Odd behavior of BUFGMUX in Virtex-2...

[Tungsten-W]

Hi, there:

I am using some 14 global clock buffers in the design, I instantiated all of
them as BUFGMUX...for example...

In simulation, it showed some gray time on clk_12m at the two transitions of
sel...how may I do correct this?

BUFGMUX CLK_BUF2( .O( clk_12m ), .I0( clk_12m_o ), .I1( clk_12m_o ), .S(
sel ) );
//1 BUFGMUX CLK_BUF2( .O( clk_12m ), .I0( clk_12m_o ), .I1( ), .S( sel ) );
//2 BUFGMUX CLK_BUF2( .O( clk_12m ), I( clk_12m_o ) );

initial
begin
sel <= 1'b0;
#1900 sel <= 1'b1;
#5000 sel <= 1'b0;
end

The manual said BUFG is implemented as BUGMUX...now when I instantiate a
BUFG CLK_BUF2( ... ), can I still use
the INST CLK_BUF2 LOC = "BUFGMUX0P" constraints?

Thanks for your advice...