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

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

Configure IS-IS with weight

Updated: September 8, 2026

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Teaches you how to configure IS-IS interface weights and verify normalized load distribution across multiple paths.

Configure IS-IS UCMP path weighting so two IPv4 paths are used with an explicit traffic-share ratio based on configured interface weights.

Procedure

  1. Configure IS-IS and set interface weights for IPv4 unicast.

    Example:

    Router(config)# router isis 1
Router(config-isis)# interface HundredGigE 0/3/0/8
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# weight 200
Router(config-isis-if-af)# exit
Router(config-isis-if)# exit
Router(config-isis)# interface HundredGigE 0/3/0/9
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# weight 300

  2. Verify CEF entry and weighted next-hop distribution.

    1. Validate expected normalized weights from output.

      Example:

      Path weights: 200 and 300
GCD: 100
Normalized weights: 200/100 = 2, 300/100 = 3
Expected ratio: 2:3
    2. Confirm the values in the CEF output.

      Example:

      Router# show cef ipv4 198.51.100.3 detail
...
...
via 1.0.0.2/32 ... weight 200
via 2.0.0.2/32 ... weight 300

Weight distribution:
slot 0, weight 200, normalized_weight 2
slot 1, weight 300, normalized_weight 3

CEF installs both next hops and applies weighted forwarding in a 2:3 ratio (from configured weights 200 and 300).