Segment Routing v6 Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Segment Routing v6 Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Path maximum transmission unit discovery for SRv6 encapsulated packets

Updated: September 8, 2026

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Explains how Path MTU Discovery identifies oversized SRv6 packets and adjusts their size to prevent silent packet loss when fragmentation is unavailable.

Path maximum transmission unit (MTU) discovery for SRv6 encapsulated packets is a network mechanism that

  • prevents silent packet loss and ensures efficient data transmission in SRv6 networks

  • dynamically adjusts encapsulated packet sizes to match the path MTU, and

  • enables routers to send ICMP error messages to the source when oversized packets are detected, addressing the critical limitation that SRv6-enabled routers do not support packet fragmentation.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

Path MTU discovery for SRv6 Packets on Ingress Provider Edge (PE) Routers, Egress (PE) Routers, and P Role Transit Nodes

Release 25.4.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100], 8010 [ASIC: A100])(select variants only*)

*This feature is supported on:

  • 8711-48Z-M

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Path MTU discovery for SRv6 Packets on Ingress Provider Edge (PE) Routers, Egress (PE) Routers, and P Role Transit Nodes

 Release 25.1.1

Introduced in this release on: Fixed Systems ( 8010 [ASIC: A100])

You can measure and monitor the packet loss information when one SRv6-enabled router sends an oversized packet to another. This functionality enables a router to send an ICMP error message to the source in such cases, prompting the sender to resend a packet whose size is within the MTU value, thus ensuring the packet moves ahead. The feature is critical for SRv6-enabled routers as these routers do not support packet fragmentation.

Previously, a router dropped oversized packets without notifying the source, resulting in packet loss.

This feature is now supported on:

  • 8011-4G24Y4H-I

Path MTU discovery for SRv6 Packets on Ingress Provider Edge (PE) Routers, Egress (PE) Routers, and P Role Transit Nodes

Release 24.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

*This feature is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 8712-MOD-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Path MTU discovery for SRv6 Packets on Ingress Provider Edge (PE) Routers, Egress (PE) Routers, and P Role Transit Nodes

Release 24.1.1

You can measure and monitor the packet loss information when one SRv6-enabled router sends an oversized packet to another. This functionality enables a router to send an ICMP error message to the source in such cases, prompting the sender to resend a packet whose size is within the MTU value, thus ensuring the packet moves ahead. The feature is critical for SRv6-enabled routers as these routers do not support packet fragmentation.

Previously, a router dropped oversized packets without notifying the source, resulting in packet loss.

The hw-module configuration is not required, this feature is enabled by default.