OSPF full spf versus partial spf

[asolapur]

I want to know when cisco routers run full spf versus partial spf. what are the circumstance under which cisco routers run partial spf?

[pkhatri]

Hi,

Firstly, you must enable incremental SPF using the command 'ispf' under the OSPF routing process. If that is not enabled, a full SPF is always performed

From CCO:

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.

Hope that helps - pls rate the post if it does.

Paresh

[Harold Ritter]

Paresh,

Incremental SPF shouldn't be confused with partial and full SPF. The explanation you provided about when partial and full are ran is correct. This behavior doesn't need incremental SPF to take place.

For more information on incremental SPF, please refer to the following document:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1829/products_feature_guide09186a008012db76.html

Hope this helps,

[pkhatri]

I'm a bit confused, Harold ... The explanation I provided was a quote from the same document you referred to. So how are partial SPF and iSPF different in this regard ?

Thanks,

Paresh.

[pkhatri]

OK, I think I get where you are coming from, Harold.

Here is the modified explanation (Correct me if I am wrong):

Partial SPF:

- executed on a change in a type-3/4/5/7 LSA (as per sections 16.5 and 16.6 of RFC2328)

- if there is a change in a type-1 or type-2 LSA, it affects the topology of the area and so a full SPF must be run

Incremental SPF:

- this is potentially run when there is a change in a type-1 or type-2 LSA

- the unchanged part of the tree is kept intact but the parts of the tree that are affected by the change are rebuilt and re-attached to the unchanged part of the tree

Paresh

[Harold Ritter]

Correct. Incremental SPF is ran in place a full SPF. In other words, it is a more efficient version of the full SPF.

Hope this helps,

[sunyoung.song]

Is partial spf default on Cisco router?

and Is it not related SPF timer?

Please see bellow debug log.

(this log is setting timer throttle spf 50 5000 10000)

Mar 27 17:17:48.584: OSPF: Detect change in LSA type 3, LSID 192.168.2.1, from 192.168.0.1 area 1

Mar 27 17:17:48.584: OSPF: Schedule partial SPF - type 3 id 192.168.2.1 adv rtr 192.168.0.1

Mar 27 17:17:48.584: OSPF: Service partial SPF 1/0/0

Mar 27 17:17:48.584: OSPF: Start partial processing Summary LSA 192.168.2.1, mask 255.255.255.255, a

dv 192.168.0.1, age 1, seq 0x80000051 (Area 1)

Mar 27 17:17:48.584: Add better path to LSA ID 192.168.2.1, gateway 0.0.0.0, dist 517

Mar 27 17:17:48.584: Add path: next-hop 10.0.200.1, interface GigabitEthernet3/10

Mar 27 17:17:48.584: OSPF: delete lsa id 192.168.2.1, type 3, adv rtr 192.168.0.1 from delete list

Mar 27 17:17:48.584: Add Summary Route to 192.168.2.1/255.255.255.255. Metric: 517, Next Hop: 10.

0.200.1

Mar 27 17:17:48.584: OSPF: insert route list LS ID 192.168.2.1, type 3, adv rtr 192.168.0.1

Mar 27 17:17:48.584: OSPF: delete lsa id 192.168.2.1, type 3, adv rtr 192.168.0.1 from delete list

Mar 27 17:17:48.584: OSPF: insert route list LS ID 192.168.2.1, type 3, adv rtr 192.168.0.1

Mar 27 17:17:48.584: OSPF: Start partial processing Summary LSA 192.168.2.1, mask 255.255.255.255, a

dv 192.168.0.2, age 31, seq 0x80000050 (Area 1)

Mar 27 17:17:48.584: OSPF: delete lsa id 192.168.2.1, type 3, adv rtr 192.168.0.2 from delete list

Mar 27 17:17:48.632: OSPF: Detect change in LSA type 3, LSID 192.168.2.1, from 192.168.0.2 area 1

Mar 27 17:17:48.632: OSPF: Schedule partial SPF - type 3 id 192.168.2.1 adv rtr 192.168.0.2

Mar 27 17:17:48.636: OSPF: Service partial SPF 1/0/0

Mar 27 17:17:48.636: OSPF: Start partial processing Summary LSA 192.168.2.1, mask 255.255.255.255, a

dv 192.168.0.1, age 1, seq 0x80000051 (Area 1)

Mar 27 17:17:48.636: Add better path to LSA ID 192.168.2.1, gateway 0.0.0.0, dist 517

Mar 27 17:17:48.636: Add path: next-hop 10.0.200.1, interface GigabitEthernet3/10

Mar 27 17:17:48.636: OSPF: delete lsa id 192.168.2.1, type 3, adv rtr 192.168.0.1 from delete list

Mar 27 17:17:48.636: Add Summary Route to 192.168.2.1/255.255.255.255. Metric: 517, Next Hop: 10.

0.200.1

Mar 27 17:17:48.636: OSPF: insert route list LS ID 192.168.2.1, type 3, adv rtr 192.168.0.1

Mar 27 17:17:48.636: OSPF: delete lsa id 192.168.2.1, type 3, adv rtr 192.168.0.1 from delete list

Mar 27 17:17:48.636: OSPF: insert route list LS ID 192.168.2.1, type 3, adv rtr 192.168.0.1

Mar 27 17:17:48.636: OSPF: Start partial processing Summary LSA 192.168.2.1, mask 255.255.255.255, a

dv 192.168.0.2, age 1, seq 0x80000051 (Area 1)

Mar 27 17:17:48.636: Add better path to LSA ID 192.168.2.1, gateway 0.0.0.0, dist 516

Mar 27 17:17:48.636: Add path: next-hop 10.0.70.1, interface GigabitEthernet3/1

e1-1#

Mar 27 17:17:48.636: Add Summary Route to 192.168.2.1/255.255.255.255. Metric: 516, Next Hop: 10.

0.70.1

Mar 27 17:17:48.636: OSPF: insert route list LS ID 192.168.2.1, type 3, adv rtr 192.168.0.2

Mar 27 17:17:48.636: OSPF: delete lsa id 192.168.2.1, type 3, adv rtr 192.168.0.2 from delete list

Mar 27 17:17:48.636: OSPF: insert route list LS ID 192.168.2.1, type 3, adv rtr 192.168.0.2

[pkhatri]

Hi,

Changes in Type-3/4/5/7 LSAs always trigger partial SPF runs - that behaviour *is* the default.

Hope that helps - pls rate the post if it does.

Paresh

[sunyoung.song]

thanks for your advice.