Timers used in OSPF

In any IGP there are a lot of timers involved in running the protocol. This post will not
describe the hello and dead timers, these are well known. This post describes the less known
timers that control how often packets are sent and how often SPF is run. Modifying timers is
not recommended unless you have a very good reason to do so.

Router(config-router)# timers pacing flood 50

Every interface running OSPF has a flood-list with the LSAs that need to be sent out that
interface. This command configures how long to wait for additional LSAs and pack them into
a single update packet. This saves resources like CPU usage but setting it too high will lead
to slower convergence.

Router(config-router)# timers pacing retransmission 100

Controls how long to wait to send LSAs that are unacknowledged using the same grouping
principle.

Router(config-router)# timers pacing lsa-group 75

Every LSA has a maxage, the default is 1 hour and LSAs are usually refreshed at half their
maxage which is 30 minutes. Earlier versions of IOS would refresh all LSAs at once regardless
of age which led to CPU spikes. Later an individual timer was implemented which led to not all
LSAs expiring at once but every LSA was sent individually. The group pacing feature looks at
LSAs that are expiring at the near same time and group these together. LSAs that
expire within 75 seconds will be grouped together, the default value is 240 seconds.

Next we will look at throttling the SPF algorithm.

timers throttle spf spf-start spf-hold spf-max-wait

This timer throttles the SPF algorithm, if there is a flapping link or something causing a lot
of LSAs being sent and requiring SPF to run this can affect performance of CPU. When the first
LSA arrives the timer will run SPF after spf-start msecs. If there is another event within spf-
hold the timer will be doubled. If Another event occurs inside this hold-time the timer is once
again doubled, it is an exponential timer. Spf-max-wait makes sure there is a roof so that the
timer is not set to high. If there are no events for 2 times the max-wait the timer will revert
back to spf-start.

Also the sending and receiving of LSAs can be throttled.

timers throttle lsa all start-interval hold-interval max-interval

Defines how long to wait before sending LSAs. By default the first LSA is sent immediately and
the timer is set to hold-interval. If another LSA needs to be generated it will be generated at
hold-interval and the hold-interval will doubled. When the topology is stable after 2 times the
max-interval the timer will revert back to start-interval. If there are several events during an
interval they will be grouped and sent at hold-interval or max-interval depending on how
many events that have occured.

timers lsa arrival milliseconds

How long to wait before accepting the same LSA. If it is received faster than this timer the LSA
will be dropped. This timer should be set to less than or equal to the hold-interval of the
timers throttle lsa all command.

ip ospf transmit-delay seconds

Interface command. Estimate of time needed to send a link-state update over
a link. Should only matter on really low bandwidth links. LSAs that are sent will
have their age incremented by the amount this command specifies before
being sent.

ip ospf retransmit-interval seconds

How long to wait before transmitting unacknowledged LSAs.