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

Explicit End DT46 SRv6 SIDs

Updated: September 8, 2026

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Explains how persistent End.DT46 SIDs support dual-stack L3VPN and BGP services and provides guidance for assigning explicit SIDs and custom uSID allocation ranges.

Explicit End.DT46 SRv6 SIDs is a SRv6 feature that allows network administrators to configure specific Segment Identifiers (SIDs) associated with SRv6-based L3VPN/Internet BGP services, ensuring these SIDs are persistent over reloads and restarts, unlike dynamically allocated SIDs.

Table 1. Feature History Table

Feature Name

Release

Description

Support for End.DT46 SRv6 Endpoint Behavior

Release 25.4.1

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

*This feature is supported on:

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Support for End.DT46 SRv6 Endpoint Behavior

Release 25.1.1

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

This feature is now supported on:

  • 8712-MOD-M

  • 8011-4G24Y4H-I

Support for End.DT46 SRv6 Endpoint Behavior

Release 24.4.1

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

*This feature is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Support for End.DT46 SRv6 Endpoint Behavior

Release 7.5.3

This feature adds support for the “Endpoint with decapsulation and specific IP table lookup” SRv6 end-point behavior (End.DT46).

The End.DT46 behavior is used for dual-stack L3VPNs. This behavior is equivalent to the single per-VRF VPN label (for IPv4 and IPv6) in MPLS.

Support for Explicit End.DT46 SRv6 SIDs

Release 25.4.1

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

*This feature is supported on:

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Support for Explicit End.DT46 SRv6 SIDs

Release 25.1.1

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

*This feature is supported on Cisco 8011-4G24Y4H-I routers.

Support for Explicit End.DT46 SRv6 SIDs

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

Support for Explicit End.DT46 SRv6 SIDs

Release 7.5.3

This feature allows you to configure explicit SIDs associated with SRv6-based L3VPN/Internet BGP services. In previous releases, these SIDs were only allocated dynamically by BGP.

Explicit End.DT46 SRv6 SIDs are persistent over reloads and restarts.

The feature introduces these changes:

CLI:

  • The segment-routing srv6 static endpoint sid prefix behavior end-udt46 command mode is introduced.

Multiple explicit uDT46 IDs allocation examples

Multiple explicit uDT46 IDs allocated from the LIB or W-LIB range can be created under the same SRv6 locator. Each ID is uniquely associated to a VRF.

Example: Explicit uDT46 (LIB)

  • Locator: fcbb:bb00:11::/48

  • Explicit functions: fe0a (VRF-A), fe0b (VRF-B), fe0c (VRF-C)

    • fcbb:bb00:11:fe0a::/64 — Explicit 16-bit DT46 function from LIB for VRF-A

    • fcbb:bb00:11:fe0b::/64 — Explicit 16-bit DT46 function from LIB for VRF-B

    • fcbb:bb00:11:fe0c::/64 — Explicit 16-bit DT46 function from LIB for VRF-C

Example: Explicit uDT46 (W-LIB)

  • Locator: fcbb:bb00:11::/48

  • Explicit functions: fff7:d (VRF-D), fff7:e (VRF-E), fff7:f (VRF-F)

    • fcbb:bb00:11:fff7:d::/80 — Explicit 32-bit DT46 function from W-LIB for VRF-D

    • fcbb:bb00:11:fff7:e::/80 — Explicit 32-bit DT46 function from W-LIB for VRF-E

    • fcbb:bb00:11:fff7:f::/80 — Explicit 32-bit DT46 function from W-LIB for VRF-F

An explicit uDT46 ID allocated from the LIB or W-LIB range can be associated to the same VRF under multiple SRv6 locators.

This association is useful when a look-up under a given VPN table is desired for a node with multiple locators (for example, unicast and Anycast locators).

The locators can be from the same ID block or different ID blocks:

  • When the locators are from the same block, the manual uDT46 IDs for a given VRF must have the same value across locators.

  • When the locators are from different blocks, the manual uDT46 IDs for a given VRF could be either the same value or different values.

We recommend using the same function ID across locators since it allows for simpler identification to the associated VRF table.

Example 1: Explicit uDT46 SIDs (LIB) with a common function ID under 2 locators of the same block-id

  • Locator 1: fcbb:bb00:10::/48 (unicast locator)

  • Locator 2: fcbb:bb00:100::/48 (Anycast locator)

  • Explicit function: fe0a

  • VRF lookup: VRF-A

fcbb:bb00:10:fe0a::/64 segment-routing srv6 endpoint behavior uDT46 vrf VRF-A
    fcbb:bb00:100:fe0a::/64 segment-routing srv6 endpoint behavior uDT46 vrf VRF-A

Example 2: Explicit uDT46 SIDs (LIB) with a common function ID under 2 locators of different block-id

  • Locator 1: fcbb:bb00:11::/48 (unicast locator – algo0)

  • Locator 2: fcbb:bb01:11::/48 (unicast locator – algo128)

  • Explicit function: fe0b

  • VRF lookup: VRF-B

    fcbb:bb00:11:fe0b::/64 segment-routing srv6 endpoint behavior uDT46 vrf VRF-B
    fcbb:bb01:11:fe0b::/64 segment-routing srv6 endpoint behavior uDT46 vrf VRF-B

