ospf timers

Answered Question
Jan 16th, 2008
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router1>sh ip route 10.138.122.0

Routing entry for 10.138.0.0/16

Known via "ospf 65040", distance 110, metric 302, type inter area

Last update from 10.132.253.22 on GigabitEthernet0/1, 07:51:38 ago

Routing Descriptor Blocks:

* 10.132.253.22, from 10.132.246.8, 07:51:38 ago, via GigabitEthernet0/1

Route metric is 302, traffic share count is 1




hi everybody,


i want know the timing mentioned in ip route (07:51:38) refreshes very often,

sometimes it start from 0:00:00 and be there for 15 seconds sometime for a minute


what does that mean

topology changes(LSA'S)

hello updates

?.....




Rgds

shiva


Correct Answer by mohammedmahmoud about 9 years 4 months ago

Hi Arav,


The Reason output can be interpreted as follows:


Record of reasons causing SPF to be executed:


•N A change in a network LSA (type 2) has occurred.

•R A change in a router LSA (type 1) has occurred.

•SA A change in a Summary autonomous system boundary router (ASBR) (SA) LSA has occurred.

•SN A change in a Summary Network (SN) LSA has occurred.

•X A change in an External Type-7 (X7) LSA has occurred.


Rick, after labing this around i consolidated that interArea routes causes SPF execution (interarea and external routes result in partial SPF calculations) and in the output of the show ip ospf statistics it is tagged as SN:


PE-4#sh ip ospf statistics


OSPF Router with ID (150.1.4.4) (Process ID 1)


Area 0: SPF algorithm executed 6 times


Summary OSPF SPF statistic


SPF calculation time

Delta T Intra D-Intra Summ D-Summ Ext D-Ext Total Reason

23:48:04 0 0 0 0 0 0 0 R,

23:47:54 0 0 0 0 0 0 0 R,

23:42:27 0 0 0 0 0 0 0 R, N,

02:36:09 0 0 0 0 0 0 0 R,

01:51:29 0 0 0 0 0 0 0 R,

00:00:18 0 0 0 0 0 0 0 R, SN,



BR,

Mohammed Mahmoud.

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mohammedmahmoud Wed, 01/16/2008 - 02:57
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Hi Shiva,


This timer specifies the last time the route was updated (from hours:minutes:seconds).


By design, OSPF requires LSAs to be refreshed as they expire after 3600 seconds. You can check out "ip ospf flood-reduction" - Suppresses the unnecessary flooding of LSAs in stable topologies - In other words suppresses the LSA refresh (DNA LSA) - Only LSAs exchanged are in case of topology change.


[edit] Sorry i drifted with the explanation, Kevin's reply is more direct.


BR,

Mohammed Mahmoud.

Kevin Dorrell Wed, 01/16/2008 - 02:58
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Shiva,


07:51:38 means the route has been stable for 7 hours 51 minutes and 38 seconds. The timing starts on topology changes. If you are seeing values very much smaller, then you probably have routes flapping.


Kevin Dorrell

Luxembourg


shiva_ial Wed, 01/16/2008 - 03:26
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hi kevin,

thanks for your reply

i have seen timer refreshing every 15 seconds

to 30 seconds(means when timer reaches 15 seconds goes back to 0 and again starts)so is it ok on normal scenario or it shouldn't be


pls clarify terminology related in detail, because i am seeing timer refreshing every 15 seconds which means there could be topology changes occuring causing the timer to restart

will it include hello update,I could not find any topology changes in my network every 15 seconds





Richard Burts Wed, 01/16/2008 - 03:47
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shiva


There seems to be some confusion about the OSPF LSAs which are sent on a periodic basis to ensure the consistency of the OSPF data base and the LSAs that are sent when a link changes state. Neither the periodic LSAs to ensure consistency nor the Hellos cause a calculation of the SPF while the LSAs for a link state change do cause a calculation of the SPF. I believe that what you are seeing indicates that the route for 10.138.0.0/16 was involved in an SPF calculation 7 hours ago because the link changed state.


You can check on this by looking at show commands to see how many times the SPF calculation has occurred and you might consider running the debug for ospf spf to see it running.


[edit] having looked more closely at the original post I see that the route is an inter-area summary route. An update of this would not cause an SPF calculation on your router but probably did cause an SPF calculation on the router where the route originated.


A slightly different suggestion about how to check on this would be to run debug ip rout and look at how often your routing table is being updated.


