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Example: Configuring the Transmission Frequency for Link-State PDUs on IS-IS Interfaces

Understanding Link-State PDU Throttling for IS-IS Interfaces

Control traffic (link-state PDU and related packets) might cause delays in user traffic (information packets) because control traffic always has precedence in terms of scheduling on the interface cards.

Unfortunately, the control traffic transmission rate does not get lower on low-bandwidth interfaces such as DS-0 or fractional T1/E1 lines. Control traffic stays the same, regardless of line bandwidth.

Junos OS does not support automated calculation of link-state PDU throttling based on available bandwidth because the lowest-speed interface cards on a Juniper Networks routing device starts at T1/E1 speeds (1.5 and 2 Mbps). It is assumed that even with link-state PDU pacing of 20 ms, the control traffic will not consume more than half of the interface bandwidth.

However, there might be fractional T1/E1 circuits (less than the full bandwidth) configured as well, where link-state PDU pacing might have to be adjusted.

Thus, the lsp-interval statement helps to resolve two issues: regulating the control-traffic-to-user-traffic ratio, and protecting neighbors during transient situations.

The traffic subject to this pacing is non-self-originated traffic, which is traffic that has been originated by other routers, not the local router. Junos OS has hard-coded rate limiting for locally generated link-state PDUs. All the link-state PDUs are paced using a 20 ms timer. Additionally, there is logic that makes sure that the adjacency is reliably up for some time before advertising the adjacency.

Example: Configuring the Transmission Frequency for Link-State PDUs on IS-IS Interfaces

This example shows how to modify the link-state PDU interval time.

• 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

To keep reachability information in the network current, link-state protocols need to originate, distribute, and revoke or time-out topology information. In IS-IS, topology information is encoded in link-state PDUs.

By default, the routing device sends one link-state PDU out an interface every 100 milliseconds. To modify this interval, include the lsp-interval statement:

To disable the transmission of all link-state PDUs, set the interval to 0.

Link-state PDU throttling by use of the lsp-interval statement controls the flooding pace to neighboring routing devices in order to not overload them and also to ensure that user traffic is not delayed on low-bandwidth links.

In this example, an IS-IS routing device on a LAN segment is configured to send link-state PDUs every 1000 milliseconds.

Figure 1 shows the topology used in this example.

Figure 1: IS-IS Link-State PDU 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 link-state PDU interval.
   [edit protocols isis interface fe-1/2/0.0]user@R1# set lsp-interval 1000
4. (Optional) Enable tracing for tracking link-state PDU operations.
   [edit protocols isis traceoptions]user@R1# set file isis-traceuser@R1# set flag lsp

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 Link-State PDU Interval
• Checking the Link-State PDU Statistics
• Checking the Trace Log

Verifying the Link-State PDU Interval

Purpose

Check the link-state PDU interval setting on the IS-IS interface.

Action

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

fe-1/2/0.0
  Index: 70, State: 0x6, Circuit id: 0x1, Circuit type: 3
  LSP interval: 1000 ms, CSNP interval: 10 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 link-state PDU interval is set to 1000 milliseconds.

Checking the Link-State PDU Statistics

Purpose

Check the number of link-state PDUs 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                  24           24            0           13            0
IIH                2467           24            0          836            0
CSNP                474          474            0            0            0
PSNP                  0            0            0            0            0
Unknown               0            0            0            0            0
Totals             2965          522            0          849            0

Total packets received: 2965 Sent: 849

SNP queue length: 0 Drops: 0
LSP queue length: 0 Drops: 0
SPF runs: 14
Fragments rebuilt: 15
LSP regenerations: 6
Purges initiated: 0

IS-IS statistics for R2:
PDU type       Received    Processed        Drops         Sent       Rexmit
LSP                  13           13            0           24            0
IIH                 828           15            0         2459            0
CSNP                  0            0            0          474            0
PSNP                  0            0            0            0            0
Unknown               0            0            0            0            0
Totals              841           28            0         2957            0

Total packets received: 841 Sent: 2957

SNP queue length: 0 Drops: 0
LSP queue length: 0 Drops: 0
SPF runs: 17
Fragments rebuilt: 26
LSP regenerations: 11
Purges initiated: 0

Meaning

The output shows the number of link-state PDUs sent and received on Device R1 and Device R2.

Checking the Trace 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 lsp command.

Jun 18 15:27:02.692031 Received L1 LSP R2.00-00, on interface fe-1/2/0.0
Jun 18 15:27:02.692753 Updating L1 LSP R2.00-00 in TED
Jun 18 15:27:44.396480 Updating L1 LSP R1.00-00 in TED
Jun 18 15:27:45.398077 Sending L1 LSP R1.00-00 on interface fe-1/2/0.0
Jun 18 15:28:44.689024 Received L1 LSP R2.02-00, on interface fe-1/2/0.0
Jun 18 15:28:44.689663 Updating L1 LSP R2.02-00 in TED
Jun 18 15:29:15.954900 Updating L2 LSP R1.00-00 in TED
Jun 18 15:29:16.955620 Sending L2 LSP R1.00-00 on interface fe-1/2/0.0
Jun 18 15:29:28.789986 Received L2 LSP R2.00-00, on interface fe-1/2/0.0
Jun 18 15:29:28.790620 Updating L2 LSP R2.00-00 in TED
Jun 18 15:30:27.727892 Received L2 LSP R2.02-00, on interface fe-1/2/0.0
Jun 18 15:30:27.728519 Updating L2 LSP R2.02-00 in TED

Meaning

The output shows that Level 1 and Level 2 link-state PDUs are being sent and received roughly every 1000 milliseconds (1 second).