Tag Archives: 3.6g

3.6.g Implement troubleshoot and optimize OSPF convergence and scalability

3.6.g [i] Metrics

OSPF convergence is extremely fast when compared to other protocols. To keep this desirable behavior fully functional in your network, you need to consider the three components that determine how long it takes for OSPF to converge:

● The length of time it takes OSPF to detect a link or interface failure

● The length of time it takes the routers to exchange routing information via LSAs, rerun the Shortest Path First algorithm, and build a new routing table

● A built-in SPF delay time of five seconds (default value)

Thus, the average time for OSPF to propagate LSAs and rerun the SPF algorithm is approximately 1 second. Then the SPF delay timer of five seconds must elapse. Therefor OSPF convergence can be a anything from 6 to 46 seconds, depending upon the type of failure, SPF timer settings, size of the network, and size of the LSA database. The worst case scenario is when a link fails but the destination is still reachable via an alternate route, because the 40 second default dead timer will need to expire before the SPF is rerun.

If OSPF interface costs are auto-calculated based on interface bandwidth then the OSPF reference bandwidth on a router should be at least twice the highest interface bandwidth configured on any of the router’s interfaces. OSPF link cost is an integer value calculated by dividing the reference bandwidth by the interface’s bandwidth value. If interface bandwidth values are large and the reference bandwidth is too small, this calculation will result in interfaces with different bandwidths being assigned a metric of 1. To avoid such issues, you can use auto-cost reference-bandwidth command.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3607-3616).  . Kindle Edition.



3.6.g Implement, troubleshoot and optimize OSPF convergence and scalability

3.6.g [ii] LSA throttling, SPF tuning, fast hello

The OSPF Link-State Advertisement (LSA) Throttling feature provides a dynamic mechanism to slow down link-state advertisement (LSA) updates in OSPF during times of network instability . It also allows faster Open Shortest Path First (OSPF) convergence by providing LSA rate limiting in milliseconds.

The use of SPF throttle timer tuning can aid in improving the convergence of the campus network to within the sub-second threshold, but is not sufficient to ensure optimal convergence times. Two factors impact the ability of OSPF to converge: the time waiting for an SPF calculation, and the time waiting for an LSA to be received indicating a network topology change. Before the 12.2S release, Cisco IOS implemented two internal timers affecting the generation of LSAs. The first was an internal delay timer that throttled the generation of router (type-1) and network (type-2) LSAs for 500 msec after a network interface change. A second timer throttled the generation of any specific updated LSA for at least five seconds after having sent the same LSA. These two timers could impact the speed at which the network was able to converge. On the detection of any interface change, OSPF would not generate an LSA indicating the link status change for 500 msec, thus preventing the SPF process from responding to the link failure for at least 500 additional msec.

After this occurred, any additional change such as link restoration was throttled for a further five seconds, also potentially impacting recovery. The presence of these delay timers, like the SPF timers, was based on a need to ensure the stability of the network and mitigate against OSPF thrashing in the event of a flapping link or other network problem.

The same design and physical factors that allow for SPF tuning in the campus environment also make it amenable to tuning of the LSA timers. The use of routed point-to-point interfaces in the campus removes the need to consider the loss of multiple logical links in the event of a single interface failure (as is the case in a multi-point WAN environment). The use of direct fiber connections between devices also reduces the probability for link loss and ensures a higher degree of accurate link status detection (no LMI or other soft WAN-like failures need to be considered). Interface-specific features such as debounce timers and IP event dampening also lessen the probability of false or flapping interface conditions. The combination of these factors serves to mitigate the factors with which the LSA timers were initially designed to address.

Tuning LSA throttle timers uses an approach similar to that described above for SPF. Three configuration values are used: an initial delay timer, a hold timer, and a maximum hold timer. Using a similar approach to that discussed above results in the use of the same timer values for the LSA configuration as for the SPF configuration.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Location 3639).  . Kindle Edition.



3.6.g Implement, troubleshoot and optimize OSPF convergence and scalability

3.6.g [iii] LSA propagation control [area types, ISPF]

By default, OSPF LSA propagation is controlled by three parameters:

● OSPF_LSA_DELAY_INTERVAL: Controls the length of time that the router should wait before generating a type 1 router LSA or type 2 network LSA. By default, this parameter is set at 500 ms.

● MinLSInterval: Defines the minimum time between distinct originations of any particular LSA. The value of MinLSInterval is set to 5 seconds.

● MinLSArrival: The minimum time that must elapse between reception of new LSA instances during flooding for any particular LSA. LSA instances received at higher frequencies are discarded. The value of MinLSArrival is set to 1 second.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3675-3678).  . Kindle Edition.



3.6.g Implement, troubleshoot and optimize OSPF convergence and scalability

3.6.g [iv] IP FRR/fast reroute [single hop]

The OSPFv2 Loop-Free Alternate Fast Reroute feature uses a pre-computed alternate next hop to reduce failure reaction time when the primary next hop fails. It lets you configure a per-prefix loop-free alternate (LFA ) path that redirects traffic to a next hop other than the primary neighbor. The forwarding decision is made and service is restored without other routers’ knowledge of the failure.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3680-3682).  . Kindle Edition.