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350-501 · Question #58

Refer to the exhibit. Which three outcomes occur if the prefix list is added to the neighbor? (Choose three)

The correct answer is C. 192.168 0.0/17 is permitted D. 192.168.0.0/16 is denied F. 192.168 0.0/19 is permitted. The prefix list BLOCK permits prefixes of exactly 192.168.X.X/17 and the specific 192.168.0.0/19 prefix, while explicitly denying the 192.168.0.0/16 prefix and any other unmatched prefixes.

Networking

Question

Refer to the exhibit. Which three outcomes occur if the prefix list is added to the neighbor? (Choose three)

Exhibit

350-501 question #58 exhibit

Options

  • A192.168 0.0/19 is denied
  • B192.168 0.0/17 is denied
  • C192.168 0.0/17 is permitted
  • D192.168.0.0/16 is denied
  • E192.168 0.0/16 is permitted
  • F192.168 0.0/19 is permitted

How the community answered

(16 responses)
  • A
    6% (1)
  • B
    6% (1)
  • C
    75% (12)
  • E
    13% (2)

Why each option

The prefix list BLOCK permits prefixes of exactly 192.168.X.X/17 and the specific 192.168.0.0/19 prefix, while explicitly denying the 192.168.0.0/16 prefix and any other unmatched prefixes.

A192.168 0.0/19 is denied

The prefix 192.168.0.0/19 explicitly matches and is permitted by `seq 10 permit 192.168.0.0/19`.

B192.168 0.0/17 is denied

The prefix 192.168.0.0/17 explicitly matches and is permitted by `seq 5 permit 192.168.0.0/16 ge 17 le 17`.

C192.168 0.0/17 is permittedCorrect

The prefix-list entry `seq 5 permit 192.168.0.0/16 ge 17 le 17` specifically permits any prefix that falls within the 192.168.0.0/16 range and has an exact prefix length of /17. Thus, 192.168.0.0/17 is permitted by this rule.

D192.168.0.0/16 is deniedCorrect

The prefix 192.168.0.0/16 does not match `seq 5` (its prefix length is 16, not 17) and does not match `seq 10` (its prefix length is 16, not 19). Therefore, it falls through to `seq 15 deny 0.0.0.0/0 le 32`, which explicitly denies it.

E192.168 0.0/16 is permitted

The prefix 192.168.0.0/16 does not match any permit statements and is ultimately denied by the explicit deny all rule in `seq 15`.

F192.168 0.0/19 is permittedCorrect

The prefix-list entry `seq 10 permit 192.168.0.0/19` explicitly permits the specific prefix 192.168.0.0/19 as it matches the exact network and prefix length.

Concept tested: IP prefix-list matching with ge/le

Source: https://www.cisco.com/c/en/us/td/docs/ios/ipaddr/configuration/guide/ipaddr_c/iad-cfg-ip-prfx-lsts.html

Topics

#Prefix-list#Route Filtering#BGP#Routing Policy

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