Hello
The IPS or IDS comes into picture when traffic hits the sensing interface and flows through the IPS... Now, you have VLANs on the local core switch , which is not in the IPS segment. Hence, the traffic between VLANs will not be seen or stopped by the IPS.
In case you have a 6500 chassis in your core, you can use a Network Analysis module and see the traffic flowing on all VLANs. Netflow can clearly give you data about the source/destination IP/port, through which you can detect attacks to a certain extent.
If you have servers, which has to be protected on the VLAN, i would advice you to have Cisco Security Agent on them, which will work as a Host ids...
Hope this helps.. let us know your views on this.
Raj
