Which Prefix List Matches /1 to /16 in a Control Policy?

300-415 By islever Editorial Last updated

Configure control policies

Short answer

A The IP prefix list 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16 in the centralized control policy.

Question

An engineer is configuring a list that matches all IP prefixes with lengths from /1 to /16 in a centralized control policy. Which list accomplishes this task?
  1. 0.0.0.0/0 le 16 Correct Answer
  2. 0.0.0.0/1 le 16
  3. 0.0.0.0/0 ge 1
  4. 0.0.0.0/16 ge 1

Key Takeaways

The question tests knowledge of the 'le' (less-than-or-equal) prefix-length operator in SD-WAN prefix lists, and the trap is assuming that a list starting at 0.0.0.0/0 must also match the /0 default route rather than beginning at /1.

In Cisco Catalyst SD-WAN centralized control policies, an IP prefix list written as 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16. This page explains why option A is the documented syntax and why the /1-based alternatives fail the requirement.

Common mistake: Option B (0.0.0.0/1 le 16) is the most common wrong pick because learners reason that /1 must be stated explicitly to cover /1 to /16; in SD-WAN prefix-list syntax the base 0.0.0.0/0 with le 16 already yields exactly the /1–/16 range Cisco documents.

Expert Analysis

Why the Answer Is Correct

Cisco documents that ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16, which is precisely the requirement in this centralized control policy question. The le keyword constrains the match to prefixes whose length is less than or equal to 16, so every concrete network prefix from a /1 up to a /16 is accepted. As FlipCCNP quoted directly from the Cisco policy-basics documentation, 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16, making option A the documented, verifiable answer. Because the base 0.0.0.0/0 entry itself carries no usable network information for a route match in this context, the effective matched range starts at /1.

Why the Other Options Are Wrong

Option B, 0.0.0.0/1 le 16, anchors the list at a /1 network boundary, which restricts matches to only the address space under that first /1 rather than the full documented /1–/16 behavior; as gladpark argued, it does not reproduce the documented semantics. Option C, 0.0.0.0/0 ge 1, uses ge (greater-than-or-equal) without an upper bound, so it matches /1 through /32 and beyond the requested /16 ceiling. Option D, 0.0.0.0/16 ge 1, is doubly wrong: it anchors on a /16 prefix and uses ge, producing neither the intended base nor the intended length ceiling. None of B, C, or D reproduce the vendor-documented /1-to-/16 match.

Community Comment Notes

Most commenters converged on A, with FlipCCNP pasting the Cisco example verbatim and CCNPCollab1297 reinforcing that the quoted line reads "ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16." A dissenting voice, gladpark, voted B on the reasoning that "0.0.0.0/0 le 16 matches all IP prefixes with lengths from /0 to /16," while isaacmejia similarly argued from broadcast-address arithmetic that "i would go with B." immphoenix raised a narrower caveat that "Technically A is flawed becasue A match the default route 0.0.0.0/0," a nuance the official example resolves in favor of A.

Official Reference

https://sdwan-docs.cisco.com/

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

Exam Strategy

When a prefix-list question cites an exact range like /1 to /16, look for the Cisco-documented 0.0.0.0/0 le <n> idiom rather than re-deriving the match yourself; memorize that le sets the upper bound and ge sets a lower bound only.

Frequently Asked Questions

Why does 0.0.0.0/0 le 16 start matching at /1 instead of /0?

Cisco documents that ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16. The /0 base carries no network information to match, so effective matches begin at /1.

Why is 0.0.0.0/1 le 16 not the right list for the /1 to /16 range?

Anchoring on 0.0.0.0/1 binds the match to a single /1 address block instead of providing the general /1–/16 prefix-length range that 0.0.0.0/0 le 16 delivers.

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Discussion 19 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 19

  1. FlipCCNP 👍 13
    This should be A. • 0.0.0.0/0—Match any prefix–length pair. • 0.0.0.0/0 le length—Match any IP prefix whose length is less than or equal to length. For example, ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16. https://sdwan-docs.cisco.com/Product_Documentation/Software_Features/SD-WAN_Release_16.3/06Policy_Basics/03Localized_Control_Policy/Configuring_Localized_Control_Policy
  2. CCNPCollab1297 👍 6
    Please stop giving wrong answers in each question's comments.
  3. CCNPCollab1297 👍 4
    A is correct. Directly from your link: "For example, ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16."
  4. ramjam 👍 4
    Agree. It should be A. example, ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16.
  5. immphoenix 👍 3
    Technically A is flawed becasue A match the default route 0.0.0.0/0, of which the prefix-length is arguably 0.
Show 14 more comments
  1. gladpark 👍 3 Answered B
    0.0.0.0/0 le 16 matches all IP prefixes with lengths from /0 to /16. The question asks for /1 to /16.
  2. isaacmejia 👍 2
    The question clearly says from /1 to /16, 0.0.0.0/0 match everything Broadcast addr is 255.255.255.255. 0.0.0.0/1 Broadcast addr is 127.255.255.255 dont get confused isnot the same. i would go with B
  3. khanda 👍 2 Answered A
    Correct answer is A.
  4. Speirsington 👍 2 Answered A
    A is correct
  5. Han2022 👍 1
    This should be A as well. • 0.0.0.0/0 le length—Match any IP prefix whose length is less than or equal to length. For example, ip-prefix 0.0.0.0/0 le 16 matches all IP prefixes with lengths from /1 through /16. Reference: https://sdwan-docs.cisco.com/Product_Documentation/Software_Features/SD-WAN_Release_16.3/06Policy_Basics/03Localized_Control_Policy/Configuring_Localized_Control_Policy
  6. colipto 👍 1
    Option A (0.0.0.0/0 le 16) also matches the criteria but it includes all IP prefixes up to /16, including those longer than /16 (e.g., /17, /18, etc.), which may not be desired. B
  7. timtgh 👍 1
    Why would you think that? You even said, "up to /16." So that does NOT include /17, /18, etc. That's what up to /16 means. The mask is not longer than /16.
  8. donciga 👍 1 Answered A
    for those doubting https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/policies/vedge-20-x/policies-book/m-localized-policy.html
  9. A_Wolf 👍 1
    no, 0.0.0.0/1 le 16 does not match "all prefixes", the /1 tells to check the first bit of the network, thus NOT all prefixes.
  10. spamke 👍 1
    A is correct of course, but 0.0.0.0/1 le 16 is also matching all prefixes from /1 to /16, so.. interesting..
  11. timtgh 👍 1
    Option B is a ridiculous answer, as it only matches routes where the first bit is a 0.
  12. Elmaestro 👍 1
    A is the correct answer as the majority agreed because the le length up to /16 only
  13. MajorSalieri 👍 1
    B is correct. le denotes any range between 0-16(inclusive)
  14. Baio 👍 1 Answered A
    Corretto A Explanation in FlipCCNP comment

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