What are main factor and related parameter influencing convergence rate of ISIS?
ISO-10589 defines SPF algorithm. When one node in network topology changes, it will re-calculate all nodes of network. The duration of calculation is long. It occupies too much CPU resource and influences convergence rate of the whole network. For optimization solution, I-SPF and PRC replace SPF algorithm. They only re-calculate changed node and related route. It can increase convergence rate of ISIS. It is defaulted to be enable in the realization of VRP5.
After routing algorithm is improved, the convergence rate is influenced if the interval of trigger route calculation is long. Especially when the network frequently changes, ISIS will often calculate and creates new LSP to notify the change. CPU is overused and convergence rate is influenced. Intelligent timer can better solve the problem. It can improve routing calculation and reduce the waste of the resource of the system to some degree by delaying SPF calculation and LSP creation. It can also avoid the delay and reduce convergence rate.
The main parameters are max-interval, init-interval and incr-interval. When LSP (calculate SPF) is initially created, the time interval is initial-interval; subsequently, when it changes once, the interval adds incremental-interval until max-interval. After max-interval occurs three times, the interval returns initial-interval.
3. Rapid advertisement of LSP
According to realization of traditional IS-IS protocol, when IS-IS receives LSP sent by other router, it will set one timer if the LSP is newer than its LSDB and advertises LSP in LSDB. The synchronization of LSDB is very slow. When the router configuring LSP rapid advertisement receives one or more newer LSP, it advertises LSP that is less than designated number before the calculation and quickens the synchronization of LSDB. It can increase convergence rate of the whole network.
The main parameter, lsp-count: the maximum count of LSP advertisement of each interface, the range is 1~15 and the default is 5. max-timer-interval interval: designate the maximum interval of LSP advertisement and the unit is millisecond. The range is 10~50000 and the default is 10 milliseconds.