The BGP Multipath Load Sharing for eBGP and iBGP feature allows you to configure multipath load balancing with both external BGP (eBGP) and internal BGP (iBGP) paths.

Vyatta OS supports both iBGP and eBGP Multipath.

Regarding the load sharing, traffic load is shared across them on a per session basis. That is, each new session is routed across the path that has the fewest active sessions. To achieve this, the route is advertised with the nexthop address being the interface address rather than the address of the best path. Once traffic is routed to the interface, it can select the path based on current traffic loads.

FYI: The behavior for Cisco IOS device for BGP multipath:

Load balancing over the multi BGP paths is performed by CEF. CEF load balancing is configured on a per-packet round robin or on a per session (source and destination pair) basis. The default load balancing mode of CEF is per-destination.