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ECN Support on MPLS Networks

MPLS ECN (Explicit Congestion Notification) support in Junos utilizes the MPLS EXP field to encode ECN codepoints, enabling explicit congestion management across MPLS networks. By integrating ECN, you enhance traffic management and ensure efficient packet flow, mitigating congestion and preventing packet loss. This feature requires configuring ECN codepoints and maintaining consistent ECN mappings across network nodes for seamless operation.

Understand ECN Support on MPLS Networks

Benefits of MPLS ECN Support

  • MPLS ECN support enhances traffic management by providing explicit congestion notifications, optimizing packet flow across MPLS networks.

  • It reduces packet loss by enabling network nodes to respond to congestion signals promptly, ensuring data integrity and improved transmission efficiency.

  • It allows for precise congestion handling through the use of ECN pairs, ensuring that congestion signals are accurately communicated and addressed.

Overview

Implementing MPLS ECN support in Junos involves using the MPLS EXP field to encode ECN codepoints, enabling explicit congestion management. By configuring ECN codepoints, network nodes like ingress label edge routers (LERs), LSRs, and egress LERs can effectively communicate congestion occurrences. These nodes use ECN pairs within the MPLS EXP field to indicate whether a packet has experienced congestion or is ECN-capable, ensuring accurate communication and response to congestion signals. This setup enhances the network's ability to manage traffic, reduce packet loss, and maintain high transmission efficiency.

To configure MPLS ECN, use mpls-ecn-map statement at the [edit class-of-service] hierarchy level to define ECN mappings. For example, set class-of-service mpls-ecn-map ecn-capable-exp 6 congestion-experienced-exp 7 specifies if a packet with EXP 6 (110) is received on an LSR and if it experiences congestion, then the outgoing EXP will be 7 (111). Verification of these configurations can be done via show class-of-service mpls-ecn-map, ensuring consistency across all network nodes. This consistency is crucial for effective congestion management and achieving the benefits of reduced packet loss and optimized traffic flow.

An important consideration when configuring MPLS ECN is the mutual exclusivity between MPLS EXP rewrite and MPLS ECN features on LSRs. There are four codepoints used for ECN marking, which are encoded using two bits of the IP header. The MPLS EXP field is the logical place to encode ECN codepoints, but with only 3 bits (8 codepoints) available, and with the same field being used to convey DSCP information as well, there is a clear incentive to conserve the number of codepoints consumed for ECN purposes. Recognizing these limitations is essential to avoid configuration conflicts that could degrade network performance. Ensure that ECN configurations are consistently applied across your network infrastructure, especially in environments running Junos on various switch variants.

Configure ECN Support on MPLS Networks

Use CLI statements like set class-of-service mpls-ecn-map to establish ECN configurations, and verify them with show class-of-service mpls-ecn-map. Acknowledging the mutual exclusivity between MPLS EXP rewrite and MPLS ECN on label-switching routers (LSRs) is crucial to avoid conflicts, ensuring robust and efficient network functionality.

Be vigilant about the mutual exclusivity constraint between MPLS EXP rewrite and MPLS ECN, as failure to do so can result in operational conflicts. These guidelines help maintain a streamlined network setup that leverages ECN for enhanced traffic management and efficient packet flow.

  1. On the ingress LER, define an ECN data forwarding class.
  2. On the ingress LER, configure a firewall filter that matches on specified ECN values and assigns the packets to the ECN data forwarding class and specified loss priority.

    Here, ingress packets that have a DSCP value of 22 (010110) and ECN value of 1 (01, ECN-capable) are assigned to the ECN data forwarding class with a low loss priority. Ingress packets that have a DSCP value of 22 and ECN value of 3 (11, congestion experienced) are assigned to the ECN data forwarding class with a high loss priority.

  3. On the ingress LER, apply the firewall filter to an ingress interface.
  4. On the ingress LER, define a rewrite rule for the ECN-marked MPLS packets.

    ECN-capable packets, those with a low loss priority, are assigned an EXP value of 110 (6). ECN packets experiencing congestion, those with a high loss priority, are assigned an EXP value of 111 (7).

  5. One the ingress LER, assign the rewrite rule to an egress interface.
  6. One the LSR, define and MPLS ECN map that maps what EXP value an ECN-capable packet should be assigned if it experiences congestion.

    Here, if a packet with EXP 6 (110) is received on the LSR and if it experiences congestion, then the outgoing EXP will be 7 (111).

    Note:

    This configuration is global and applies to all ports.

Run the show class-of-service mpls-ecn-map command to confirm your configuration:

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