OSPF LSA Throttling

Lessons Discussion
17

Hello Joseph

OSPF LSA group pacing feature instructs a router to group OSPF LSAs and pace the refreshing, checksumming, and ageing functions. What is meant by “pacing” is a more uniform distribution of these operations over time. The result is that LSA refreshing, checksumming, and ageing operations will be grouped in such a way that they will take place at more regular intervals over time. The result is a more efficient use of the router’s resources. LSA group pacing is enabled by default on modern Cisco IOS devices.

Before this feature, all OSPF LSAs were on a single timer. This meant that all OSPF LSAs refreshed at once. For routers containing several thousand LSAs, this resulted in spikes of CPU and memory usage. Even if LSAs had different ages, refreshing them on a single timer actually synchronized them, resulting in excessive resource usage at the time of synchronization.

Group pacing allows LSAs to each use their own independent timer, which allows the refreshing of the LSAs to be staggered, and spread out over time, increasing efficiency.

The commands that you shared in your post shouldn’t need to be used except in very rare circumstances. As stated, pacing is enabled by default and should be sufficient for most implementations. However, in the rare case where it is needed, you can tweak these timers to more appropriately space out packet retransmissions and inter-packet timers to achieve a more efficient usage of resources. You can find out more about these commands at the following CIsco documentation:

As for these commands:

…they are not associated with pacing. These commands are used to tweak the way in which LSA updates are sent. Unlike pacing, these commands refer only to the transmission and retransmission of each particular LSA, independent of the sending of other LSAs. Specifically:

  • The ip ospf transmit-delay command is used to set the estimated time needed to send an LSA update packet (including processing time, transmission time, and propagation delay). OSPF will increment the LSA age time by the transmit delay amount before transmitting the LSA update. This is done to ensure that the age time will take into account situations where excessively large link state updates are sent that may take several seconds or even tens of seconds to send.
  • The ip ospf retransmit-interval command is used to set the time between LSA retransmissions. When a router sends an LSA to its neighbor, it keeps the LSA until it receives an acknowledgment message from the neighbor. If the router receives no acknowledgment within the retransmit interval, the local router resends the LSA. This is used to ensure that you are allowing enough time for the acknowledgement to be sent in situations where there is an increase in delay.

I hope this has been helpful!

Laz

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