SPF algorithm -- OSPF

Unanswered Question
Sep 15th, 2009
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Hi All,


We have the below output of


SW01#sh ip ospf


Router is not originating router-LSAs with maximum metric


SPF algorithm last executed 16:22:47.016 ago SPF algorithm executed 778 times


What does this mean?

What causes SPF to execute?


Regards,

Naidu.



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Giuseppe Larosa Wed, 09/16/2009 - 01:59
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Hello Naidu,

I hope you are fine

>> Router is not originating router-LSAs with maximum metric


this simply means that local node is originating its router LSA(s) (one for each area it is taking part) with normal OSPF costs.

There is a feature that allows a just started OSPF router to advertise its OSPF data structure with metric 65534 (infinite metric) at startup until BGP converges or some time (minutes).


you network is stable last time SPF was executed was 16 hours 22 min 47 secs ago.


SPF is executed when a topology change in the OSPF area happens like for example a link goes down, the node(s) that were connected to that link will send an updated version of their router LSA that is flooded in all the area.

Routers receiving the new LSA have to execute again the SPF for all known prefixes to adapt to network changes.


To be noted LSAs are refreshed every 30 minutes (on average) but this is a reason to recalculate.


Changes in other OSPF areas or in OSPF external routes can be handled with partial recalculation in more modern IOS images


Hope to help

Giuseppe


ilnaiduccna Wed, 09/16/2009 - 04:21
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Hi Giuseppe,


Thank you very much for your explaination.


OK, if the Router is not originating router-LSAs with maximum metric, what precautions I need to take, what commands need to excute. Please suggest me how to proceed.


Regards,

Naidu.

Richard Burts Wed, 09/16/2009 - 04:30
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Naidu


The OSPF maximum metric is an optional feature. Before you are concerned about how to implement it I believe you should consider whether you need this optional feature. The message that your router is not generating maximum metric LSAs is an informational observation about what your router is or is not doing. It is not necessarily an indication that you SHOULD generate maximum metric.


HTH


Rick

Giuseppe Larosa Wed, 09/16/2009 - 04:35
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Hello Naidu,

it is a normal condition to send out LSAs without maximum metric.

The feature is optional and it is needed only on scenarios with OSPF + BGP if for example you don't use BGP you don't need it


Hope to help

Giuseppe


ilnaiduccna Wed, 09/16/2009 - 06:12
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Hi Rick & Giuseppe,


Thank you very much for your responses.

Yes I am not using BGP only OSPF is there configured, but BGP routes redistributed here.


Experts, I have another Post in this same blog is "Network Freeze between HQ and DC" Can you please help me out in that with.


Regards,

Naidu.

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