In our previous post, we discussed how OSPF can be tuned to prevent frequent computation of the SPF in times of network instability by using OSPF throttling.

By using this scheduling technique, if a large number of LSAs are received, triggered by events such a link or OSPF neighbor down, the SFP rather than immediately start the SFP calculation for every received LSA it will wait for more LSAs to be added to the LSDB before triggering the SFP calculation.

This solution is suitable for this situation, but what about self-originated such the aged-out or corrupted (like the ones with invalid checksum) LSAs?

There is where LSA and SPF pacing enters to play.   Before going into detail, let me emphasize something:  Modifying the OSPF timers is not recommended unless you fully understand and know what you are doing.   The incorrect tuning or long timers of SPF pacing may lead to slow convergence.

Now that you were warned let’s talk about it.

Since IOS version 12.2, Cisco has introduced three types of packet-pacing delay timers to optimize the LSA flooding procedure.

• Group packet-pacing timer
• Flood packet-pacing timer
• Retransmission packet-pacing timer

OSPF Group packet-pacing Timer

In Cisco implementations of OSPF (before Cisco IOS 12.0), the aging of every LSA’s was individual, meaning that only LSAs close to 1800 seconds were re-flooded.   This process was better than the original “full” refresh done every 30 minutes, but still inefficient because it produces multiple quick floods due to independent aging during the interval.   The Group packet-pacing timer, instead of processing independent LSAs, group the ones ready to be refreshed and flood them all at once every 240 seconds by default.

To configure the OSPF group packet-pacing timer use the timers pacing lsa-group {seconds} OSPF process command.

OSPF Flood packet-pacing Timer

The flood pacing timer operates on a per-interface basis; it applies only if a router has more than a single LSA to be flooded out an individual interface.   The way it works is that if a router has a large number of LSAs to be flooded out an interface, the timer will prevent flooding the LSAs immediately one after the other one. Instead, it’s added to the flooding list. Then after 33 milliseconds (default packet-pacing timer), the LSAs included in the list are flooded.

To configure the OSPF flood packet-pacing timer use the timers pacing flood {milliseconds} OSPF process command.

OSPF Retransmission packet-pacing Timer

This timer works the same way as group packet-pacing timer but only takes care of the unacknowledged LSAs.   In other words, if an LSAck is not received, the LSA is placed in a flooding list instead flooding the LSA at once.   The default timer in Cisco implementations is 66 milliseconds.

To configure the OSPF flood packet-pacing timer use the timers pacing retransmission {milliseconds} OSPF process command.

It is time to close this post.  In the next one, we will discuss OSPF incremental SPF.

Thank you for visiting.