their adjacency had been established.
o Link state request—This message requests LSAs from
other OSPF neighbor routers.
o Link state update—This message responds to (and sends)
LSAs to neighbor router link state requests. Link state
announcements are what OSPF uses to send the actual
link state information. There are five link state
advertisements:
� Router links advertisements—These are sent by all
OSPF routers. They are used to describe the state
and cost of the connected interface within the given
area.
� Network links advertisements—Designated routers
on multi-access networks generate this message.
The messages list all routers connected to the
network.
� Summary links advertisements—ABRs send these
messages; they provide summary information for
networks inside and outside the OSPF network.
� Autonomous system external links
advertisements—These LSAs are sent by AS