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Example: Configuring ECN

Requirements

• One Juniper Networks QFX Series switch
• Junos OS Release 13.2X51-D25 or later for the QFX Series

Overview

ECN enables end-to-end congestion notification between two endpoints on TCP/IP based networks. The two endpoints are an ECN-enabled sender and an ECN-enabled receiver. ECN must be enabled on both endpoints and on all of the intermediate devices between the endpoints for ECN to work properly. Any device in the transmission path that does not support ECN breaks the end-to-end ECN functionality

A weighted random early detection (WRED) packet drop profile must be applied to the output queues on which ECN is enabled. ECN uses the WRED drop profile thresholds to mark packets when the output queue experiences congestion.

ECN reduces packet loss by forwarding ECN-capable packets during periods of network congestion instead of dropping those packets. (TCP notifies the network about congestion by dropping packets.) During periods of congestion, ECN marks ECN-capable packets that egress from congested queues. When the receiver receives an ECN packet that is marked as experiencing congestion, the receiver echoes the congestion state back to the sender. The sender then reduces its transmission rate to clear the congestion.

ECN is disabled by default. You can enable ECN on best-effort traffic. ECN should not be enabled on lossless traffic queues, which uses priority-based flow control (PFC) for congestion notification, and ECN should not be enabled on strict-high priority traffic queues.

To enable ECN on an output queue, you not only need to enable ECN in the queue scheduler, you also need to:

• Configure a WRED packet drop profile.
• Configure a queue scheduler that includes the WRED drop profile and enables ECN. (This example shows only ECN and drop profile configuration; you can also configure bandwidth, priority, and buffer settings in a scheduler.)
• Map the queue scheduler to a forwarding class (output queue).
• Add the forwarding class to a forwarding class set (priority group, for hierarchical scheduling).
• Associate the queue scheduler with a traffic control profile (priority group scheduler for hierarchical scheduling).
• Apply the traffic control profile and the forwarding class set to an interface. On that interface, the specified output queue (the queue mapped to the forwarding class) uses the scheduler that is mapped to the traffic control profile, which enables ECN on the queue and applies the WRED drop profile to the queue.

Table 1 shows the configuration components for this example.

Configuration

CLI Quick Configuration

To quickly configure the drop profile, scheduler with ECN enabled, and to map the scheduler to an output queue on an interface, copy the following commands, paste them in a text file, remove line breaks, change variables and details to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level:

Configuring ECN

Step-by-Step Procedure

1. Configure the WRED packet drop profile be-dp. This example uses a drop start point of 30 percent, a drop end point of 75 percent, a minimum drop rate of 0 percent, and a maximum drop rate of 80 percent:
   [edit class-of-service]
user@switch# set drop-profile be-dp interpolate fill-level 30 fill-level 75 drop-probability 0 drop-probability 80

   
   

2. Create the scheduler be-sched with ECN enabled and associate the drop profile be-dp with the scheduler:
   [edit class-of-service]
user@switch# set schedulers be-sched explicit-congestion-notification
user@switch# set schedulers be-sched drop-profile-map loss-priority low protocol any drop-profile be-dp
user@switch# set be-sched transmit-rate percent 25
user@switch# set be-sched shaping-rate percent 50
user@switch# set be-sched buffer-size percent 25
user@switch# set be-sched priority low

   
   

3. Map the scheduler be-sched to the best-effort forwarding class (output queue 0) using scheduler map be-map:
   [edit class-of-service]
user@switch# set scheduler-maps be-map forwarding-class best-effort scheduler be-sched

   
   

4. Add the forwarding class best-effort to the forwarding class set be-pg:
   [edit class-of-service]
user@switch# set forwarding-class-sets be-pg class best-effort

   
   

5. Associate the scheduler map be-mapwith the traffic control profile be-tcp:
   [edit class-of-service]
user@switch# set traffic-control-profiles be-tcp scheduler-map be-map

   
   

6. Associate the traffic control profile be-tcp and the forwarding class set be-pg with the interface on which you want to enable ECN on the best-effort queue:
   [edit class-of-service]
user@switch# set interfaces xe-0/0/20 forwarding-class-set be-pg output-traffic-control-profile be-tcp

   
   

Verification

To verify that ECN is enabled, perform the following task:

• Verifying That ECN Is Enabled

Verifying That ECN Is Enabled

Purpose

Verify that ECN is enabled in the scheduler be-sched by showing the configuration for the scheduler map be-map.

Action

Scheduler map: be-map, Index: 12240

  Scheduler:be-sched, Forwarding class: best-effort, Index: 115
    Transmit rate: 25 percent, Rate Limit: none, Buffer size: 25 percent,
    Buffer Limit: none, Priority: low
    Excess Priority: unspecified, Explicit Congestion Notification: enable
    Drop profiles:
      Loss priority   Protocol    Index    Name
      Low             any          3312    be-dp           
      Medium-high     any          1       <default-drop-profile>
      High            any          1       <default-drop-profile>

Meaning

• The scheduler associated with the scheduler map is be-sched.
• The scheduler map applies to the forwarding class best-effort (output queue 0).
• The scheduler be-sched has a transmit rate of 25 percent, a queue buffer size of 25 percent, and a drop priority of low.
• Explicit congestion notification state is enable.
• The WRED drop profile used for low drop priority traffic is be-dp.