Junos OS supports unequal-cost load balancing for BGP by utilizing the BGP link bandwidth extended community. This feature allows a router to advertise its link's bandwidth to a BGP peer. The receiving router can then be configured with a policy to use this advertised bandwidth information to distribute traffic proportionally across the available paths. This ensures that the 10 Gigabit Ethernet link carries a proportionally higher volume of traffic than the 1 Gigabit Ethernet link, fulfilling the requirement to use both links as efficiently as possible according to their capacity.

B. Disabling multipath would force the use of only a single best path, leaving the second link unused, which is contrary to the stated goal.

C. The metric-out parameter (MED) is used to influence how a remote AS sends traffic to the local router, not how the local router load-balances its outbound traffic.

D. Per-prefix load balancing is a method for distributing traffic, but it requires multiple paths to be considered equal and eligible for multipath first, which is not the case here due to different IGP costs.

1. Juniper Networks TechLibrary

Junos OS Routing Protocols Library for Routing Devices

"Understanding BGP Link Bandwidth": This document states

"The BGP link bandwidth extended community enables BGP to perform unequal-cost load balancing... The link bandwidth information is carried in an optional

non-transitive BGP extended community attribute. This allows BGP to load-balance traffic across different external BGP (EBGP) peers in proportion to the bandwidth of the links."

2. Juniper Networks TechLibrary

Junos OS Routing Protocols Library for Routing Devices

"Example: Configuring BGP Link Bandwidth": This configuration example demonstrates the exact solution. It shows creating a community bw-community and applying it via an export policy. The document notes

"To enable unequal-cost load balancing

you configure a BGP export policy that includes the bandwidth community."

3. Juniper Networks TechLibrary

Junos OS Routing Protocols Library for Routing Devices

"multipath (Protocols BGP)": The documentation for the multipath statement clarifies that for standard BGP multipath

several attributes

including the IGP cost to the BGP next hop

must be identical. The bandwidth community feature is the specific mechanism to override this requirement for proportional

unequal-cost load balancing.