Technical Tip: How to validate the last time that SPF was executed on OSPF protocol

[ojacinto]

Description	This article describes how to check the last time the SPF algorithm was executed on the OSPF protocol.
Scope	FortiOS v7.2.0 and later, v7.4.0 and later, 7.6.0 and later.
Solution	After OSPF dynamic routing is configured on the FortiGate according to this KB article: Technical Tip: Basic OSPF configuration and MTU settings in OSPF  It is important to monitor the 'SPF execution algorithm' because this counter tracks the number of times that the SPF algorithm has been run since the OSPF protocol was started on the FortiGate. It indicates how many times the FortiGate has had to recalculate the SPF for the OSPF protocol. For example: FGT-HUBDC1 (root) # get router info ospf status Routing Process "ospf 0" with ID 10.254.0.99 Process uptime is 1 hour 13 minutes Process bound to VRF default Conforms to RFC2328, and RFC1583Compatibility flag is disabled Supports only single TOS(TOS0) routes Supports opaque LSA Do not support Restarting SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Refresh timer 10 secs Number of incomming current DD exchange neighbors 0/5 Number of outgoing current DD exchange neighbors 0/5 Number of external LSA 0. Checksum 0x000000 Number of opaque AS LSA 0. Checksum 0x000000 Number of non-default external LSA 0 External LSA database is unlimited. Number of LSA originated 2 Number of LSA received 5 Number of areas attached to this router: 1    Area 0.0.0.0 (BACKBONE)     Number of interfaces in this area is 3(3)     Number of fully adjacent neighbors in this area is 2     Area has no authentication     SPF algorithm last executed 01:04:12.370 ago     SPF algorithm executed 4 times     Number of LSA 2. Checksum 0x013788 The above output shows that OSPF process uptime is 1 hour 13 minutes; however, the last time that the SPF algorithm was run was 1 hour 4 minutes and 12 seconds ago. Monitoring this counter is an important part of network health and performance because it could indicate network stability. If this counter is frequently changing it can indicate a part of the network as unstable (it could be a router, link or network that is flapping): FGT-HUBDC1 (root) # get router info ospf status Routing Process "ospf 0" with ID 10.254.0.99 Process uptime is 1 days 16 hours 27 minutes ... SPF algorithm last executed 00:00:04.120 ago < -- SPF algorithm executed 25661 times  Getting the same output after some minutes: FGT-HUBDC1 (root) # get router info ospf status Routing Process "ospf 0" with ID 10.254.0.99 Process uptime is 1 days 16 hours 32 minutes ... SPF algorithm last executed 00:00:03.640 ago < --- SPF algorithm executed 25724 times  This change has an impact on the FortiGate performance because the SPF algorithm is a CPU-intensive operation. A best practice under a similar scenario where SPF is constantly changing is to use the 'spf-delay' and 'spf-holdtime' timers. Related article: Technical Tip: How to Implement OSPF SPF scheduling and throttling