Because RSMLT is based on SMLT, all SMLT configuration rules apply. In addition, RSMLT is enabled on the SMLT aggregation switches for each VLAN. The VLAN must be a member of SMLT links and vIST's L2VSN. For more information about how to configure SMLT in a Layer 2 environment, see MultiLink Trunking and Split MultiLink Trunking.
The VLAN also must be routable (IP address configured) and you must configure an Interior Gateway Protocol (IGP) such as OSPF on all four routers, although it is independent of RSMLT. All routing protocols, even static routes, work with RSMLT.
The RSMLT pair switches provide backup for each other. As long as one of the two routers of an vIST pair is active, traffic forwarding is available for both next hops R1/R2 and R3/R4.