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Configuring OSPF Timers

OSPF routers constantly track the status of their neighbors, sending and receiving hello packets that indicate whether the neighbor still is functioning, and sending and receiving link-state advertisement and acknowledgment packets. OSPF sends packets and expects to receive packets at specified intervals.

You can perform the following tasks when modifying the OSPF timers:

Modifying the Hello Interval

Routers send hello packets at a fixed interval on all interfaces, including virtual links, to establish and maintain neighbor relationships. This interval, which must be the same on all routers on a shared network, is advertised in the hello interval field in the hello packet. By default, the router sends hello packets every 10 seconds.

To modify how often the router sends hello packets out of an interface, include the hello-interval statement:

hello-interval seconds;

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

On nonbroadcast networks, the router sends hello packets every 120 seconds until active neighbors are detected by default. This interval is long enough to minimize the bandwidth required on slow WAN links. To modify this interval, include the poll-interval statement:

poll-interval seconds;

Note: The poll-interval statement is valid for OSPFv2 only.

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

Once the router detects an active neighbor, the hello packet interval changes from the time specified in the poll-interval statement to the time specified in the hello-interval statement.

Controlling the LSA Retransmission Interval

When a router sends link-state advertisements to its neighbors, the router expects to receive an acknowledgment packet from the neighbor within a certain amount of time. If the router does not receive an acknowledgment, it retransmits the advertisement.

Note: You must configure LSA retransmit intervals to be equal or greater than 3 seconds to avoid triggering a retransmit trap because the JUNOS software delays LSA acknowledgments by up to 2 seconds.

By default, the router waits 5 seconds for an acknowledgment before retransmitting the link-state advertisement. To modify this interval, include the retransmit-interval statement:

retransmit-interval seconds;

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

Modifying the Router Dead Interval

If a router does not receive a hello packet from a neighbor within a fixed amount of time, the router modifies its topological database to indicate that the neighbor is nonoperational. The time that the router waits is called the router dead interval. By default, this interval is 40 seconds (four times the default hello interval).

To modify the router dead interval, include the dead-interval statement. This interval must be the same for all routers on a shared network.

dead-interval seconds;

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

Specifying the Transit Delay

Before a link-state update packet is propagated out of an interface, the router must increase the age of the packet. If you have a very slow link (for example, one with an average propagation delay of multiple seconds), the age of the packet must be increased by a similar amount. Doing this ensures that you do not receive a packet back that is younger than the original copy.

The default transit delay is 1 second. You should never have to modify the default value. However, if you need to specify the approximate transit delay to use to age update packets, include the transit-delay statement:

transit-delay seconds;

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.


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