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Example: Configuring the Transmission Frequency for CSNPs on IS-IS Interfaces

Understanding the Transmission Frequency for CSNPs on IS-IS Interfaces

When IS-IS is activated on a routing device’s interface, the device first sends some IS-IS hello packets (IIHs) to its neighbors to ensure that the circuit is capable of transporting packets in both directions. In the IIHs, the router embeds information about the designated router (also called the designated intermediate system or DIS). One of the designated router roles on an IS-IS broadcast circuit is to synchronize the link-state databases on LANs. The designated router does this by periodically sending a directory of its link-state database, which is received by all the routing devices on a LAN.

If the routing device is the designated router on a LAN, IS-IS sends CSNPs every 10 seconds. If the routing device is on a point-to-point interface, it sends CSNPs every 5 seconds. The general recommendation is to use the default values or to increase the CSNP interval if there are a large number of broadcast circuits that need to be supplied with fresh CSNPs. Increasing the interval can help protect against CSNP flooding.

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.

• Requirements
• Overview
• Configuration
• Verification

Requirements

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

Overview

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. If the routing device is on a point-to-point interface, it sends CSNPs every 5 seconds. You might want to modify the default interval to protect against CSNP flooding.

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 Topology

This example describes the steps on Device R1.

Configuration

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.

Step-by-Step Procedure

1. Configure the interfaces
   [edit interfaces]user@R1# set fe-1/2/0 unit 0 description to-R2user@R1# set fe-1/2/0 unit 0 family inet address 10.0.0.1/30user@R1# set fe-1/2/0 unit 0 family isouser@R1# set lo0 unit 0 family inet address 192.168.0.1/32user@R1# set lo0 unit 0 family iso address 49.0002.0192.0168.0001.00
2. Enable IS-IS on the interfaces.
   [edit protocols isis]user@R1# set interface fe-1/2/0.0 user@R1# set interface lo0.0
3. Modify the CSNP interval.
   [edit protocols isis interface fe-1/2/0.0]user@R1# set csnp-interval 30
4. (Optional) Enable tracing for tracking CSNP operations.
   [edit protocols isis traceoptions]user@R1# set file isis-traceuser@R1# set flag csn

Results

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.

Verification

Confirm that the configuration is working properly.

• Verifying the CSNP Interval
• Checking the CSNP Statistics
• Checking the IS-IS Log

Verifying the CSNP Interval

Purpose

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

Action

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

IS-IS interface database:
fe-1/2/0.0
  Index: 70, State: 0x6, Circuit id: 0x1, Circuit type: 3
  LSP interval: 100 ms, CSNP interval: 30 s, Loose Hello padding
  Adjacency advertisement: Advertise
  Level 1
    Adjacencies: 1, Priority: 64, Metric: 10
    Hello Interval: 9.000 s, Hold Time: 27 s
    Designated Router: R2.02 (not us)
  Level 2
    Adjacencies: 1, Priority: 64, Metric: 10
    Hello Interval: 9.000 s, Hold Time: 27 s
    Designated Router: R2.02 (not us)

Meaning

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

Checking the CSNP Statistics

Purpose

Checking the number of CSNPs sent and received.

Action

From operational mode, enter the show isis statistics command.

IS-IS statistics for R1:
PDU type       Received    Processed        Drops         Sent       Rexmit
LSP                   5            5            0            3            0
IIH                  94           20            0           43            0
CSNP                  6            6            0            0            0
PSNP                  0            0            0            0            0
Unknown               0            0            0            0            0
Totals              105           31            0           46            0

Total packets received: 105 Sent: 46

SNP queue length: 0 Drops: 0
LSP queue length: 0 Drops: 0
SPF runs: 5
Fragments rebuilt: 5
LSP regenerations: 0
Purges initiated: 0
-----

IS-IS statistics for R2:
PDU type       Received    Processed        Drops         Sent       Rexmit
LSP                   3            3            0            5            0
IIH                  35           11            0           86            0
CSNP                  0            0            0            6            0
PSNP                  0            0            0            0            0
Unknown               0            0            0            0            0
Totals               38           14            0           97            0

Total packets received: 38 Sent: 97

SNP queue length: 0 Drops: 0
LSP queue length: 0 Drops: 0
SPF runs: 7
Fragments rebuilt: 7
LSP regenerations: 0
Purges initiated: 0

Meaning

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

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

Checking the IS-IS Log

Purpose

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

Action

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

Jun 18 14:36:19.504064 Received L1 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:36:19.523065 Received L2 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:36:48.904120 Received L1 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:36:48.916425 Received L2 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:37:14.954447 Received L1 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:37:14.971329 Received L2 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:37:44.227106 Received L1 CSN, source R2, interface fe-1/2/0.0
Jun 18 14:37:44.244181 Received L2 CSN, source R2, interface fe-1/2/0.0

Meaning

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