IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

IS-IS auto-cost reference bandwidth

Updated: September 8, 2026

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Introduces IS-IS auto-cost reference bandwidth, detailing how metric calculation works, providing reference information, explaining interface bandwidth tracking, recommending auto-cost usage, and guiding configuration procedures.

The IS-IS auto-cost reference bandwidth feature is a routing metric mechanism that

  • provisions IS-IS metrics based on the physical bandwidth of a link

  • adjusts IS-IS metrics for bundle interfaces when bandwidth changes, and

  • optimizes route selection by lowering the cost for high-speed links.

This feature enhances network adaptability and ensures that IS-IS routing decisions accurately reflect the current link capacities.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

IS-IS auto-cost reference bandwidth

 Release 25.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: Q200, P100], 8700 [ASIC: P100, K100], 8010 [ASIC: A100]); Centralized Systems (8600 [ASIC: Q200]); Modular Systems (8800 [LC ASIC: Q100, Q200, P100])

The IS-IS auto-cost reference bandwidth feature automates IS-IS metric provisioning based on physical link bandwidth, optimizing path selection and reducing operational overhead. This feature allows you to configure a reference bandwidth, which IS-IS then uses to automatically calculate interface metrics. It also dynamically adjusts metrics for bundle interfaces when member links change, ensuring accurate and efficient routing without manual intervention.

CLI:

How auto-cost metric calculation works

Summary

The auto-cost metric calculation process dynamically determines and updates interface costs within IS-IS, ensuring routing decisions reflect current link bandwidths.

The key components involved in the process are:

  • IS-IS: The routing protocol that performs the calculation and updates.

  • Interface bandwidth: The actual capacity of a link, which is a primary input for the metric calculation.

  • Configured reference bandwidth: A user-defined value that scales interface bandwidths into IS-IS metrics.

  • Link State PDU (LSP) generation: The creation of data packets to propagate updated link-state information.

  • Shortest Path First (SPF) run: The algorithm execution to re-calculate optimal paths based on new metrics.

The auto-cost metric calculation process involves IS-IS using interface bandwidth and a configured reference bandwidth to dynamically determine and update interface costs. This process leverages LSP generation and SPF runs to propagate updates and recalculate optimal routing paths.

Workflow

These stages describe how auto-cost metric calculation works:

  1. Condition check: IS-IS verifies that auto-cost reference bandwidth is configured on an active interface or under the IS-IS router process, and that a default metric is not explicitly configured.

  2. Initial metric determination: IS-IS calculates the interface cost based on its current bandwidth and the configured reference bandwidth.

  3. Bandwidth change detection: IS-IS receives a notification indicating a change in the interface's bandwidth.

  4. Metric recalculation: IS-IS updates the auto-cost reference bandwidth metric using the new interface bandwidth.

  5. Routing update initiation: The updated metric triggers an LSP PDU generation.

  6. Path re-evaluation: An SPF run is initiated to re-calculate routing paths.

  7. Routing table update: The new metric is reflected in the routing table, influencing routing decisions.

Result

The IS-IS routing table dynamically reflects current link capacities, optimizing route selection based on real-time bandwidth conditions.

IS-IS auto-cost metric information

The metric calculates automatically using reference bandwidth to support optimal, real-time path selection.

  • The IS-IS auto-cost metric dynamically assigns routing metrics based on an interface's physical bandwidth.

  • This feature optimizes route selection by lowering the cost for high-speed links and ensures routing decisions accurately reflect current link capacities.

  • The auto-cost metric is automatically calculated using the configured reference bandwidth, supporting optimal path selection by reflecting real-time link capacities.

Metric calculation logic

When granularity_bandwidth is less than or equal to link_bandwidth:

  • The effective bandwidth is determined by rounding down the link_bandwidth to the nearest multiple of granularity_bandwidth.

  • The metric is calculated by dividing the reference_bandwidth by this effective bandwidth.

  • Formula:

    
metric = reference_bandwidth / (link_bandwidth - (link_bandwidth % granularity_bandwidth))

  • When granularity_bandwidth is greater than link_bandwidth or not configured:

  • The metric is calculated by dividing the reference_bandwidth directly by the actual link_bandwidth.

  • Formula:

    metric = reference_bandwidth / link_bandwidth

Key components

  • reference_bandwidth: Defined in kilobits per second (kbps), serves as the baseline for scaling interface bandwidths into IS-IS metrics.

  • link_bandwidth: The actual physical capacity of the interface, measured in kbps.

