CompTIA Network+ (N10-009)Network ImplementationHard
An enterprise network uses iBGP between two routers within the same autonomous system. Both routers receive routes to the same external prefix from different eBGP peers, and all other BGP attributes are equal except LOCAL_PREF. Router A advertises the prefix with a LOCAL_PREF of 200, while Router B advertises it with a LOCAL_PREF of 100. Which router's path will be preferred by BGP's best path selection algorithm?
- ARouter B's path, because a lower LOCAL_PREF value indicates a more preferred exit point
- BRouter A's path, because a higher LOCAL_PREF value indicates a more preferred exit point
- CThe router will load-balance equally between both paths regardless of LOCAL_PREF
- DNeither path will be selected until AS_PATH length is evaluated first
Show answer & explanationAnswer & explanation
Correct answer: B. Router A's path, because a higher LOCAL_PREF value indicates a more preferred exit point
In BGP's best path selection process, LOCAL_PREF is evaluated early (after weight, for Cisco implementations) and a higher value is preferred, indicating the organization's preferred egress point for outbound traffic. LOCAL_PREF is compared before AS_PATH length in the decision process, so Router A's path with LOCAL_PREF 200 wins.
Why the other options are wrong
- A. This reverses the rule; higher LOCAL_PREF, not lower, is preferred.
- C. BGP does not load-balance by default when attributes clearly differentiate paths.
- D. LOCAL_PREF is evaluated before AS_PATH length in the BGP decision process.
BGP LOCAL_PREF
A BGP path attribute used within an AS to indicate the preferred exit point for outbound traffic; higher values are preferred and it is exchanged only via iBGP.
- Higher LOCAL_PREF wins
- Evaluated early in BGP best path algorithm (after Weight)
- Only propagated within the AS via iBGP, not to eBGP peers
Memory trick: Higher LOCAL_PREF, higher priority for the exit door.