Some of the most common reasons for SPF running constantly are:
- Interface flaps within the area
- Neighbor interface flaps within the area
- Duplicate router ID
Topology changes in an OSPF network cause SPF calculations within that
area. The SPF is not recalculated if the topology change is in another area.
Actually, OSPF distributes interarea (between areas) topology information using
a distance-vector method. The ABRs forward routing information between areas
using a distance vector technique similar to that of RIP
The following
is an example of SPF running constantly due to an interface flap within the
network. This is a common problem in OSPF. Whenever there is a link flap in an
area, OSPF runs SPF within that area. So, if a network has unstable links, it
can cause constant SPF calculations. SPF itself is not a problem because OSPF
is just adjusting to the change in the database through calculating SPF. The
real problem occurs if there are small routers in the area and a constant SPF
run might cause a CPU spike in a router.
Because R1 is also included in
area 0, any link flap in area 0 causes all routers in area 0 to run SPF.

Debugs
and Verification
A link flap in an area causes SPF to run. If a link is
flapping constantly, this can increase the number of SPF calculations in an
area. A constant number of SPF calculations in not a problem, but if the number
is incrementing constantly, it is an indication of a problem.
The output
of show ip ospf, shows that there is a huge counter for SPF
in area 0.

The easiest way to find out which particular LSA is flapping is to turn on
debug ip ospf monitor. This debug
command in Figure
shows that a
router LSA is flapping in area 0.
The next step is to determine which
router LSA is flapping and check the log for any interface flap. The
show log command in Figure
shows the log of
the router with router ID 192.168.1.129, the ID displayed in the output of the
debug ip ospf monitor command. The log shows that a serial
link keeps going up and down. Whenever there is an interface flap, it causes
SPF to run.
Solution
There are two solutions to this problem:
- Fix the link that is flapping.
- Redefine the area boundaries.
Sometimes, the first solution might not be manageable because the link
is flapping as the result of a telco outage that is outside the network
boundary. One way to fix this temporarily is to manually shut down the
interface.
The second solution requires some redesigning. If the link
flap is happening too often, it might be possible to redefine the area, exclude
this router from the area, and make it a member of a totally stubby area.
Sometimes, this is also difficult to implement, depending upon the physical
location of this link.