  • granularity_bandwidth: Configured in kbps, determines the increment for rounding down the link_bandwidth before metric calculation.

  • % (Modulus Operator): Returns the remainder of the division; used to find how much link_bandwidth exceeds the nearest lower multiple of granularity_bandwidth.

Additional notes

  • The final calculated metric is always an integer.

  • Fractional results from the division are typically rounded up to the next whole number to ensure conservative metric assignment, preventing high-bandwidth links from receiving an undeservedly low metric.

How IS-IS tracks interface bandwidth changes

Summary

IS-IS dynamically monitors interface bandwidth fluctuations to ensure that routing metrics are current, optimizing path selection and informing Traffic Engineering components of changes.

The key mechanisms involved in the process are:

  • IS-IS: The routing protocol responsible for monitoring, checking configurations, and initiating updates.

  • Interface bandwidth change notification: An event signaling a change in the physical capacity of a link.

  • Auto-cost reference bandwidth configuration: The setting that enables dynamic metric calculation based on bandwidth.

  • Higher-priority metric configurations: Other metric settings, such as metric-fallback and explicit default metrics, that can override auto-cost.

  • LSP PDU generation: The process of creating and sending routing updates.

  • SPF run: The algorithm that re-calculates optimal paths in the routing table.

  • Traffic Engineering (TE) component: A system that utilizes routing information for traffic management.

Workflow

These stages describe how IS-IS tracks interface bandwidth changes:

  1. Receive notification: IS-IS receives an alert indicating a change in an interface's bandwidth.

  2. Check auto-cost status: IS-IS determines if auto-cost reference bandwidth is enabled for the affected interface.

  3. Evaluate metric priority: If auto-cost is enabled, IS-IS verifies that no other higher-priority metric configurations, such as metric-fallback or explicit default metrics, are active on the interface.

  4. Recalculate metric: If auto-cost is enabled and has the highest priority, IS-IS recalculates the metric for the interface using the new bandwidth information.

  5. Initiate routing updates: IS-IS triggers an LSP PDU generation to broadcast the updated metric.

  6. Re-evaluate paths: IS-IS initiates an SPF run to re-calculate optimal routing paths based on the new metric.

  7. Update routing table: The routing table is updated to reflect the new metric, influencing future routing decisions.

  8. Notify TE component: IS-IS informs the Traffic Engineering (TE) component about any changes to TE tunnel metrics.

Result

Interface metrics are dynamically adjusted in response to bandwidth changes, leading to optimized routing decisions and accurate information for traffic engineering.

Guidelines for auto-cost reference bandwidth

Consider these guidelines when deploying and managing the auto-cost reference bandwidth feature:

  • Configure auto-cost reference bandwidth only on active interfaces that have bandwidth information available.

  • Avoid configuring auto-cost reference bandwidth on passive interfaces, as this feature does not apply to passive interfaces.

  • Avoid configuring both an explicit metric under router address family or interface address family and auto-cost reference bandwidth on the same interface. If an explicit metric is configured, the auto-cost reference bandwidth setting is ignored.

  • Avoid relying on the auto-cost reference bandwidth metric if you have configured other metric types, such as metric-fallback or explicit default metrics, on the interface. The system will always prioritize these explicitly configured metrics over auto-cost.

  • Monitor Bundle-Ether interfaces for changes in link status. The effective bandwidth for these interfaces is determined by the total bandwidth of all active member links. Any change in link status, such as a link coming up or going down, automatically triggers a recalculation of the auto-cost metric.

  • Operate the auto-cost reference bandwidth feature entirely within the router. This feature has no impact on interoperability with other IS-IS implementations or RFC conformance.

  • Expect no significant increase in CPU usage or impact on router performance when using this feature.

Configure auto-cost reference bandwidth

Enable automatic IS-IS metric provisioning based on link bandwidth.

This task involves configuring the auto-cost reference bandwidth globally for all interfaces or specifically for an individual interface. The interface-specific configuration overrides the global setting.

Before you begin

Ensure you have access to the router's command-line interface and are in privileged EXEC mode.

Follow these steps to configure the auto-cost reference bandwidth:

Procedure

  1. Apply a global auto-cost reference bandwidth for all interfaces.

    Example:

    Router(config)# router isis 1
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-af)# auto-cost reference-bandwidth 30000000 granularity 100000

    The range for reference-bandwidth keyword is from 1 through 4294967295 kilobit per second.

    The range for granularity keyword is from 1 through 4294967295 kilobit per second.

