OSPF Fast Convergence Tools - Event Processing

OSPF Event Processing is the third post about OSPF fast convergence tool, earlier posts have already covered OSPF Event Detection and OSPF Event Propagation.

OSPF event processing is all about how to throttle SPF calculations and increase the OSPF convergence time. Event processing is having three different tools as below listed:-
• OSPF SPF Calculations
• OSPF SPF Throttling
• OSPF Incremental SPF

OSPF SPF Calculations:- OSPF full calculations are only triggered by LSA1 and LSA2 where as partial SPF is triggered by LSA 3, LSA 4, LSA 5 and LSA 7.

OSPF SPF Throttling:- OSPF Event propagation deals with the LSA throttling whereas OSPF Event processing deals with SPF throttling. SPF and iSPF(increment SPF) cab be rate limited to delay calculations and save lot of network resources during when the network is not stable. We can throttle only full SPF and iSPF not the partial SPF. Below is the command line to configure SPF throttling:-
router ospf 10
timers throttle spf (start) (hold) (max)

start:[default = 5000 ms] Initial SPF schedule delay in milliseconds.
Recommendation = 10 ms

hold:[default=10000 ms] Minimum hold time between two consecutive SPF calculations. Range is 1 to 600000 milliseconds.
Recommendation = 100 ms

max:[default = 10000 ms] Default ‘timers spf spf-holdtime’ value was 10s. Setting max-interval to 10000 ms provides response equivalent to pre SPF throttle.
Recommendation = 5000 ms


OSPF Incremental SPF:- Before the introduction of iSPF, Dijkstra Algorithm is run by all the routers whenever there is any state change occurs in the OSPF routing area. This requires lot of CPU and memory and wastes lot of resources.

So iSPF helps Dijkstra to run the algorithm only at the change portion of the OSPF tree for LSA 1 and LSA 2 only. This process saves lot of CPU and memory resources consequences reduce OSPF network convergence.

router ospf 10
network 0.0.0.0 255.255.255.255 area 0
ispf

(Read More About OSPF High Availability)

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