IS-IS fast convergence is an extended feature of IS-IS implemented to speed up route convergence. It includes the following concepts:
Incremental SPF (I-SPF) recalculates only the routes of the changed nodes rather than the routes of all nodes when the network topology changes, which speeds up the calculation of routes.
Partial route calculation (PRC) calculates only those routes which have changed when the network topology changes.
Link State PDUs (LSP) fast flooding
LSP fast flooding speeds up LSP flooding.
The first timeout period of the timer is fixed. If an event that triggers the timer occurs before the set timer expires, the next timeout period of the timer increases.
The intelligent timer applies to LSP generation and SPF calculation.
In ISO 10589, the Dijkstra algorithm was adopted to calculate routes. When a node changes on the network, the algorithm recalculates all routes. The calculation requires a long time to complete and consumes a significant amount of CPU resources, reducing convergence speed.
I-SPF improves the algorithm. Except for the first time the algorithm is run, only the nodes that have changed rather than all nodes in the network are used in the calculation. The SPT generated using I-SPF is the same as that generated using the previous algorithm. This significantly decreases CPU usage and speeds up network convergence.
Similar to I-SPF, PRC calculates only routes that have changed. PRC, however, does not calculate the shortest path. It updates routes based on the SPT calculated by I-SPF.
In route calculation, a leaf represents a route, and a node represents a device. Either an SPT change or a leaf change causes a routing information change. The SPT change is irrelevant to the leaf change. PRC processes routing information as follows:
For example, if a new route is imported, the SPT of the entire network remains unchanged. In this case, PRC updates only the interface route for this node, thereby reducing the CPU usage.
PRC working with I-SPF further improves network convergence performance and replaces the original SPF algorithm.
On the NetEngine 8000 F, only I-SPF and PRC are used to calculate IS-IS routes.
When an IS-IS device receives new LSPs from other devices, it updates the LSPs in the LSDB and periodically floods the updated LSPs based on a timer. Therefore, the synchronization of all LSDBs is slow.
With lSP fast flooding, when the router receives LSPs that can trigger route calculation or route update, it floods these LSPs before route calculation occurs, which speeds up network convergence and LSDB synchronization throughout the entire network.
LSP fast flooding is supported by default and does not need to be configured.
Although the route calculation algorithm is improved, the long interval for triggering route calculation also affects the convergence speed. A millisecond-level timer can shorten the interval. Frequent network changes, however, also consume too much CPU resources. The SPF intelligent timer addresses these problems.
In most cases, an IS-IS network running normally is stable. The frequent changes on a network are rather rare, and IS-IS does not calculate routes frequently. Therefore, a short period (within milliseconds) can be configured as the first interval for route calculation. If the network topology changes frequently, the interval set by the intelligent timer increases with the calculation times to reduce CPU consumption.
The LSP generation intelligent timer is similar to the SPF intelligent timer. When the LSP generation intelligent timer expires, the system generates a new LSP based on the current topology. The original mechanism uses a timer with fixed intervals, which cannot meet both the fast convergence and low CPU usage requirements. Therefore, the LSP generation timer is designed as an intelligent timer to respond to emergencies (for example, the interface goes Up or Down) quickly and speed up network convergence. In addition, when the network changes frequently, the interval for the intelligent timer becomes longer to reduce CPU consumption.