SOC encounter CTS and routing

Hello,

i have some doubts regarding clock tree generation and routing. I am just a beginner in this field, so certain question might seem trivial:

1. Is the CTS done before the routing process or after the routing process.

2. I was trying the optimisation option in SOC encounter before routing and it inserts buffers for the two clocks which I have in the design. Can i do the optimisation after the CTS as I donot want these buffers to be added before CTS as it doesnot detect the entire path when I do the CTS. Only till the first buffer.

3. I have a pad
BD3CRUDQP_1V2_SF_LIN PAD_N_Clk ( .A(1'b0), .TA(1'b0), .TM(1'b0), .EN(1'b1),
.TEN(1'b0), .IO(Clk_PAD), .PDN(1'b1), .PUN(1'b0), .ZI(Clk) );
While doing the CTS, in the .ctstch file should i mention PAD_N_CLK/ZI in the AutoCTSRootPin option or is there any other command for IO pads?

4. I tried doing CTS after the rooting and found a lot of DRC errors when I did the verify geometry. What may be the problem? Should I now run the optimisation of the design command?

Thank you

The flow in SoC, is floor plan, placement, optimization -prects, cts, optimization -postcts, optimization -postcts -hold, routing, optimization post route, setup &hold, optimisations si setup & hold.

Thank you for the reply.
Actually, when I do optimisation -preCTS it adds buffers to the clock and hence if I run run CTS for the Clock net I get only a partial clock tree till the first buffer is placed