IS-IS Supported Cost Types: Wide vs Narrow

H12-821 By islever Editorial Last updated

IS-IS Principles and Configuration

Short answer

BD IS-IS supports wide and narrow cost types for metric configuration.

Question

Which cost types are supported in IS-IS? (Choose all that apply.)
  1. default
  2. wide Correct Answer
  3. ToS
  4. Narrow Correct Answer

Key Takeaways

The question tests knowledge of IS-IS metric encodings; the trap is confusing compatibility modes or non-existent types like 'ToS' with the actual supported metric styles.

Learn which cost types are supported in IS-IS routing protocol. The correct answers are Wide and Narrow metrics, essential for configuring scalable network paths.

Common mistake: Choosing 'default' or 'ToS' because they sound like standard networking terms, whereas IS-IS specifically distinguishes between Narrow (6-bit) and Wide (24-bit) metric styles.

Expert Analysis

IS-IS Cost Styles and Configuration

In the IS-IS routing protocol, the metric used for path selection is called the 'cost'. The way this cost value is encoded in Link State PDUs (LSPs) defines its range and format. This encoding method is referred to as the "cost style" or "cost type".

Supported Cost Types in Huawei Implementations According to Huawei's documentation and general IS-IS standards (RFC 1195), there are two primary cost styles supported:

  1. Narrow Metric: This uses a 6-bit field, allowing cost values from 1 to 63. It is an older standard that limits scalability.
  2. Wide Metric: This uses a 24-bit field, allowing cost values from 1 to 16,777,215. This is necessary for large networks with many links or high-speed interfaces where narrow metrics would be exhausted.

Huawei devices support both narrow and wide cost styles via the cost-style command. While compatibility modes exist (like wide-compatible or narrow-compatible) to allow interoperability between routers using different styles, the fundamental supported types are Narrow and Wide.

Analysis of Options

  • A. default: There is no distinct "default" cost type separate from narrow/wide. The default behavior depends on implementation, but it isn't a named type like "default".
  • B. wide: Correct. Wide metrics are fully supported.
  • C. ToS: Incorrect. Type of Service (ToS) is not an IS-IS cost type. IS-IS does not use ToS for metric calculation in the same way IGMP or some other protocols might use differentiated services fields.
  • D. Narrow: Correct. Narrow metrics are fully supported.

The comment correctly identifies that Narrow and Wide are the supported styles, noting that Narrow is often the default initial state on some platforms, but both are explicitly configurable types.

Official Reference

https://support.huawei.com/enterprise/en/doc/EDOC1100067849/is-is-cost-style

This link opens the official vendor documentation in a new tab.

Exam Strategy

When asked about protocol-specific features like metrics or encapsulations, focus on the RFC-defined standards. For IS-IS, remember the distinction between 6-bit (Narrow) and 24-bit (Wide) metrics. Eliminate options that belong to other protocols (like ToS) or generic terms (like Default) unless explicitly defined in the vendor's context.

Frequently Asked Questions

What is the difference between Narrow and Wide metrics in IS-IS?

Narrow metrics use a 6-bit field (range 1-63), while Wide metrics use a 24-bit field (range 1-16,777,215). Wide metrics are required for large, modern networks.

Is there a 'default' cost type in IS-IS?

No, 'default' is not a specific cost type name. The default behavior usually defaults to Narrow on older implementations, but Wide is the standard for modern deployments.

More H12-821 FAQ →

Community Votes

B
50%
D
50%

50% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 2 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 2

  1. js293 👍 2
    huawei doc
  2. brunopfreire 👍 1
    Based on Huawei IS-IS configuration documentation, the supported cost types (styles) configured via the cost-style command are narrow and wide (along with compatibility modes like wide-compatible, compatible, and narrow-compatible) 1. From the options provided: wide: Supported. The cost range is 1 to 16,777,215 2. Narrow: Supported. The cost range is 1 to 63 2. This is also the default cost type 1. default: This is not a distinct cost type parameter; rather, "narrow" is the default setting. ToS: While IS-IS supports Traffic Engineering and ToS metrics (RFC 4915), "ToS" is not a primary cost style configured via the cost-style command in the same manner as Narrow or Wide. Therefore, the correct cost types from the list are: wide Narrow

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