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

PDF

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

Limitations for uSID

Updated: September 8, 2026

Want to summarize with AI?

Log in

Identifies the endpoint behaviors, encapsulation options, packet attributes, and hardware capabilities supported or restricted when deploying uSIDs.

Ensure that you follow these limitations when working with SRv6 uSID behaviors:

Supported SRv6 uSID endpoint behaviors

The SRv6 network programming is extended with new types of SRv6 SID endpoint behaviors:

  • uN—uN is the short notation for the NEXT-CSID (Compressed SID) End behavior with a pseudocode of shift-and-lookup, and PSP/USD flavors.

  • uA—uA is the short notation for the NEXT-CSID End.X behavior with a pseudocode of shift-and-xconnect, and PSP/USD flavors.

  • uDT—uDT is the short notation for the NEXT-CSID End.DT behavior with the same pseudocode as End.DT4/End.DT6/End.DT46/End.DT2U/End.DT2M.

    Starting from Cisco IOS XR Release 7.5.3, End.DT46 endpoint is supported.

  • uDX—uDX is the short notation for the NEXT-CSID End.DX behavior with the same pseudocode as End.DX4/End.DX6/End.DX2.

Supported SRv6 uSID headend behaviors

  • H.Encap.Red (1 uSID carrier with up to 6 uSIDs)

  • H.Insert.Red : Starting from IOS XR Release 24.3.1, H.Insert.Red is supported only on Cisco Silicon One P100-based routers.

Limitations for encapsulation capabilities and Parameters

Ensure that you adhere to these requirements when configuring IPv6 header fields for SRv6 encapsulated packets.

  • Source address: Supports a single source address (SA) for SRv6 encapsulated packets. This source address is derived from the SRv6 global configuration or, if not configured, from the IPv6 Loopback address.

  • Hop limit:

    • Do not manually configure the hop-limit value in the outer IPv6 header, as this action is not supported.

    • Overlay encapsulation does not propagate the hop limit by default. Use the hop-limit propagate command to enable propagation from the inner header to the outer header.

    • Underlay encapsulation (TI-LFA) behavior is always in propagate mode, regardless of the CLI.

  • Traffic-class:

    • By default, traffic-class propagation is disabled for overlay encapsulation unless you enable it with the traffic-class propagate command, which allows propagation from the inner header to the outer header.

    • Manual configuration of the traffic-class value in the outer IPv6 header is not supported.

    • Underlay encapsulation (TI-LFA) always uses propagate mode, regardless of the CLI configuration.

  • Flow Label:

    • Cisco 8000 series routers use the flow-label from the incoming IPv6 header. During USD operations, flow-label is used from the inner IPv6 header.

    • During H.Encap.Red operations, if the inner packet has a flow label (non-zero value), the Cisco 8000 series routers propagate it to the outer IPv6 header. If the flow label is not present (zero), it is computed.

  • Underlay H-Encapsulation (P Role): A maximum of 6 SIDs (Segment Identifiers) per carrier is used for SRH underlay encapsulation. This applies to P devices responsible for maintaining the underlay transport.

  • Underlay H-Insertion (PE Role):A maximum of 3 SIDs (1 carrier with 3 SIDs per carrier) is used for SRH underlay insertion. This applies to PE devices integrating underlay routing.

  • Overlay H-Encapsulation (PE Role): A maximum of 3 SIDs (1 carrier with 3 SIDs per carrier) is used for SRH overlay encapsulation. This applies to PE devices managing overlay routing.