You must check the capabilities of your Cisco router. It probably supports
RADIUS authentication. If it does, you should install a RADIUS server that
supports an LDAP backend. Both OpenRadius and FreeRadius are useful
alternatives, and I am sure others will do the job as well.

Because you did not tell us the operating system on which you run your
LDAP server, it's difficult to give more details.

Be prepared to read some RADIUS documentation so that the Cisco VPN router
and the RADIUS server agree on what is expected and what is provided. Both
of these are out of the scope of the OpenLDAP list.

Your system may also support TACACS+, which may be an optional mechanism.
The approach will be almost the same, but you will need to figure out the
necessary software (or get help from someone else on this or another
list).

Good luck!

--
Bjørn