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

L2 and L3 services with remote SIDs from W-LIB

Updated: September 8, 2026

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Explains how headends install remote 32-bit W-LIB service SIDs to steer Layer 2 and Layer 3 traffic and support service-function chaining.

An L2 and L3 service with remote SIDs from Wide Local ID Block (W-LIB) is a SRv6 feature that

uses SRv6 to enable a headend node to receive and install remote Segment Identifiers (SIDs) with wide (32-bit) functions
supports both Layer and Layer service steering through these remote SIDs, and
facilitates precise packet forwarding and service function chaining by embedding service SIDs into packet headers to steer traffic along specific paths including the intended service functions

This capability is enabled by default and allows the source node to insert a service SID into the packet header, which uniquely identifies a specific service function and directs the packet through the network accordingly.

Table 1. Feature History Table
Feature Name	Release Information	Feature Description
SRv6 Services: L2 and L3 Services with Remote SIDs from Wide Local ID Block	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
SRv6 Services: L2 and L3 Services with Remote SIDs from Wide Local ID Block	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
SRv6 Services: L2 and L3 Services with Remote SIDs from Wide Local ID Block	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
SRv6 Services: L2 and L3 Services with Remote SIDs from Wide Local ID Block	Release 7.9.1	This feature enables an SRv6 headend node to receive and install remote SIDs with Wide (32-bit) functions (Remote W-LIB). The Remote W-LIB is supported for Layer 3 (VPN/BGP global) and Layer 2 EVPN services (ELINE/ELAN). This capability is enabled by default.

How the L2 and L3 services using remote SIDs from W-LIB work

Summary

The SRv6 headend node, source node, and receiver are the key components involved in SRv6 services with remote SIDs from W-LIB. Together, these components manage the identification, insertion, signaling (through transposition and SRv6 SID Structure Sub-Sub-TLV), and reassembly of SRv6 Service SIDs. This collaborative effort ensures precise Layer 2 and Layer 3 traffic steering for service function chaining and optimizes the compression of service prefix NLRIs in BGP update messages.

Workflow

These stages describe how the L2 and L3 services using remote SIDs from W-LIB work.

The source node inserts an SRv6 Service SID into the packet header to identify and steer the packet through the intended service function path.
The headend node signals the Service SID by transposing variable parts of the SRv6 SID value (function, argument, or both) into existing label fields, which optimizes compression of service prefix NLRIs in BGP updates.
The SRv6 SID Structure Sub-Sub-TLV (SSTLV) carries appropriate length fields that enable the receiver to accurately reassemble the split SRv6 Service SID parts. The Transposition Offset specifies the bit position, and the Transposition Length specifies how many bits are extracted from the SRv6 SID and placed into the high-order bits of the label field.
The receiver reassembles the split SRv6 Service SID parts based on the SSTLV information.
The headend node steers the packet along the specific path that includes the required service function, using the reassembled Service SID.

Remote W-LIB uSID structure with SRv6 SID SSTLV

A remote W-LIB uSID is defined by specific parameters within the SRv6 SID SSTLV that describe the composition and transposition of the SID value and label.

The example uSID fcbb:bb00:0200:fff0:0001:: is characterized as follows:

Block length (BL) of 32 bits = fcbb:bb00
Node length (NL) of 16 bits = 0200
Function length (FL) of 32 bits = fff0:0001
Argument length (AL) of 0
Transposition length (TPOS len) of 16 bits = 0001
Transposition offset (TPOS offset) of 64 bits = fcbb:bb00:0200:fff0:

From these parameters:

The SID value is constructed as fcbb:bb00:0200:fff0::, which includes the block, node, and function parts without the transposed argument.
The Label value extracted through transposition is 0x0001, representing the transposed bits from the original SID.

Interpret BGP VPNv4 route table output for a prefix learned from multiple egress PEs

Understand the details of BGP paths for a VPNv4 prefix learned from three egress Provider Edge (PE) routers

This task helps network engineers analyze BGP route entries, including path attributes and Segment Routing (SR) information, to troubleshoot or verify VPNv4 routing

Procedure

View the BGP route table for a VPNv4 prefix learned from three egress PEs.

