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Example: Configuring the Transmission Frequency for CSNP Packets on IS-IS Interfaces

This example shows how to modify the complete sequence number PDU (CSNP) interval on IS-IS interfaces.


Before you begin, configure IS-IS. See Example: Configuring IS-IS for information about the sample IS-IS configuration.


CSNPs contain a complete list of all link-state PDUs in the IS-IS database. CSNPs are sent periodically on all links, and the receiving systems use the information in the CSNP to update and synchronize their link-state PDU databases. The designated router multicasts CSNPs on broadcast links in place of sending explicit acknowledgments for each link-state PDU.

If the routing device is the designated router on a LAN, IS-IS sends CSNPs every 10 seconds. You might want to modify the default interval to protect against CSNP flooding.


The csnp-interval statement is effective only when configured on LAN interfaces.

To modify the CSNP interval, include the csnp-interval statement:

The time can range from 1 through 65,535 seconds.

To configure the interface not to send any CSNPs, specify the disable option:

In this example, an IS-IS routing device on a LAN segment is configured to send CSNPs every 30 seconds.

Figure 1 shows the topology used in this example.

Figure 1: IS-IS CSNP Interval TopologyIS-IS CSNP Interval Topology

This example describes the steps on Device R1.



CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure the CSNP interval:

  1. Configure the interfaces.

  2. Enable IS-IS on the interfaces.

  3. Modify the CSNP interval.

  4. (Optional) Enable tracing for tracking CSNP operations.


From configuration mode, confirm your configuration by entering the show interfaces and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode. Repeat the configuration on Device R2.


Confirm that the configuration is working properly.

Verifying the CSNP Interval


Check the CSNP interval setting on the IS-IS interface.


From operational mode, enter the show isis interface extensive command.


The output shows that the CSNP interval is set to 30 seconds.

Checking the CSNP Statistics


Checking the number of CSNPs sent and received.


From operational mode, enter the show isis statistics command.


The output shows the number of CSNPs sent and received on Device R1 and Device R2.


On broadcast links, only the designated intermediate system (DIS) sends CSNPs.

Checking the IS-IS Log


Check the IS-IS trace log to view the interval between packets.


From operational mode, enter the show log isis-trace | match csn command.


The output shows that Level 1 and Level 2 CSNPs are being received roughly every 30 seconds.