This topology is an example of a "non trivial" redundant coupling scheme. The shortest path rule indicates that 6 transmission zeros can be realized, the number of free parameters is equal to 9+(6/2)=12 which is enough to adjust all symmetric responses with 6 symmetric zeros. However, a quick analysis indicates that the response associated to such a topology always admits a double transmission zero at zero frequency, whatever the couplings values are. For short, one of the pair of transmission zeros is not adjustable, which makes this topology quite useless in practice (at least for pass-band filters). Removing coupling (5,8) and adding a coupling between (2,3) yields a non-redundant topology (see database) that accommodates all symmetric responses with 6 transmission zeros.
In general "redundant" for a topology means that the set of admissible responses is not "as big" as the set of freely adjustable couplings (in math. terms, big means here with respect to its dimension as algebraic variety). The redundancy property can be tested by computing the rank of a Jacobian matrix.
