GLB multi-link support on IP Fabric

This feature extends Global Load Balancing (GLB) to support multiple paths between spine and top-of-rack switches on a 3-stage CLOS IP fabric. To enable GLB for multi-link on a 3-CLOS IP fabric, include the glb-multilink-mode max-val|avg-val statement at [edit forwarding-options enhanced-hash-key] hierarchy level. By default, the spine advertises the average quality of all links. Make sure you enable GLB globally at [edit protocols bgp] hierarchy level.