BGP Path 1 from next-hop 7::1 and a 32-bit uDT4 function (0xfff0 4002) allocated from W-LIB
BGP Path 2 from next-hop 9::1 and a 16-bit uDT4 function (0x4002) allocated from LIB
BGP Path 3 from next-hop 8::1 and a 16-bit uDT4 function (0x4002) allocated from LIB

The example shows output of a BGP route table for a VPNv4 prefix learned from three egress PEs:

Example:

Router# show bgp vpnv4 unicast rd 100:2 2.2.0.1/32 detail 

BGP routing table entry for 2.2.0.1/32, Route Distinguisher: 100:2
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                5314         5314
    Flags: 0x20061292+0x00060000; multipath; backup available; 
Last Modified: Jan 20 14:37:59.189 for 00:00:19
Paths: (3 available, best #1)
  Not advertised to any peer
 Path #1: Received by speaker 0
  Flags: 0x2000000085070005+0x00, import: 0x39f
  Not advertised to any peer
  Local
    7::1 (metric 20) from 2::1 (192.0.0.1), if-handle 0x00000000
      Received Label 0x40020
      Origin IGP, localpref 150, valid, internal, best, group-best, multipath, import-candidate, imported
      Received Path ID 1, Local Path ID 1, version 5314
      Extended community: RT:100:2 
      Originator: 192.0.0.1, Cluster list: 2.0.0.1
      PSID-Type:L3, SubTLV Count:1, R:0x00,
       SubTLV:
        T:1(Sid information), Sid:fccc:cc00:7001:fff0::, F:0x00, R2:0x00, Behavior:63, R3:0x00, SS-TLV Count:1
         SubSubTLV:
          T:1(Sid structure):
           Length [Loc-blk,Loc-node,Func,Arg]:[32,16,32,0], Tpose-len:16, Tpose-offset:64
      Source AFI: VPNv4 Unicast, Source VRF: VRF_2, Source Route Distinguisher: 100:2
  Path #2: Received by speaker 0
  Flags: 0x2000000084060005+0x00, import: 0x096
  Not advertised to any peer
  Local
    9::1 (metric 20) from 2::1 (192.0.0.3), if-handle 0x00000000
      Received Label 0x40020
      Origin IGP, localpref 100, valid, internal, backup(protect multipath), add-path, import-candidate, imported
      Received Path ID 2, Local Path ID 5, version 5314
      Extended community: RT:100:2 
      Originator: 192.0.0.3, Cluster list: 2.0.0.1
      PSID-Type:L3, SubTLV Count:1, R:0x00,
       SubTLV:
        T:1(Sid information), Sid:fccc:cc00:9001::, F:0x00, R2:0x00, Behavior:63, R3:0x00, SS-TLV Count:1
         SubSubTLV:
          T:1(Sid structure):
           Length [Loc-blk,Loc-node,Func,Arg]:[32,16,16,0], Tpose-len:16, Tpose-offset:48
      Source AFI: VPNv4 Unicast, Source VRF: VRF_2, Source Route Distinguisher: 100:2
  Path #3: Received by speaker 0
  Flags: 0x2000000084070005+0x00, import: 0x296
  Not advertised to any peer
  Local
    8::1 (metric 20) from 2::1 (192.0.0.2), if-handle 0x00000000
      Received Label 0x40020
      Origin IGP, localpref 150, valid, internal, multipath, backup, add-path, import-candidate, imported
      Received Path ID 3, Local Path ID 4, version 5314
      Extended community: RT:100:2 
      Originator: 192.0.0.2, Cluster list: 2.0.0.1
      PSID-Type:L3, SubTLV Count:1, R:0x00,
       SubTLV:
        T:1(Sid information), Sid:fccc:cc00:8001::, F:0x00, R2:0x00, Behavior:63, R3:0x00, SS-TLV Count:1
         SubSubTLV:
          T:1(Sid structure):
           Length [Loc-blk,Loc-node,Func,Arg]:[32,16,16,0], Tpose-len:16, Tpose-offset:48
      Source AFI: VPNv4 Unicast, Source VRF: VRF_2, Source Route Distinguisher: 100:2

Note the following fields in the output:

Function length of 16 bits for LIB and 32 bits for W-LIB
Transposition offset (Tpose-offset) value of 48 bits for LIB and 64 bits for W-LIB
Transposition length (Tpose-len) value of 16 bits for LIB/W-LIB

L3 services with local SIDs from W-LIB

L3 services with local SIDs from W-LIB are network services that

use SRv6 service SIDs to identify specific service functions
steer packets along defined paths including those service functions, and
enable flexible and efficient service delivery by allocating uSIDs from the W-LIB space.

Table 2. Feature History Table
Feature Name	Release	Description
SRv6-Services: L3 Services with Local SIDs from W-LIB	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
SRv6-Services: L3 Services with Local SIDs from W-LIB	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
SRv6-Services: L3 Services with Local SIDs from W-LIB	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
SRv6-Services: L3 Services with Local SIDs from W-LIB	Release 7.11.1	This feature enables an SRv6 headend node to allocate and advertise local SIDs with Wide (32-bit) functions (Local W-LIB). The headend router utilizes the local W-LIB functionality to define and implement SR policies using SRv6 SIDs. The Local W-LIB is supported for Layer 3 (VPNv4/VPNv6/BGPv4/BGPv6 global) services. This feature introduces the usid allocation wide-local-id-block command.

