Hello Interval; Retransmit Interval - 3Com corebuilder 3500 Implementation Manual

Hello Interval

Retransmit Interval

The delay value that you specify for an interface also increases the age of
all LSAs that are transmitted over the interface by the same value. This
setting may also affect how soon the LSA is flushed from an area router's
database. Reasons that an LSA is flushed from a router's link state
database include:
LSA is overwritten by a newer instance of the LSA — For
example, when a router receives similar LSAs (LSAs that have identical
sequence and checksums), it then compares the ages of each LSA,
and stores the LSA that has the least age value in the LSA database.
This LSA is then used for routing table calculations.
LSA ages out — When an LSA reaches the maximum age allowed by
the system, the router first refloods the LSA onto the network. When
it is no longer needed to ensure database synchronization (for
example, when the LSA is no longer contained in neighbor LSAs), it is
then flushed from the database.
The Hello interval (in seconds) determines how often the interface
transmits Hello packets to other routers. The hello interval value must be
identical among all routers that are attached to a common network. Hello
packets notify other routers that the sending router is still active on the
network. If a router does not send Hello packets for the period of time
that is specified by the dead interval, that router is considered inactive by
its neighbors, and all participating OSPF routers within the affected areas
converge on the new topology. Therefore, the smaller the Hello interval,
the faster that topological changes are discovered; as a result, however,
more routing traffic occurs. The default value for the Hello interval is
10 seconds.
When a router sends a link state advertisement to its neighbor, it keeps a
copy of the LSA until the neighbor acknowledges receipt of the LSA with
a link state acknowledgment packet. If the sending router does not
receive a link state acknowledgment from its neighbor, it then retransmits
the LSA. The retransmit interval (in seconds) determines how long the
sending router waits for an acknowledgement before retransmitting the
LSA to its neighbors.
To prevent needless retransmissions, the value that you specify must be
greater than the roundtrip delay between any two routers on the
attached network.
OSPF Interfaces 383