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

Key concepts of SRv6 IS-IS flexible algorithm

Updated: September 8, 2026

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Describes how Flexible Algorithm and SRv6 support customized IS-IS shortest-path computation.

SRv6 Flexible Algorithm enables operators to customize IGP shortest-path computation according to their network requirements.

Operators can assign custom SR prefix SIDs for forwarding beyond link-cost-based SPF. The IGP then computes a traffic-engineered path to a destination reachable through the IGP.

SRv6 IS-IS flexible algorithm guidelines and restrictions

Determines the supported locator and algorithm configuration values.

You can configure up to 8 locators to support SRv6 Flexible Algorithm:

  • All locators prefix must share the same SID block (first 40-bits).

  • The Locator Algorithm value range is 128 to 255.

Configure SRv6 IS-IS flexible algorithms

Set up SRv6 IS-IS flexible algorithms on your Cisco network device to enable low-latency and best-effort traffic engineering based on programmatically defined metrics

SRv6 IS-IS flexible algorithms allow you to create customized path calculations for specific traffic classes, such as low-latency applications.

Before you begin

Complete the configure SRv6 before you configure flexible algorithm.

Follow these steps to configure SRv6 IS-IS flexible algorithms:

Procedure

  1. Configure SRv6 IS-IS flexible algorithms

    Example:

    Router(config)# segment-routing srv6
Router(config-srv6)# locators
Router(config-srv6-locators)# locator Loc1-BE // best-effort
Router(config-srv6-locator)# prefix 2001:db8:0:a2::/64
Router(config-srv6-locator)# exit
Router(config-srv6-locators)# locator Loc1-LL // low latency
Router(config-srv6-locator)# prefix 2001:db8:1:a2::/64
Router(config-srv6-locator)# algorithm 128
Router(config-srv6-locator)# exit
Router(config-srv6)# exit

  2. Configure IS-IS for SRv6 flexible algorithms.

    Example:

    Router(config)# router isis test-igp 
  Router(config-isis)# address-family ipv6 unicast 
  Router(config-isis-af)# segment-routing srv6 
  Router(config-isis-srv6)# locator Loc1-BE 
  Router(config-isis-srv6-loc)# exit 
  Router(config-isis-srv6)# locator Loc1-LL 
  Router(config-isis-srv6-loc)# exit

  3. Enable flexible algorithm for the low-latency.

    Configure flexible algorithm definition with delay objective and static delay per interface.

    Example:

    Router(config)# router isis test-igp 
  Router(config-isis)# flex-algo 128 
  Router(config-isis-flex-algo)# metric-type delay 
  Router(config-isis-flex-algo)# exit 
  Router(config-isis)# interface GigabitEthernet0/0/0/0 
  Router(config-isis-if)# address-family ipv6 unicast 
  Router(config-isis-if-af)# root 

  Router(config)# performance-measurement 
  Router(config-perf-meas)# interface GigabitEthernet0/0/0/0 
  Router(config-pm-intf)# delay-measurement 
  Router(config-pm-intf-dm)# advertise-delay 100 
  Router(config-pm-intf-dm)# commit

  4. Verify locator status and the IS-IS flexible algorithm definition.

    Example:

    Router# show segment-routing srv6 locator 
Mon Aug 12 20:54:15.414 EDT
Name                  ID       Algo  Prefix                    Status 
--------------------  -------  ----  ------------------------  -------
Loc1-BE               17       0     2001:db8:0:a2::/64        Up     
Loc1-LL               18       128   2001:db8:1:a2::/64        Up


Router# show isis flex-algo 128 
Mon Aug 12 21:00:54.282 EDT

IS-IS 200 Flex-Algo Database
  
Flex-Algo 128:

Level-2:
        Definition Priority: 128
        Definition Source: SRv6-LF1.00, (Local)
        Definition Equal to Local: Yes
        Disabled: No


Level-1:
        Definition Priority: 128
        Definition Source: SRv6-LF1.00, (Local)
        Definition Equal to Local: Yes
        Disabled: No


    Local Priority: 128
    FRR Disabled: No
    Microloop Avoidance Disabled: No