CompTIA Network+ (N10-009)Network ImplementationHard
A network administrator is configuring a new routing policy on an edge router that connects the company's autonomous system (AS 65000) to two different Internet Service Providers (ISPs). The administrator wants to ensure that outbound traffic destined for a specific public prefix (203.0.113.0/24) always prefers ISP A, even if ISP B advertises a shorter AS_PATH. Which BGP attribute should the administrator manipulate on the router to achieve this routing preference?
- ALOCAL_PREF (Local Preference)
- BMED (Multi-Exit Discriminator)
- COrigin
- DAS_PATH
Show answer & explanationAnswer & explanation
Correct answer: A. LOCAL_PREF (Local Preference)
LOCAL_PREF is a BGP attribute used to influence outbound traffic from an autonomous system. A higher LOCAL_PREF value is preferred, overriding other attributes like AS_PATH length, allowing the administrator to force traffic out through a specific ISP.
Why the other options are wrong
- B. MED is used to influence inbound traffic to an AS, telling external ASes which entry point is preferred. It does not influence outbound traffic from the local AS.
- C. Origin is a BGP attribute that indicates the origin of the route (e.g., IGP, EGP, Incomplete) and is considered later in the BGP path selection process, after LOCAL_PREF and AS_PATH.
- D. AS_PATH is used by BGP to determine the shortest path to a destination (fewer AS hops are preferred). While it influences routing, LOCAL_PREF takes precedence over AS_PATH in BGP's path selection process.
BGP LOCAL_PREF
A BGP attribute used within an autonomous system (AS) to influence the preferred outbound path for traffic. A higher LOCAL_PREF value indicates a more preferred path.
- Only exchanged between iBGP peers within the same AS.
- Does not affect how external ASes view your AS.
- Takes precedence over AS_PATH length in the BGP path selection algorithm for outbound traffic.
Memory trick: LOCAL_PREF is like a company's internal 'favorite route' sticker, always chosen first for leaving the building.