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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.

Policies

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?

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)
  • A
    6% (2)
  • B
    9% (3)
  • C
    82% (27)
  • D
    3% (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.

A0.0.0.0/1 le 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.

B0.0.0.0/0 ge 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.

C0.0.0.0/0 le 16Correct

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.

D0.0.0.0/16 ge 1

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

Topics

#Centralized Control Policy#IP Prefix Lists#Prefix Length Matching#SD-WAN Policies

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