SPF Running Constantly-Cause: Interface Flap Within the Network

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SPF Running Constantly ‚ Cause: Interface Flap Within the Network

This is a common problem in OSPF. Whenever there is a link flap in an area, OSPF runs SPF. So, if a network has unstable links, it can cause constant SPF run. SPF itself is not a problem because OSPF is just adjusting the change in database through calculating SPF. The real prob-lem occurs if there are small routers in the network and a constant SPF run might cause a CPU spike in a router. A link flap is shown in Figure 9-99. Because R1 also is included in area 0, any link flap in area 0 causes all routers in area 0 to run SPF.

Figure 9-99. A Link Flap Causes SPF in Area 0

graphics/09fig99.gif

Figure 9-100 shows the flowchart to follow to solve this problem.

Figure 9-100. Problem-Resolution Flowchart

graphics/09fig100.gif

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 is not a problem, but if the number is incrementing constantly, it is an indication of a problem.

Example 9-264 shows the output of show ip ospf, which shows that there is a huge counter for SPF in area 0.

Example 9-264 Determining How Often SPF Is Running
 R1#  show ip ospf  Routing Process "ospf 1" with ID 192.168.254.13  Supports only single TOS(TOS0) routes  It is an area border  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 8. Checksum Sum 0x48C3E  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 3. 2 normal 1 stub 0 nssa     Area BACKBONE(0)         Number of interfaces in this area is 1         Area has no authentication  SPF algorithm executed 2668 times  

The easiest way to find out which particular LSA is flapping is to turn on debug ip ospf monitor. This debug shows exactly which LSA is flapping. Example 9-265 shows the output of debug ip ospf monitor and reveals that a router LSA is flapping in area 0.

Example 9-265 debug ip ospf monitor Output Pinpoints Route Flap
 R1#  debug ip ospf monitor  OSPF: Schedule SPF in area 0.0.0.0  Change in LS ID 192.168.1.129, LSA type R,  OSPF: schedule SPF: spf_time 1620348064ms wait_interval 10s 

The next step is to go on that router whose router LSA is flapping and check the log for any interface flap. Example 9-266 shows the log of the router with router ID 192.168.1.129. The log shows that a serial link keeps going up and down. Whenever there is an interface flap, it causes SPF to run.

Example 9-266 Router Log Pinpoints the Interface Causing Route Flap
 R3#  show log   *Mar 29 01:59:07: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state   to down  *Mar 29 01:59:09: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state           to up *Mar 29 01:59:30: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state           to down *Mar 29 02:00:03: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed           state to up 

Solution

Actually two solutions exist in this case:

  • Fix the link flap.

  • Redefine the area boundaries.

Sometimes, the first solution might not be manageable because the link is flapping as the result of some telco outage beyond your control. One way to fix this temporarily is to manually shut down that 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.

In short, link flaps are realities; if there are too many link flaps, the number of routers in an area should be decreased so that fewer routers are affected.

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Troubleshooting IP Routing Protocols
Troubleshooting IP Routing Protocols (CCIE Professional Development Series)
ISBN: 1587050196
EAN: 2147483647
Year: 2002
Pages: 260

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