On 4500 BackBone Switch ; we have 15 VLANS, running OSPF as the routing protocol, we need to create another new VLAN (vlan 120) and the requirement is that this VLAN should not be allowed to communicate with other vlans.
As per requirement I have not added this vlan range in OSPF routing process but still other vlan are able to communicate..
Can someone explain why this is happening...
Not running OSPF on the new vlan interface simply means that LSUs from this router will not include information regarding the new vlan in its updates to the OSPF neighbors.
That means that a user sitting, say, 3 hops away, will not have a route to that network.
The reason that users on the other vlans that are configured on that 4500 switch can communicate with the new vlan is that they are all directly connected routes. You need a router/L3 switch to pass traffic from one vlan to another (inter-vlan routing). Creating SVIs for several vlans on the same switch satisfies that requirement, thereby allowing users in each of these vlans to communicate with each other.
To isolate the new vlan, you can look into using vlan maps or traditional ACLs and applying them to the vlan's SVI.