11-03-2010 10:32 AM - edited 03-04-2019 10:21 AM
Hi all from Cisco Community,
I'm doing a research on Cisco incremental SPF. So far I know that there is Full SPF and Partial SPF (Which is run when type 3, 4, 5, or 7 LSAs is received -- this actually is the incremental SPF mention in RFC 2328). However, some Cisco IOS support a feature known as Incremental SPF (iSPF for short), configured by:
router ospf id
ispf
I've searched around for quite a bit and haven't found anywhere that explain the exact process (in which how LSA is evaluated, the process flow etc.). Would anyone kindly point me to the appropriate document? (I've also try and read the debug log but it doesn't make much sense to me)
Thanks in advance!
Solved! Go to Solution.
11-03-2010 01:45 PM
Hello,
Information about the incremental SPF are quite rare, unfortunately. One of the better articles can be found here:
http://blog.ine.com/2009/12/31/tuning-ospf-performance/#more-3213
The article seems to suggest that the incremental SPF has been described in an semiannual report by Eric Rosen et al. - it can be found here (requires an access to ACM portal - I can download the document for you if you are interested).
http://portal.acm.org/citation.cfm?id=205453&dl=ACM&coll=DL&CFID=108592207&CFTOKEN=20336747
Best regards,
Peter
11-04-2010 12:51 AM
Hello,
I've sent you the e-mail. Also, I've found an apparently Cisco-authored presentation at the ripe.net that also talks about the incremental SPF:
http://www.ripe.net/ripe/meetings/ripe-41/presentations/eof-isis/sld049.html
Best regards,
Peter
11-03-2010 11:39 AM
Which kind of document are you looking for ? Cisco doesn't disclose their code internals unless necessary, so if you want to enable the feature, you have to rely on what the public documentation says, in this case:
OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.
Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.
11-03-2010 01:05 PM
Thanks for the reply,
I'm looking for an explanation of what steps is taken in incremental SPF.
For example, you can partly refer to RFC 2328 (http://tools.ietf.org/html/rfc2328#page-160) to know what are the steps of a full SPF Algorithm. In this case, I'd like to know how the affected part of the tree is found and recompute, etc.
Cisco does include the process in their debug mode (debug ip ospf spf), but it would be really hard to get the concept from there, and it's hard to get an overview and all.
Is there any document (Cisco or non-Cisco) that thoroughly explain the process? (Not code internals but only the process in more details)
Regards
11-03-2010 01:45 PM
Hello,
Information about the incremental SPF are quite rare, unfortunately. One of the better articles can be found here:
http://blog.ine.com/2009/12/31/tuning-ospf-performance/#more-3213
The article seems to suggest that the incremental SPF has been described in an semiannual report by Eric Rosen et al. - it can be found here (requires an access to ACM portal - I can download the document for you if you are interested).
http://portal.acm.org/citation.cfm?id=205453&dl=ACM&coll=DL&CFID=108592207&CFTOKEN=20336747
Best regards,
Peter
11-03-2010 10:58 PM
Hello Peter,
I've already read the article on ine.com. I'll be very grateful if you can download the document from ACM. Please do.
Please post it here (if it's legal) or please email me the document.
Thanks again.
Regards,
N.
11-04-2010 12:51 AM
Hello,
I've sent you the e-mail. Also, I've found an apparently Cisco-authored presentation at the ripe.net that also talks about the incremental SPF:
http://www.ripe.net/ripe/meetings/ripe-41/presentations/eof-isis/sld049.html
Best regards,
Peter
11-04-2010 12:58 AM
Hello,
Thank you. This helps me a lot!
Regards,
N.
01-05-2011 01:16 AM
Hello Peter,
I'm also doing a research on Cisco incremental SPF , as you said information about the incremental SPF are quite rare, would you please email me the document?
Tanks a lot.
Regards.
01-05-2011 01:24 AM
Hello,
Please contact me via e-mail - you can find my address in my profile.
Best regards,
Peter
02-12-2011 07:13 AM
To all who may be interested,
The presentation at the RIPE has moved to a new URL:
http://meetings.ripe.net/ripe-41/presentations/eof-isis/index.html
The iSPF part starts at
http://meetings.ripe.net/ripe-41/presentations/eof-isis/sld049.html
Best regards,
Peter
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