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Example: Reducing Jitter in Hierarchical CoS Queues

This example shows how to reduce jitter in the output queues for VLAN ports hosted on a hierarchical queuing MPC.

Requirements

This example uses the following Juniper Networks hardware and Junos OS software:

  • MX960 router in an IPv4 network and running Junos OS Release 13.2 or later.

  • Available Gigabit Ethernet port hosted on FPC slot 2, PIC slot 0, port 0.

  • Available Gigabit Ethernet port hosted on port 0 of a Gigabit Ethernet Modular Interface Card (MIC) in PIC slot 0 of an MPC2 Q Modular Port Concentrator (MPC) in FPC slot 5.

Before you begin configuring this example, make sure that the maximum number of queues allowed for the hierarchical queuing MPC in slot 5 has not yet been configured. When you enter the show chassis fpc 5 command from configuration mode, the max-queues statement should not display.

Overview

In this example you configure hierarchical scheduling on a VLAN port hosted on a hierarchical queuing MPC. To reduce jitter in the queues for all egress ports hosted on the MPC, reduce the maximum number of queues allowed for MPC.

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.

Baseline Configuration

Step-by-Step Procedure

Configure hierarchical scheduling at xe-5.0.0.

  1. To configure the VLAN 1 input and output at xe-2/0/0.0 and xe-5/0/0.0:

  2. Map each of four queues to a forwarding class.

  3. Assign a packet-scheduling priority value to each forwarding class.

  4. Customize the default IEEE 802.1p classifier (BA classifier based on Layer 2 header) by defining different values for iEEE 802.1p code points.

  5. Apply the BA classifier to the input of the logical units on xe-2/0/0.

  6. Configure the scheduler map smap_jitter to map the forwarding classes to the schedulers.

  7. Configure the traffic control profile tcp to combine the scheduler map smap_jitter (that maps the forwarding classes to the schedulers for port-based scheduling) with a shaping rate (for hierarchical scheduling).

  8. Apply the traffic control profile to the router output at xe-5/0/0.

  9. If you are done configuring the device, commit the configuration.

Results

Confirm your configuration by entering show interfaces and show cloass-of-service commands from configuration mode. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

Confirm that the configuration is working properly

Measuring End-to-End Jitter to Establish the Baseline

Purpose

Establish a baseline measurement by noting the amount of jitter that occurs when the hierarchical queuing line card hosting the egress port is configured with the default maximum number of queues.

Action

To measure jitter:

  1. Pass traffic through the VLAN.

  2. Measure the variation in packet delay for selected packets in the data flow.

Configuring Jitter Reduction

Purpose

Reduce jitter in the VLAN port output queues.

Action
  1. Configure a reduced maximum number of queues for egress ports on the hierarchical queuing MPC in slot 5, thereby reducing the jitter in the port queues.

  2. If you are done configuring the device, commit the configuration.

Measuring End-to-End Jitter to Verify Jitter Reduction

Purpose

Measure the amount of jitter that occurs when the hierarchical queuing line card hosting the egress port is configured with a reduced maximum number of queues.

Action

To measure jitter:

  1. Pass traffic through the VLAN.

  2. Measure the variation in packet delay for selected packets in the data flow.