300-415 · Question #98
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?
The correct answer is C. 0.0.0.0/0 le 16. The prefix-list entry '0.0.0.0/0 le 16' matches all IP prefixes that are subnets of 0.0.0.0/0 with a prefix length up to and including /16, effectively matching all prefixes from /0 through /16.
Question
Options
- A0.0.0.0/1 le 16
- B0.0.0.0/0 ge 1
- C0.0.0.0/0 le 16
- D0.0.0.0/16 ge 1
How the community answered
(33 responses)- A6% (2)
- B9% (3)
- C82% (27)
- D3% (1)
Why each option
The prefix-list entry '0.0.0.0/0 le 16' matches all IP prefixes that are subnets of 0.0.0.0/0 with a prefix length up to and including /16, effectively matching all prefixes from /0 through /16.
0.0.0.0/1 le 16 restricts the base to /1, which would miss prefixes in the upper half of the IP address space that fall under 128.0.0.0/1.
0.0.0.0/0 ge 1 matches all prefixes from /1 to /32, which is far broader than the intended /1 to /16 range.
In Cisco SD-WAN centralized control policy, '0.0.0.0/0 le 16' uses the base prefix 0.0.0.0/0 (which is a supernet of all IPv4 space) combined with the 'le 16' qualifier to match any prefix whose length is 16 or shorter. This captures all prefixes from /0 to /16, which is the closest match for the requirement of /1 to /16 among the available options.
0.0.0.0/16 ge 1 is an invalid combination because 'ge' must be greater than or equal to the base prefix length (/16), making this match only /16 through /32, not /1 to /16.
Concept tested: Cisco SD-WAN centralized policy prefix-list ge/le matching
Source: https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/policies/ios-xe-17/policies-book-xe/centralized-policy.html
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