Simple OSPF question

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Aug 30th, 2007
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Hi all,

Hope some one can help? This morning I have migrated all of my routers from RIP to OSPF. If I do a 'show ip route' the OSPF routes are listed, and if I do 'show ip protocol' I get the servers listed in Routing Information Services.

My question is this, all of the Last Update times are now well over an hour, is this normal? I can't remember if OSPF updates every 30mins or only if a change happens? A packet sniffer shows lots of OSPF hello packets but no acknowledgments...

Thanks for your help.



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rajinikanth Thu, 08/30/2007 - 02:27
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ospf updates its LSDB at interval of every 30 minutes if a change has not occurred within that 30 minutes.

If not it updates whenever there is a change.



Pavel Bykov Thu, 08/30/2007 - 03:34
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1. Last Update is the time of last change of link state, not when the router received an timed update packet (as per your mentioned 30 minutes)

2. It's not that simple with Acks. OSPF sends helloes which are acked back by helloes. LS-Ack is sent when router acknowledges receipt of LSU, but it doesn't have to be like that. It can send an LSU back with received routes, and that will count as acknowledgement too.

3. OSPF sessions are maintained by HELLO packets. If there would be no communication for DEAD INTERVAL, the OSPF relation will go down.

gctnet2006 Thu, 08/30/2007 - 03:41
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Thanks very much for your response, I was worried that I may loose my routes at some point, but it sounds like everything is set up ok.



purohit_810 Thu, 08/30/2007 - 03:59
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Hello packet has no ack...

Only in reply of tern it gives Hello packet each other.

Second.. it's normal.



Harold Ritter Thu, 08/30/2007 - 05:01
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The LSA refresh that happens at ~30 minutes interval will not cause the "Last Update" time to be reset. Only an SPF recalculation will do that. The "Last Update" time should roughly correspond to the last event in the "show ip ospf statistics" output. So if your network is stable, it is normal for the time since last update to be high.

Hope this helps,


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