OSPF LSA Throttling

This topic is to discuss the following lesson:

Nice work Rene, just wondering in terms of the default values, the lsa-hold value will never double as the lsa max value is also 5 seconds. So it can never wait longer than 5 seconds before generating a new LSA?

Hello Chris

Yes, this is the case for the default configuration. lsa-hold will not actually double unless you tweak the lsa-max value accordingly.

I hope this has been helpful!

Laz

Hello: Why is there a flood sat 35 and 80. I do not see any outstanding LSAs

Hello Ram

I’m not sure I understand your question. Can you please clarify? Thanks!

Laz

image
Why is there a flood sent at time 35? There has not been any LSA between 15 and 35.
Thanks

Hi Ram,

I understand the confusion, I’ll see if I’ll remove it. The third “flood” arrow is when normally a flood would occur, if an LSA was generated during the 2x LSA hold timer.

Rene

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hello rene, good job , i am just confuse about lsa arrival timers in command (timers lsa arriver) why i need to set it & how reciever router recieve more than one lsa during 10000, & I am alreay set timers lsa throttle 5000 10000 60000 at sender neighbor router which already postpond flooding operation till 10000 lsa hold times expire.
high appreciated your feedback

Hello Saif

Yes you are correct, that if you configure the first router in this particular topology with lsa throttle 5000 10000 60000 you will never have a case where you will receive the same LSA within 10000 milliseconds. However, this command is still useful especially if you are connecting your network with that of another entity using OSPF. You don’t have control over what the other side does, and frankly, you have no reason to trust them. So in order to protect your network, you can and should adjust the lsa arrival timer on your end to be safe.

I hope this has been helpful!

Laz

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Thanks appreciate your nice explanation

Hello,

If I understood correctly, it’s not exactly the generation of LSAs that is delayed but their flooding, right?

Also, some diagrams illustrate the flooding of LSAs, even though there was no LSA generation, during that interval. Is this correct?

Regards,
LP

Hello Luis

First we must determine what the terms “LSA generation” and “flooding” mean, and what the arrows in the diagrams mean.

Under normal circumstances, LSA generation and flooding occur at the same time. In actuality, they are one and the same thing. This means that whenever there is an event that “triggers” LSA generation, the LSA is generated and flooded. It is not possible to generate an LSA and hold on to it until the time comes to flood it. So generation and flooding are one and the same event.

In the diagrams, the red arrows marked “LSA” are the points in time where an LSA should have been generated and flooded, but is not. The purple “Flood” arrows are the times when the LSA is generated and flooded, as a result of the timers. There is no concept of creating an LSA and holding on to it until the time comes to flood it.

For this reason, whenever you see a “Flood” arrow, it means actual generation and flooding. Whenever you see an “LSA” arrow, it means “would have been generated and flooded” under normal circumstances.

I hope this has been helpful.

Laz

Hi Laz,

Thanks.

Regarding my second question, why is there a Flood arrow at 35 secs if there were no LSAs (red arrows) between 15 and 35 secs?

image

Thanks.

Hello Luis

Yes indeed you are correct, the Flood arrow should not be there since there was no event during the 2x lsa-hold duration that would require a generation and flooding of an LSA. So no purple arrow should appear. I’ve spoken to @ReneMolenaar about this and he too has confirmed this, so he will make the necessary modifications.

Thanks, and I hope this has been helpful!

Laz

Hi Guys,

Could you explain what LSA pacing and LSA retransmission for me please? I’m finding it hard to find information on this topic. What exactly is LSA pacing or group pacing?

What would these commands achieve:

 timers pacing flood
 timers pacing retransmission

 ip ospf transmit-delay
 ip ospf retransmit-interval

Thank you

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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