  2. Configure auto-cost reference bandwidth for a specific interface.

    Example:

    Router(config-isis-af)# interface Bundle-Ether1
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-af)# auto-cost reference-bandwidth 20000000 granularity 100000

  3. Use the show running-config router isis to verify the configured auto cost metric.

    Example:

    Router# show running-config router isis 1

Sun Aug 31 19:10:22.072 UTC
router isis 1
 is-type level-2-only
 net 49.0001.0000.0000.00.0600.00
 nsr
 distribute link-state
 nsf ietf
 log adjacency changes
 trace mode enhanced
 lsp-refresh-interval 65000
 max-lsp-lifetime 65535
 affinity-map RED bit-position 3
 address-family ipv4 unicast
  metric-style wide
 auto-cost reference-bandwidth 32000000000 granularity 100000000 segment-routing mpls sr-prefer ! address-family ipv6 unicast metric-style wide auto-cost reference-bandwidth 32000000000 granularity 100000000 microloop avoidance segment-routing microloop avoidance rib-update-delay 65535 segment-routing mpls sr-prefer
!

  4. Use the show isis database detail command to display detailed IS-IS database information. You can verify the calculated autocost metric from this show output. In this show output, the calculated auto-cost metric is 80.

    Example:

    Router# show isis database detail

Sun Aug 31 19:10:24.442 UTC

IS-IS 1 (Level-1) Link State Database

LSPID       LSP Seq Num     LSP Checksum     LSP Holdtime/Rcvd     ATT/P/OL
r1.00-00    * 0x00000010    0xf36d           65507/*                0/0/0

Area Address: 49.0001
LSP MTU: 1492
NLPID: 0xcc
NLPID: 0x8e
MT: Standard (IPv4 Unicast)
MT: IPv6 Unicast 0/0/0
IP Address: 192.0.2.1
IPv6 Address: 2001:DB8::1
Hostname: r1
Router Cap: 192.0.2.1 D 5:0
Metric: 80 IS-Extended r2.00 Metric: 80 IS-Extended r2.00
Metric: 80 IS-Extended r3.00
Metric: 80 MT (IPv6 Unicast) IS-Extended r2.00
Metric: 80 MT (IPv6 Unicast) IS-Extended r2.00
Metric: 80 MT (IPv6 Unicast) IS-Extended r3.00
Metric: 80 IP-Extended 198.51.100.0/24
Metric: 80 IP-Extended 198.51.100.1/24
Metric: 80 IP-Extended 198.51.100.2/24
Metric: 80 IP-Extended 198.51.100.3/24
Metric: 10 IP-Extended 192.0.2.1/32
Metric: 80 MT (IPv6 Unicast) IPv6 2001:DB8:1::/64
Metric: 80 MT (IPv6 Unicast) IPv6 2001:DB8:2::/64
Metric: 80 MT (IPv6 Unicast) IPv6 2001:DB8:3::/64
Metric: 80 MT (IPv6 Unicast) IPv6 2001:DB8:4::/64
Metric: 10 MT (IPv6 Unicast) IPv6 2001:DB8::1/128

  5. Use the show isis interface command to verify the interface bandwidth and the calculated auto-cost metric. In this output, the bandwidth is 400000000 and the calculated auto-cost metric is 80.

    Example:

    
Router# show isis interface

FourHundredGigE0/0/0/0 Enabled
Adjacency Formation: Enabled
Prefix Advertisement: Enabled
Bandwidth: 400000000 Circuit Type: level-1 (Configured: level-1-2)
Media Type: P2P
Circuit Number: 0
Extended Circuit Number: 35
Last IIH Received: 19:10:25 (5.36 sec ago), 1497 octets
Last IIH Sent: 19:10:28 (2.07 sec ago), 1497 octets
Sending next P2P IIH in: 7 s IPv4 BFD: Disabled
IPv6 BFD: Disabled Level-1
Adjacency Count: 1
Adjacency Flaps: 0
LSP Pacing Interval: 33 ms
PSNP Entry Queue Size: 0
Hello Interval: 10 s
Hello Multiplier: 3 CLNS I/O
Protocol State: Up
MTU: 1497
SNPA: 7837.b7ad.0400
Layer-2 Multicast: Listening
All ISSs: IPv4 Unicast Topology: Enabled
Adjacency Formation: Running
Prefix Advertisement: Running
Policy (L1/L2): -/-
Metric (L1/L2): 80/80
Metric fallback:
Bandwidth (L1/L2): Inactive/Inactive
Anomaly (L1/L2): Inactive/Inactive