Table Of Contents
IS-IS Fast-Flooding of LSPs Using the fast-flood Command
The IS-IS Fast-Flooding of LSPs Using the fast-flood Command feature improves Intermediate System-to-Intermediate System (IS-IS) convergence time when new link-state packets (LSPs) are generated in the network and shortest path first (SPF) is triggered by the new LSPs. This document introduces the new fast-flood command.
Feature History for the IS-IS Fast-Flooding of LSPs Using the fast-flood Command Feature
This feature was introduced.
This feature was integrated into Cisco IOS Release 12.3(7)T.
Note Software images for Cisco 12000 series Internet routers have been deferred to Cisco IOS Release 12.0(27)S1.
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Information About IS-IS Fast-Flooding of LSPs Using the fast-flood Command
Before using the fast-flood command, you should understand the concept in the following section:
Benefits of Fast-Flooding
If you are using the SPF and if very short values are used for the initial delay required (less than 40 milliseconds), SPF may start before the LSP that triggered SPF is flooded to neighbors. The router should always flood (at least) the LSP that triggered SPF before the router runs the SPF computation.
We recommend that you enable the fast-flooding of LSPs before the router runs the SPF computation, to ensure that the whole network achieves a faster convergence time.
How to Use the IS-IS Fast-Flooding of LSPs Using the fast-flood Command Feature
This section contains the following procedure:
•Enabling Fast-Flooding (optional)
Perform this task to enable fast-flooding.
2. configure terminal
3. router isis [tag]
4. fast-flood lsp-number
6. show running-configuration
The following sections provide references related to the IS-IS Fast-Flooding of LSPs Using the fast-flood Command feature.
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This section documents the new fast-flood command:
To fast-flood Intermediate System-to-Intermediate System (IS-IS) link-state packets (LSPs), use the fast-flood command in router configuration mode. To disable the fast-flooding, use the no version of this command.
no fast-flood lsp-number
(Optional) The number of LSPs from 1 to 15 to be flooded before shortest path first (SPF) is started. The default is 5 LSPs.
Fast-flooding is disabled. Five LSPs are flooded before SPF is started.
This command was introduced.
This command was integrated into Cisco IOS Release 12.3(7)T.
The fast-flood command tells the router to flood a certain number of LSPs, as configured by the lsp-number argument (if no lsp-number value has been specified, the default value of 5 is used), that invoke SPF before running SPF. When you speed up the LSP flooding process, you improve overall network convergence time.
If you are running SPF and if you have configured very short values (less than 40 milliseconds) for the initial delay that is set by the seconds argument of the incremental-spf command, the SPF computation might start before the LSP that triggered SPF is flooded to neighbors. The router should always flood (at least) the LSP that triggered SPF before the router runs the SPF computation.
We recommend that you enable the fast-flooding of LSPs before the router runs the SPF computation, in order to achieve a faster convergence time.
In the following example, the fast-flood command is entered to configure the router to flood the first seven LSPs that invoke SPF, before the SPF computation is started. When the show running-configuration command is entered, the output confirms that fast-flooding has been enabled on the router.Router# clear isis rib redistribution 10.1.0.0 255.255.0.0Router> enableRouter# configure terminalRouter(config)# router isis firstRouter(config-router)# fast-flood 7Router(config-router)# endRouter# show running-configurationfast-flood 7
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