3Com 4007, Руководство по внедрению, Страница: 471 / 684

OSPF Interfaces 471
Delay The transmit delay is the estimated time (in seconds) that it takes for the
system to transmit a link state update packet on the interface. The system
increases the age of the link state advertisements (LSAs) that are
contained in the update packets by the value that you specify for the
delay.
This delay setting has more significance for interfaces that are connected
to very low speed links because, on slower speed links, it is more
probable that the router may send out back-to-back data packets more
quickly than other routers and hosts can receive them. To avoid this
situation, set the transmit delay to configure the router to wait a specified
number or seconds between transmissions.
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.
Hello Interval 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.