RFC 10005 defines a BGP extended community that lets a router attach link bandwidth information to a route. Another router can use that value when it spreads traffic across multiple BGP paths. RFC 10005 matters because links are not always of the same capacity. This RFC provides routers with a standard way to carry bandwidth information along the route, enabling traffic to be weighted toward the higher-capacity path. Right now this RFC is just a draft. https://www.rfc-editor.org/rfc/rfc10005.html
A router may have two usable paths toward the Capital I internet or a peer. One path may sit behind a 100G handoff, while another path only has 10 Gig. Equal load balancing can push traffic into both paths without knowing which link will fill much sooner.
RFC 10005 defines the BGP Link Bandwidth Extended Community for that case. The value is tied to the BGP next hop. It does not replace the normal best-path policy. It helps the forwarding plane weight traffic after BGP has already selected multiple usable paths.
Local preference, AS path, MED, and other BGP policy still decide which routes are eligible. The link bandwidth community only helps once multipath is in play. Used correctly, the 100G path mentioned earlier can carry more traffic than the 10G path, rather than both paths sharing the same amount of bandwidth.
The RFC also defines transitive and non-transitive versions of the community. A route reflector, edge router, or downstream BGP speaker can pass, remove, or regenerate the value depending on next-hop behavior and local policy. The bandwidth value is encoded in bytes per second.
For an ISP- or exchange-connected network, the value has been needed for a while. A router with multiple exits should not overload a smaller handoff just because BGP sees both paths as valid. RFC 10005 helps the forwarding table treat a 100G path like 100G and a 10G path like 10G.
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