Example 3: Explicit uDT46 SIDs (W-LIB) with a common function ID under 2 locators of the same block-id

  • Locator 1: fcbb:bb00:10::/48 (unicast locator)

  • Locator 2: fcbb:bb00:100::/48 (Anycast locator)

  • Explicit function: fff7:d

  • VRF lookup: VRF-D

fcbb:bb00:11:fff7:d::/80 segment-routing srv6 endpoint behavior uDT46 vrf VRF-D
    fcbb:bb00:100:fff7:d::/80 segment-routing srv6 endpoint behavior uDT46 vrf VRF-D

Example 4: Explicit uDT46 SIDs (W-LIB) with a common function ID under 2 locators of different block-id

  • Locator 1: fcbb:bb00:11::/48 (unicast locator – algo0)

  • Locator 2: fcbb:bb01:11::/48 (unicast locator – algo128)

  • Explicit function: fff7:e

  • VRF lookup: VRF-E

fcbb:bb00:11:fff7:e::/80 segment-routing srv6 endpoint behavior uDT46 vrf VRF-E
    fcbb:bb01:11:fff7:e::/80 segment-routing srv6 endpoint behavior uDT46 vrf VRF-E

Configure explicit End DT46 SRv6 SIDs

This task explains how to configure explicit End.DT46 SRv6 Segment Identifiers (SIDs) to enable L3VPN/Internet BGP services with persistent SIDs.

Procedure

Configure explicit uDT46 IDs allocated from the LIB or W-LIB range.

Use the allocation-context vrf command to associate an explicit uDT46 ID allocated from the LIB or W-LIB to a VRF. Use the forwarding keyword if VRF-lite (the deployment of VRFs without BGP/MPLS) is enabled.

Example:

Router(config)# segment-routing
Router(config-sr)# srv6
Router(config-srv6)# static
Router(config-srv6-static)# endpoint
Router(config-srv6-static-endpoint)# sid fcbb:bb00:10:fe0a:: behavior end-udt46
Router(config-srv6-static-sid)# allocation-context vrf VRF-A
Router(config-srv6-static-sid)# forwarding
Router(config-srv6-static-sid)# exit
Router(config-srv6-static-endpoint)# sid fcbb:bb00:11:fff7:d:: behavior end-udt46
Router(onfig-srv6-static-sid)# allocation-context vrf VRF-D

Configure explicit SRv6 uSID allocation start range

Use this task to modify the starting value for the explicit Local ID Block (LIB) and explicit Wide-Local ID Block (W-LIB) ranges used in SRv6 uSID allocation.

SRv6 micro-SIDs (uSIDs) utilize Local ID Blocks (LIB) and Wide-Local ID Blocks (W-LIB) for allocation. The explicit LIB and W-LIB ranges are designated for manually assigned SIDs. Modifying the start of these explicit ranges can impact the number of available IDs in the dynamic LIB and W-LIB ranges. The default explicit LIB start value is

0xfe00, and the default explicit W-LIB start value is 0xfff7.

Before you begin

  • The usid-f3216 SID format must be configured or enabled by default.

Procedure

  1. Use the show segment-routing srv6 manager command to display the default LIB and W-LIB start values.

    Example:

    
Router# show segment-routing srv6 manager 
Fri Sep  9 18:31:06.033 UTC
Parameters:
  SRv6 Enabled: Yes
  SRv6 Operational Mode: None
  Encapsulation:
    Source Address:
      Configured: ::
      Default: ::
    Hop-Limit: Default 
    Traffic-class: Default 
  SID Formats:
    f3216 <32B/16NFA> (2)
      uSID LIB Range:
        LIB Start   : 0xe000
        ELIB Start  : 0xe064 (configured)
      uSID WLIB Range:
        EWLIB Start : 0xfff0 (configured)

. . .

  2. Use the segment-routing srv6 formats format usid-f3216 usid wide-local-id-block explicit start command to modify the start value for the explicit W-LIB.

    Example:

    
RP/0/RP0/CPU0:ios(config)# segment-routing
RP/0/RP0/CPU0:ios(config-sr)# srv6
RP/0/RP0/CPU0:ios(config-srv6)# formats
RP/0/RP0/CPU0:ios(config-srv6-fmts)# format usid-f3216
RP/0/RP0/CPU0:ios(config-srv6-fmt)# usid local-id-block explicit start 0xE064
RP/0/RP0/CPU0:ios(config-srv6-fmt)# usid wide-local-id-block explicit start 0xFFF0

  3. Use the show segment-routing srv6 manager command tto display the configured explicit LIB and W-LIB starting values.

    Example:

    
RP/0/RP0/CPU0:ios# show segment-routing srv6 manager 
Fri Sep  9 18:31:06.033 UTC
Parameters:
  SRv6 Enabled: Yes
  SRv6 Operational Mode: None
  Encapsulation:
    Source Address:
      Configured: ::
      Default: ::
    Hop-Limit: Default 
    Traffic-class: Default 
  SID Formats:
    f3216 <32B/16NFA> (2)
      uSID LIB Range:
        LIB Start   : 0xe000
        ELIB Start  : 0xe064 (configured)
      uSID WLIB Range:
        EWLIB Start : 0xfff0 (configured)

. . .