HTH


Rick

mohammedmahmoud Wed, 01/16/2008 - 04:14
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Hi Rick,


Always a fortune having you around :), totally agree with you about the SPF recalculation, Type 3 LSA should only cause SPF recalculations in its original area, this means that most probably the area that this route belongs to is suffering topology changes, we can check this via the "show ip ospf statistics".


BR,

Mohammed Mahmoud.

aravindhs Wed, 01/16/2008 - 04:53
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Rick, as usual, real nice observation about the inter-area summary route and hence the non-requirement of an SPF calculation on this router.


Mohammed, a very useful command that shows the ospf statistics. However, how do we read the 'Reason' column in the command output ?


Cheers

Arav

Correct Answer
mohammedmahmoud Wed, 01/16/2008 - 05:47
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Hi Arav,


The Reason output can be interpreted as follows:


Record of reasons causing SPF to be executed:


•N A change in a network LSA (type 2) has occurred.

•R A change in a router LSA (type 1) has occurred.

•SA A change in a Summary autonomous system boundary router (ASBR) (SA) LSA has occurred.

•SN A change in a Summary Network (SN) LSA has occurred.

•X A change in an External Type-7 (X7) LSA has occurred.


Rick, after labing this around i consolidated that interArea routes causes SPF execution (interarea and external routes result in partial SPF calculations) and in the output of the show ip ospf statistics it is tagged as SN:


PE-4#sh ip ospf statistics


OSPF Router with ID (150.1.4.4) (Process ID 1)


Area 0: SPF algorithm executed 6 times


Summary OSPF SPF statistic


SPF calculation time

Delta T Intra D-Intra Summ D-Summ Ext D-Ext Total Reason

23:48:04 0 0 0 0 0 0 0 R,

23:47:54 0 0 0 0 0 0 0 R,

23:42:27 0 0 0 0 0 0 0 R, N,

02:36:09 0 0 0 0 0 0 0 R,

01:51:29 0 0 0 0 0 0 0 R,

00:00:18 0 0 0 0 0 0 0 R, SN,



BR,

Mohammed Mahmoud.

marikakis Wed, 01/16/2008 - 06:05
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From your addition placed in parentheses, it seems that you agree with my earlier post :-)

mohammedmahmoud Wed, 01/16/2008 - 06:15
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Maria,


I edited my post before seeing your post :) i think this makes me an honest believer of your opinion :)


BR,

Mohammed Mahmoud.

marikakis Wed, 01/16/2008 - 06:18
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Ok, Mohammed, that's fine and I am glad we have a genuine agreement, because I really like the people in this forum and wouldn't like it to be any other way :-)


Edit: By the way, the output you posted is very useful for future reference.

mohammedmahmoud Wed, 01/16/2008 - 06:34
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Maria, thank you very much, i wouldn't dare to ask about the other way :) but rest assured that around here we only have one way.

aravindhs Wed, 01/16/2008 - 07:10
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Hi Mohammed, That's v helpful. You get a 5! I couldn't make out what SA was. But now I can, from your post.


And I reckon D-Intr and D-Summ Ext7 & D-Ext7 from the command output 'show ip ospf stat detail' just mean Database-IntraArea and Database-Summary routes for NSSA external routes respectively. Correct me if I am wrong. And your exercise about the partial SPF sounds reasonable.


cheers

Arav

shiva_ial Wed, 01/16/2008 - 20:44
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hi everyone,

(especially Richard burts & Mohammed)i have to thank you,since the answer from you to the posts were very detail and easy understanding.


everyone in this forum gives a wonderful solutions making this forum """"Rocking"""""


rgds

shiva



Richard Burts Wed, 01/16/2008 - 21:07
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shiva


Thank you for the compliment. It is gratifying to know that my posts have been helpful.


The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.


HTH


Rick

shiva_ial Wed, 01/16/2008 - 21:19
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sure rick,

i started learning things from

this forum,lot of questions unanswered were

answered in this forum makes me interesting.

I learnt new things from the post uncovered in books and probelms which could happen on realtime.


I request all the experts participating in this forum to keep up their good work on sharing their knowledge and experiences in this forum.


thanks a lot.


rgds

shiva


marikakis Wed, 01/16/2008 - 05:45
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Hello,


I am under the impression that inter-area and external information are involved in partial SPF runs. I mean, this type of information are seen as leaves in the SPF tree anyway, so no full SPF run is required, but they must be somehow glued to the body of the SPF tree.


Kind Regards,

M.

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