SRv6 service SID and BGP uSID allocation

An SRv6 service SID uniquely identifies a specific service function within a network. The source node inserts this service SID into the packet header to steer the packet along a defined path that includes the targeted service function. This mechanism provides enhanced flexibility and control over packet processing, enabling efficient service delivery across the network.

Key points regarding uSID allocation and BGP transposition include:

By default, BGP instructs the SID-Manager to allocate uSIDs from the LIB space only. When enabled, BGP can enforce uSID allocation from the W-LIB space.
For VPN services, BGP performs transposition of the service SID into the label part of the NLRI as specified in IETF RFC 9252. In the LIB implementation, the 16-bit function is transposed to the NLRI label field.
In the W-LIB implementation, BGP transposes the last 16 bits of the 32-bit W-LIB function to the NLRI label field for VPNv4 and VPNv6 routes.
There is no transposition for BGPv4/BGPv6 global routes.

Limitations of L3 services with local SIDs from W-LIB

The usid allocation wide-local-id-block command cannot be used together with the alloc mode route-policy command in BGP. The wide-local-id-block SID allocation does not support dynamic allocation from locators based on route-policy selection.

Support for Cisco Silicon One ASICs

This feature is supported on Cisco 8000 Series Routers and Line Cards with Cisco Silicon One Q200 and P100 ASICs.
This feature is not supported on Cisco 8000 Series Routers and Line Cards with Cisco Silicon One Q100 ASICs.

Configure L3 services with local SIDs from W-LIB

Enable the allocation and advertisement of an SRv6 service SID using the wide-local-id-block (W-LIB) mode to support L3 services

The W-LIB allocation mode applies precedence rules at various configuration levels within BGP to control how uSIDs are allocated and advertised.

Procedure

Apply W-LIB uSID allocation globally under BGP

Example:

router bgp 1
  segment-routing srv6
    usid allocation wide-local-id-block
  !

Apply W-LIB uSID allocation at the IPv4 and IPv6 address family levels under BGP.

Example:

router bgp 1
  address-family ipv4 unicast
    segment-routing srv6
      usid allocation wide-local-id-block
  !
  address-family ipv6 unicast
    segment-routing srv6
      usid allocation wide-local-id-block

Apply W-LIB uSID allocation for all VPNv4 and VPNv6 address families.

Example:

router bgp 1
  address-family ipv4 unicast
    segment-routing srv6
      usid allocation wide-local-id-block
   !
  address-family ipv6 unicast
    segment-routing srv6
      usid allocation wide-local-id-block

Apply W-LIB uSID allocation at the VRF level for IPv4 and IPv6 address families.

Example:

router bgp 1
  address-family vpnv4 unicast
    vrf all
      segment-routing srv6
        usid allocation wide-local-id-block
    !
  address-family vpnv6 unicast
    vrf all
      segment-routing srv6
        usid allocation wide-local-id-block

Use the show commands to verify the W-LIB uSID allocation.

Verify the W-LIB uSID allocation.

Example:

RP/0/0/CPU0:PE1# show bgp ipv4 unicast process

BGP Process Information:
BGP is operating in STANDALONE mode
Autonomous System number format: ASPLAIN
Autonomous System: 100
Router ID: 192.168.0.1
Default Cluster ID: 192.168.0.1
Active Cluster IDs:  192.168.0.1
Fast external fallover enabled
Platform Loadbalance paths max: 16
Platform RLIMIT max: 2147483648 bytes
Maximum limit for BMP buffer size: 409 MB
Default value for BMP buffer size: 307 MB
Current limit for BMP buffer size: 307 MB
Current utilization of BMP buffer limit: 0 B
Neighbor logging is enabled
Enforce first AS enabled
AS Path multipath-relax is enabled
Use SR-Policy admin/metric of color-extcomm Nexthop during path comparison: disabled
Default local preference: 100
Default keepalive: 60
Graceful restart enabled
Restart time: 120
Stale path timeout time: 360
RIB purge timeout time: 600
Non-stop routing is enabled
ExtComm Color Nexthop validation: RIB

Update delay: 120
Generic scan interval: 15
Configured Segment-routing Local Block: [0, 0]
In use Segment-routing Local Block: [15000, 15999]
Platform support mix of sr-policy and native nexthop: No
Segment Routing SRv6 Locator Name: LOC2
Segment Routing SRv6 uSID WLIB allocation: Enforced

