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.
Question
Refer to the exhibit. Which three outcomes occur if the prefix list is added to the neighbor? (Choose three)
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)- A6% (1)
- B6% (1)
- C75% (12)
- E13% (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.
The prefix 192.168.0.0/19 explicitly matches and is permitted by `seq 10 permit 192.168.0.0/19`.
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`.
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.
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.
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`.
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
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