Avaya Configuring IP Services Manual de usuario Pagina 134

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Configuring IP Services
5-4 114065 Rev. A
The Hello protocol is responsible for ensuring that communication between
neighbors is bidirectional. Periodically, OSPF routers send out Hello packets over
all interfaces. Included in these Hello packets are
The router’s priority
The router’s Hello Timer and Dead Timer value
A list of routers that have sent this router Hello packets on this interface
The router’s choice for designated router and backup designated router
Bidirectional communication is determined when one router sees itself listed in
the neighbor’s Hello packet.
Neighbor Adjacencies
Neighbors may form an adjacency for the purpose of exchanging routing
information. When two routers form an adjacency, they go through a process
called database exchange to synchronize their topological databases. When their
databases are synchronized, the routers are said to be fully adjacent. From this
point on, only routing change information is passed between the adjacencies, thus
conserving bandwidth.
All routers connected by a point-to-point network or a virtual link will always
form an adjacency. Also, every router on a multiaccess network forms an
adjacency relationship with the designated router and the backup designated
router.
Designated Routers
To further reduce the amount of routing traffic, the Hello protocol elects a
designated router and a backup designated router on each multiaccess network.
Instead of neighboring routers forming adjacencies and swapping link-state
information with each other (which on a large network can mean a lot of routing
protocol traffic), all routers on the network form an adjacency with the designated
router and the backup designated router only and send link state information to
them. The designated router then redistributes the information from each router to
every other router.
The Hello protocol always elects a backup designated router along with the
designated router. Its responsibility is to take over all of the designated router’s
functions should the designated router fail.
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