What is BGP Weight?

Understanding BGP Weight

BGP Weight is a Cisco-proprietary path attribute used to influence outbound route selection on a single router. It is locally significant, meaning it is never advertised to any BGP neighbor and affects only the router on which it is set. When multiple paths to the same destination exist, the route with the highest weight is preferred, making weight the very first tiebreaker in the BGP best-path algorithm.

Weight is a 16-bit value ranging from 0 to 65,535. Routes originated by the local router default to a weight of 65,535, while routes learned from neighbors default to 0. Administrators set weight using neighbor route-maps or the neighbor weight command. Because it is evaluated before AS-path length, local preference, and other attributes, weight gives an operator a powerful, router-specific lever to force traffic out a chosen exit.

For security and resilience, weight matters in traffic-engineering and failover designs and in containing routing incidents. Since weight stays local, it cannot be abused by external peers to influence your selection, unlike attributes that propagate. However, misconfigured weight can silently override intended routing policy, sending sensitive traffic through an unintended or untrusted path, so it must be documented and reviewed in routing security audits.

For example, a multihomed enterprise router connects to two ISPs and wants all outbound traffic to prefer ISP-A while ISP-B serves only as backup. The operator applies a route-map setting weight 200 on routes received from ISP-A and leaves ISP-B routes at the default 0. The router now sends traffic via ISP-A; if that session drops, the ISP-A routes disappear and the router automatically falls back to ISP-B. Because weight is local, the neighboring router or another router in the same AS is unaffected and would need its own policy to behave the same way.