CompTIA Network+ (N10-009)Network ImplementationHard
An edge router receives the same route to 203.0.113.0/24 from two BGP neighbors. All BGP path attributes are equal except the AS_PATH attribute: one route has AS_PATH '65010 65020 65030' and the other has AS_PATH '65010 65040'. Assuming no other tie-breakers apply first, which route will BGP select as the best path?
- AThe route with the lower next-hop IP address is always chosen regardless of AS_PATH
- BBoth routes are installed for load balancing since AS_PATH length is not a factor
- CThe route with AS_PATH '65010 65040', because it has the shorter AS path length
- DThe route with AS_PATH '65010 65020 65030', because it has more hops for stability
Show answer & explanationAnswer & explanation
Correct answer: C. The route with AS_PATH '65010 65040', because it has the shorter AS path length
BGP's path selection process prefers routes with a shorter AS_PATH length, as this typically indicates fewer autonomous systems traversed. The path '65010 65040' has 2 AS hops compared to 3 for '65010 65020 65030', making it the preferred route when other attributes like local preference and weight are equal.
Why the other options are wrong
- A. Next-hop IP is a much later tiebreaker in the BGP decision process, not the primary factor here.
- B. BGP does not automatically load balance across differing AS_PATH lengths.
- D. BGP prefers shorter, not longer, AS paths in its decision process.
BGP AS_PATH Selection
BGP's best path algorithm prefers routes with shorter AS_PATH attributes when higher-priority attributes like Weight and Local Preference are tied.
- AS_PATH lists the autonomous systems a route has traversed
- Shorter AS_PATH is generally preferred
- BGP decision process checks Weight and Local Preference before AS_PATH
Memory trick: Shorter road, faster BGP victory.