Address family: IPv4 Unicast
Dampening is enabled
Client reflection is enabled in global config
Dynamic MED is Disabled
Dynamic MED interval : 10 minutes
Dynamic MED Timer : Running, will expire in 342 seconds
Dynamic MED Periodic Timer : Running, will expire in 42 seconds
Scan interval: 60
Total prefixes scanned: 42
Prefixes scanned per segment: 100000
Number of scan segments: 1
Nexthop resolution minimum prefix-length: 0 (not configured)
IPv6 Nexthop resolution minimum prefix-length: 0 (not configured)
Main Table Version: 44
Table version synced to RIB: 44
Table version acked by RIB: 44
IGP notification: IGPs notified
RIB has converged: version 0
RIB table prefix-limit reached ?  [No], version 0
Permanent Network Unconfigured
Segment Routing SRv6 Alloc Mode: 0
Segment Routing SRv6 uSID WLIB allocation: Enforced

RP/0/0/CPU0:PE1# show bgp vrf all ipv4 unicast process

VRF: foo
-------

BGP Process Information: VRF foo
BGP Route Distinguisher: 23:1

BGP is operating in STANDALONE mode
Autonomous System number format: ASPLAIN
Autonomous System: 100
Router ID: 192.168.0.1
Default Cluster ID: 192.168.0.1
Active Cluster IDs:  192.168.0.1
Fast external fallover enabled
Platform Loadbalance paths max: 16
Platform RLIMIT max: 2147483648 bytes
Maximum limit for BMP buffer size: 409 MB
Default value for BMP buffer size: 307 MB
Current limit for BMP buffer size: 307 MB
Current utilization of BMP buffer limit: 0 B
Neighbor logging is enabled
Enforce first AS enabled
iBGP to IGP redistribution enabled
AS Path multipath-relax is enabled
Use SR-Policy admin/metric of color-extcomm Nexthop during path comparison: disabled
Default local preference: 100
Default keepalive: 60
Graceful restart enabled
Restart time: 120
Stale path timeout time: 360
RIB purge timeout time: 600
Non-stop routing is enabled
ExtComm Color Nexthop validation: RIB

Update delay: 120
Generic scan interval: 15
Configured Segment-routing Local Block: [0, 0]
In use Segment-routing Local Block: [15000, 15999]
Platform support mix of sr-policy and native nexthop: No
Segment Routing SRv6 Locator Name: LOC2 (WLIB allocation enforced)
Segment Routing SRv6 uSID WLIB allocation: Enforced

VRF foo Address family: IPv4 Unicast
Dampening is enabled
Client reflection is not enabled in global config
Dynamic MED is Disabled
Dynamic MED interval : 10 minutes
Dynamic MED Timer : Not Running
Dynamic MED Periodic Timer : Not Running
Scan interval: 60
Total prefixes scanned: 85
Prefixes scanned per segment: 100000
Number of scan segments: 1
Nexthop resolution minimum prefix-length: 0 (not configured)
IPv6 Nexthop resolution minimum prefix-length: 0 (not configured)
Main Table Version: 152
Table version synced to RIB: 152
Table version acked by RIB: 152
IGP notification: IGPs notified
RIB has converged: version 1
RIB table prefix-limit reached ?  [No], version 0
Permanent Network Unconfigured
Segment Routing SRv6 uSID WLIB allocation: Enforced

Verify the advertised SRv6 W-LIB uSID for the default VRF.

Example:

RP/0/0/CPU0:PE1# show bgp ipv4 unicast 192.168.4.1/32

BGP routing table entry for 192.168.4.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 419          419
    SRv6-VPN SID: fccc:cccc:a:fff0:4::/80
Last Modified: Apr  3 10:35:41.000 for 136y10w
Paths: (1 available, best #1)
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    192::4
  Path #1: Received by speaker 0
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    192::4
  Local
    0.0.0.0 from 0.0.0.0 (192.168.0.1)
      Origin incomplete, metric 0, localpref 100, weight 32768, valid, redistributed, best, group-best
      Received Path ID 0, Local Path ID 1, version 419

Verify the advertised SRv6 W-LIB uSID for a specific VRF (foo).

Example:

RP/0/0/CPU0:PE1# show bgp vrf foo 192.168.7.1/32 

BGP routing table entry for 192.168.7.1/32, Route Distinguisher: 23:1
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 439          439
    SRv6-VPN SID: fccc:cccc:a:fff0:4::/80
Last Modified: Apr  3 10:31:00.000 for 00:00:44
Paths: (1 available, best #1)
  Advertised to PE peers (in unique update groups):
    192::4
  Advertised to CE peers (in unique update groups):
    10.10.10.2
  Path #1: Received by speaker 0
  Advertised to PE peers (in unique update groups):
    192::4
  Advertised to CE peers (in unique update groups):
    10.10.10.2
  Local
    0.0.0.0 from 0.0.0.0 (192.168.0.1)
      Origin incomplete, metric 0, localpref 100, weight 32768, valid, redistributed, best, group-best, import-candidate
      Received Path ID 0, Local Path ID 1, version 439
      Extended community: RT:23:23

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