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Home > Archive > CCNP > February 2002 > OSPF updates every 0 seconds
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OSPF updates every 0 seconds
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| Wavy Davy 2002-02-10, 5:23 am |
| Why is it with the OSPF protocol, that when a sh ip protocols command is issued it shows the updates as zero?
Routing Protocol is "ospf 1"
Sending updates every 0 seconds
I thought OSPF sends incremental updates as well as an update every 30 minutes.
Anyone care to enlighten me?
Cheers!!
Wavy | |
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|  OSPF will send an incremental update as a result of a network change - triggered update. 30 minutes later OSPF will send out a compressed verion of its routing table. The value of 0 seconds for sending updates in the 'show ip protocols' command indicates that OSPF will never send out regular updates, unlike RIP v1, which does so every 30 seconds.
Cheers
Hippo
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| Wavy Davy 2002-02-11, 10:04 am |
| Thanks Hippo for the explanation, I just thought that the 30 minute "update" would be shown with the #sh ip protocols command.
Cheers,
Wavy | |
| dmaftei 2002-02-12, 10:01 am |
| quote:
Originally posted by Hippo
30 minutes later OSPF will send out a compressed verion of its routing table.
AFAIK OSPF doesn't send its routing table; it sends LSAs. And an OSPF router will refresh its own LSAs every 30 min, provided the advertised LS has not changed during the last 30 min (if it changed, the change would've been advertised in a previous update, which update causes the refresh clock to reset). | |
| Hippo 2002-02-12, 11:17 am |
|  Dmaftei, I am not doubting you here, but I quoted my reply direct from the CCNP Routing Exam Cert Guide - Cisco Press, by Claire Gough. More precisely Chapter 4, sub heading Link State Routing Protocols, Table 4-6.
Since this is my current study and next exam, I could sure do with some clarification.
Cheers
Hippo
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| dmaftei 2002-02-12, 12:04 pm |
| quote:
Originally posted by Hippo
Dmaftei, I am not doubting you here...
Hmm, you should; I have a history of being wrong... However, in this case I think I'm right. I don't have the book you're referring to, so I rely on rfc 2328. Here's a couple of quotes:
quote:
RFC 2328, page 123 (12.4. Originating LSAs)
(1) The LS age field of one of the router's self-originated LSAs reaches the value LSRefreshTime. In this case, a new instance of the LSA is originated, even though the contents of the LSA (apart from the LSA header) will be the same. This guarantees periodic originations of all LSAs. This periodic updating of LSAs adds robustness to the link state algorithm. LSAs that solely describe unreachable destinations should not be refreshed, but should instead be flushed from the routing domain (see Section 14.1).
quote:
RFC 2328, page 216 (Appendix B. Architectural Constants)
LSRefreshTime
The maximum time between distinct originations of any particular LSA. If the LS age field of one of the router's self-originated LSAs reaches the value LSRefreshTime, a new instance of the LSA is originated, even though the contents of the LSA (apart from the LSA header) will be the same. The value of LSRefreshTime is set to 30 minutes.
Now, OSPF is too big a subject to be covered in one post, but if you think I can be of help with any specific part of it, let me know. | |
| SurfinUSA 2002-02-12, 12:59 pm |
| Dmaftei is right on this one. | |
| Wavy Davy 2002-02-12, 3:47 pm |
| Thank you all for the responses- what I thought was going to be a straight forward answer has turned out to be rather more involving, but at the end of the day I know a lot more about the OSPF protocol.
Cheers,
Wavy | |
| The Ghost 2002-02-12, 5:02 pm |
| Dmaftei is right about the 30 minutes update if anyone has any doubts left. Ospf does not send routing tables except for the scheduled one. | |
| dmaftei 2002-02-13, 7:49 am |
| quote:
Originally posted by The Ghost
Ospf does not send routing tables except for the scheduled one.
OSPF does not send routing tables, period. Every OSPF router in a domain sends its own LSAs, which are flooded throughout the domain (well, area, but let's keep it simple), and so every router learns about the LSAs of all the other routers. The end result is that each router has the same picture of the domain (the LS database). Each router then builds the routing table from that picture. When something changes (for instance a link goes down on a router) a new LSA is produced to reflect the change; the new LSA is flooded; all the routers get the new picture; all the routers recompute their routing tables. There is no routing table (or part of) passed around. | |
| Wavy Davy 2002-02-13, 8:02 am |
| Hence that is why the "sending update equals zero" is shown. Thank you dmatfei.
I must admit though, the quote that Hippo showed us does seem to refer to a routing table update. However after dmafttei's excellent explanation and reading the rfc myself I convinced it is an LSA update.
Once again, many thanks everyone!!
Wavy |
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