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430 CHAPTER 29: CONFIGURING OSPF
Specifying the Hello
Interval
The Hello packet is periodically sent to the neighboring router to find and maintain
OSPF neighbor relationship, and to elect the DR and BDR in the NBMA and
broadcast networks. When one router is started, it only sends hello packets to the
neighbors whose precedences are larger than 0, that is, the routers can possibly be
elected as DR or BDR. You can configure the interval for sending hello packets. If
the interval is too short, the network change can be easily found but the network
load will be greatly increased. An appropriate value must be selected for the
specific network conditions.
On NBMA and point-to-multipoint networks, the poll-interval attribute should be
configured to specify the period of sending polling hello packet before this
interface sets up a neighbor relationdship with the adjacent routers.
The poll-interval attribute must be at least 3 times the value for hello-timer.
Perform the following configurations in interface view.
Table 494 Specify Hello Intervall
By default, the hello-timer on the p2p interface is 10 seconds and the hello timer
on the
p2mp amd nbma interfaces on the same network segment must be
identical.
Specifying the Dead
Interval
The expiration time of a neighboring router means that if a hello packet of the
neighbor router (peer) is not received within a certain period, the neighbor router
is invalid. You can specify the dead-timer, the period where the peer route fails.
The value of the dead-timer must be at least 4 times the value of the hello-timer.
Perform the following configurations in interface view.
Table 495 Specify Dead Interval
By default, the dead-timer on the p2p interface is 40 seconds and on p2mp and
nbma
interface the dead-timer is 120 seconds, ranging from 1 to 65535 seconds.
Note that:
■ The dead-timer of the router on the same network segment must be the same.
■ When you modify the network type, the hello-timer and dead-timer are both
restored to their default values.
Operation Command
Set the time interval for the interface to
send hello packets
ospf timer hello seconds
Return to the default hello interval time undo ospf timer hello
Specify the length of poll-interval on
NBMA and point-to-multipoint network
type
ospf timer poll seconds
Return to the default poll interval time undo ospf timer poll
Operation Command
Specify the expiration duration of the
OSPF neighbor
ospf timer dead seconds
Return to the default value of dead
interval
undo ospf timer dead