How to solve this equation with allsolutions

Hi all,

I have this equation that I can not get all solutions symbolically:

restart:

eq1 := cos(lambda*ln(r1))*cos(lambda*ln(r2))+sin(lambda*ln(r1))*sin(lambda*ln(r2))-1 = 0:

solve(eq1, lambda, allsolutions) assuming r1>0, r2>0, r2>r1

when r1:=1: r2:=2: I get the solution

2*Pi*Z/ln(2)

when r1:=1.1: and r2:=2.1: # no solutions

How to get symbolique solution

Thanks