300-510 · Question #31
Refer to the exhibit. XR1 and XR2 are sending the prefix 10.11.11.0/24 to XR3. A configured policy on XR1 is incorrectly prepending AS path 11 11 12 12 onto this prefix. A network operator wants to…
The correct answer is D. route-policy NO_PREPEND if as-path passes-through '11' then drop else pass endif end-policy. XR1 is falsely prepending AS 11 and AS 12 onto the 10.11.11.0/24 prefix before advertising to XR3. The goal is to block only that falsely prepended route without collateral damage to other prefixes. In Cisco IOS-XR RPL, the 'as-path passes-through' condition checks whether a…
Question
Refer to the exhibit. XR1 and XR2 are sending the prefix 10.11.11.0/24 to XR3. A configured policy on XR1 is incorrectly prepending AS path 11 11 12 12 onto this prefix. A network operator wants to add a policy onto XR3 that will not allow the falsely prepending prefix from being installed. Which policy configuration applied to the XR3 neighbor configuration for XR1 can accomplish this requirement without impact to other or future received routes? A. B. C. D.
Exhibit
Options
- Aroute-policy NO_PREPEND if as-path passes-through '11' then pass else drop endif end-policy
- Broute-policy NO_PREPEND if as-path prepends drop else pass endif end-policy
- Croute-policy NO_PREPEND if as-path passes-through '1' then pass else drop endif end-policy
- Droute-policy NO_PREPEND if as-path passes-through '11' then drop else pass endif end-policy
How the community answered
(25 responses)- A12% (3)
- B4% (1)
- C4% (1)
- D80% (20)
Explanation
XR1 is falsely prepending AS 11 and AS 12 onto the 10.11.11.0/24 prefix before advertising to XR3. The goal is to block only that falsely prepended route without collateral damage to other prefixes. In Cisco IOS-XR RPL, the 'as-path passes-through' condition checks whether a specified AS number appears anywhere in the AS path. Choice D drops the route if AS 11 appears in the path (which it will on the tampered prefix from XR1), and passes all other routes-this is the targeted, non-disruptive approach. Choice A has the logic inverted (it would pass AS 11 routes and drop everything else). Choice B uses invalid syntax; there is no 'as-path prepends' condition in IOS-XR RPL. Choice C uses '1' instead of '11', which would inadvertently match any AS number containing the digit 1, causing unintended drops.
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