ISIS Fast Convergence

IS-IS, fast convergence is achieved through the following mechanisms:
                (1) Incremental SPF (ISPF)
                (2) Partial route calculation (RPC)
                (3) LSP fast flooding
(4) Intelligent timer (including the SPF intelligent timer and intelligent timer for    generating LSPs)
In addition, fast fault detection can be achieved through BFD for IS-IS, and optimizing the IS-IS network can achieve fast convergence.
Configurations of IS-IS Fast Convergence:
isis 1
 is-level level-2
 cost-style wide
 timer lsp-generation 1 50 50 level-2   //Set the interval for generating the same LSP fragment.
 flash-flood 15 level-2         //Configure LSP fast flooding.
 bfd all-interfaces enable   //Enable BFD for IS-IS to configure BFD for all IS-IS interfaces.
 network-entity 22.0000.0000.0001.00
timer spf 1 50 50             //Set the interval for SPF calculation.
traffic-eng level-2            //Configure IS-IS extension to enable TE on the entire network.
#
interface GigabitEthernet1/0/0
 undo shutdown
ip address 22.1.3.1 255.255.255.0
isis enable 1
isis bfd enable      //Enable BFD for IS-IS for a single interface.
interface GigabitEthernet1/0/1
 undo shutdown
ip address 22.1.2.1 255.255.255.0
isis circuit-type p2p      //Change the type of a broadcast interface to P2P to avoid DIS election and thus speed up route convergence.
isis enable 1
isis bfd enable

Notes: The ISPF algorithm and RPC algorithm are the default algorithms of IS-IS and are enabled by default. Therefore, they do not need to be configured.
The timer lsp-generation command is used to set the interval for generating the same LSP fragment.
The parameter init-interval specifies the initial interval for generating the same LSP fragment. The parameter incr-interval specifies the incremental interval for generating the same LSP fragment. One incr-interval is added each time the network topology changes.
When SPF calculation is performed for the first time, the interval is init-interval. Each time the routes change, the interval is added by incr-interval until the interval reaches max-interval. After the interval reaches max-interval for three times, the interval is reduced to init-interval.
In IS-IS, a device recalculates the shortest path when the LSDB changes. Frequent route calculations consume a lot of resources and thus degrade the system performance. Delaying SPF calculation can improve efficiency in route calculation and reduce the consumption of system resources. If the delay in route calculation is too long, route convergence becomes slow.
To speed up route convergence without affecting the efficiency of routers, you can use an intelligent timer in SPF calculation. This timer automatically adjusts the interval according to the frequency of changes in the LSDB.

Parameters of IS-IS Fast Convergence:

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