Hello Packet And Timer; Ospf Neighbor; Adjacency And Designated Routers - Zte ZXR10 5900E Series User Manual

Hello Packet and Timer

Hello protocol serves several purposes:
�
It is the means by which neighbors are discovered.
� It advertises several parameters on which two routers must
agree before they can become neighbors.
� Hello packets act as keepalives between neighbors.
�
It ensures bi-directional communication between neighbors.
� It elects designated routers and backup designated routers on
Broadcast and Nonbroadcast Multiaccess (NBMA) networks.
OSPF uses three types of timers related to the Hello Packet.
HelloInterval is an attribute of an interface. It defines the length
of time between Hello Packets that the router sends on the inter-
face. Default HelloInterval depends on the network type. On the
broadcast and point-to-point networks the default HelloInterval is
10 seconds. On NBMA and point-to-multipoint networks it is 30
seconds. The router's neighbor routers must agree on the Hel-
loInterval to enable them to become neighbors.
It is number of seconds before the router's neighbors will declare
it down. When they stop receiving the router's Hello Packets. The
default RouterDeadInterval is four times as long as the HelloInt-
erval, which applies to all the network types.
PollInterval is only used on the NBMA network.

OSPF Neighbor

OSPF neighbors are a group of routers on the same network. They
have some of the same configuration parameters. Routers must
first be neighbors before they can set up adjacency relationship.
Routers analyze the Hello Packets from each other when they be-
come neighbors to make sure the required parameters are stipu-
lated. Parameters include area ID, area flag, authentication infor-
mation, HelloInterval and RouterDeadInterval.

Adjacency and Designated Routers

Two routers when set up adjacency relationship. They can ex-
change routing information. Whether two routers can set up an
adjacency relationship depends on the type of the network con-
necting the routers.
There are only two routers in point-to-point network and virtual
links. Routers set up an adjacency relationship automatically. The
point-to-multipoint network can be regarded as a set of point-to-
point networks. Each pair of routers set up an adjacency relation-
ship automatically.
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Chapter 3 OSPF Configuration
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