OSPF LSAs and LSDB flooding Tutorial


(Rene Molenaar) #1

This topic is to discuss the following lesson:


OSPF LSA sequence numbers
(system) #2

May I modify the “Sequence no different” to “Is sequence different” ?
Again, may I modify the “Sequence no higher” to “Is sequence higher” ?

Dennis


(Rene Molenaar) #3

Hi Dennis,

Sure, but which exact sentences do you mean? :slight_smile:

Rene


(system) #4

Good tutorial, sir. Cleared my concept. All the best to u!.


(system) #5

Hi, Actually Link ID is not RID. It is a bug in Cisco IOS and Link ID actually shows the Link State ID which is dependent on the type of LSA. Like for example in your output the Network LSA shows Link ID as 192.168.12.2 which is the DR interface IP and the RID is 2.2.2.2


(Rene Molenaar) #6

Hi Rizwan,

Thanks for sharing this!

Rene


(system) #7

Hi Rene

Thank you so much for sharing this. It helps to clear up so many questions I have. Great website too!!!


(Rene Molenaar) #8

You are welcome Toqa!


(Michael M) #9

Very awesome flow chart. Question. i thought the LSAck happened first then added to the LSDB. Can you confirm?


(Rene Molenaar) #10

Hi Michael,

I’m 99% sure that the LSack happens after adding it to the LSDB but if you want to know for sure, I’d have to debug it.

Rene


(Michael M) #11

Rene…I trust you :slight_smile:
But i can also verify. I’ll fire up GNS3.

I really appreciate your efforts in creating and maintaining this site. Going for my written it is great to reference this site.


(Rene Molenaar) #12

Glad to hear you like it :slight_smile: Let me know what you see, if you still have doubts let me know and i’ll take a look.


(Crytsuone N) #13

I think, he suggest you to change the word on the map for clearer understanding.


(adil k) #14

HI Rene,

Hope your well?..Wanted to ask you a question what is the difference between those LSA’s which expire and renew every 30minutes (1800 seconds) compared to MaxAge Lsa which i think is 1 hours expiration (3600 seconds).

If lsa renew after 30 minutes how does it get to the max age stage and what is the maxage used for? thanks


(adil k) #15

As we have some lsa’s within out network which have exceeded the 1800seconds mark and also seeing periodically maxAge Lsa’s.


(Rene Molenaar) #16

Hi Adil,

I’m doing great, hope you as well?

The max age timer is to set a limit to the LSAs. Once you reach those 3600 seconds, the LSA will be flushed from the LSDB.

To prevent this from happening, the LSA has to be refreshed. The creator of the LSA has to reflood it every 1800 seconds.

On Cisco IOS, the implemented the reflooding of LSAs a bit differently. Instead of refreshing one LSA exactly at 30 minutes, the router will check for LSAs that have about the same age and will group them together. Instead of reflooding a single LSA, it will flood multiple LSAs that have about the same age. This is called “LSA group pacing”.

Rene


(adil k) #17

I’m well to, thank you.

Sorry but how does it get to MaxAge lsa when they are primarily re flooded every 1800 seconds?

thanks


(Rene Molenaar) #18

Hi Adil,

Normally it doesn’t, only when the router that is responsible for the LSA doesn’t reflood it then it will hit the max age timer. When this happens, something is wrong with the router that originated the LSA.

Rene


(adil k) #19

thanks again for this clarification much appreciated :slight_smile:

need to look into this as we are getting a lot of MaxAge Lsa approx ever hour or more.


(husam s) #20

Rene ,
thanks for this article you need to add video for it .