bstunRouteSerial 1.3.6.1.4.1.9.9.35.1.4.1.1.5

If bstunRouteType is serial then this is the serial interface index of the point-to-point link to the remote partner; if bstunRouteType is serialDirect then the partner is in the local BSTUN. if bstunRouteType is ip then this field is 0.