Click the exhibit. Router R6 is a route reflector with clients R1, R2, R4 and R5. R4 receives three routes from router R6 and is configured with ECMP 3. Given the following BGP configuration on…

Question

Options

• ATwo primary paths and one backup path.
• BOne primary path and two backup paths.
• CThree primary path and one backup path.
• DThree primary paths.

• A
  8% (2)
• B
  4% (1)
• C
  8% (2)
• D
  80% (20)

Explanation

Option D is correct because R4 receives exactly 3 routes from R6, and with multipath 3 configured alongside add-paths ipv4 receive, R4 can install all 3 received paths as equal-cost primary paths in the BGP routing table. The backup-path feature only installs a backup when there exists a path that wasn't selected as a primary - since all 3 received paths fit within the multipath limit of 3, there are no remaining paths to promote to backup, leaving the backup slot empty.

Why distractors fail:

• A (2 primary, 1 backup): Incorrect because multipath 3 allows all 3 paths to be primary simultaneously, not just 2.
• B (1 primary, 2 backup): Doubly wrong - backup-path only ever installs a single backup path (not two), and multipath means more than one path can be primary anyway.
• C (3 primary, 1 backup): The most tempting wrong answer. It would require a 4th path to exist as the backup candidate, but R4 only received 3 paths total, leaving nothing leftover for backup.

Memory tip: Think of backup-path as needing an "overflow" path - a path that lost the multipath election. If multipath absorbs all received paths, backup has nothing to work with. Ask yourself: "Are there more received paths than my multipath limit?" If no, backup slot stays empty.

No community discussion yet for this question.