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Example: Enabling Packet Checksums on IS-IS Interfaces for Error Checking

This example shows how to enable packet checksums for IS-IS interfaces.


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


Junos OS supports IS-IS checksums as documented in RFC 3358, Optional Checksums in Intermediate System to Intermediate System (ISIS).

IS-IS protocol data units (PDUs) include link-state PDUs, complete sequence number PDUs (CSNPs), partial sequence number PDUs (PSNPs), and IS-IS hello (IIH) packets. These PDUs can be corrupt due to faulty implementations of Layer 2 hardware or lack of checksums on a specific network technology. Corruption of length or type, length, and value (TLV) fields can lead to the generation of extensive numbers of empty link-state PDUs in the receiving node. Because authentication is not a replacement for a checksum mechanism, you might want to enable the optional checksum TLV on your IS-IS interfaces.

The checksum cannot be enabled with MD5 hello authentication on the same interface.

Figure 1 shows the topology used in this example.

Figure 1: IS-IS Checksum TopologyIS-IS Checksum 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 IS-IS checksums:

  1. Enable checksums.

  2. (Optional) Enable tracing for tracking checksum operations.


From configuration mode, confirm your configuration by entering the 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.


Confirm that the configuration is working properly.

Verifying Checksums


Verify that checksums are performed.


From operational mode, enter the show log isis | match checksum command.


The output shows that checksum information is captured in the IS-IS trace log file.