Modern switches generally have the same capabilities as routers (if you have the right image), they are just much faster and much more expensive and normally only do ethernet.

MLS has a very limited layer 3 capabilities, it does not have any support for the WAN interfaces and protocols, BGP, very limited QoS, and generally routing capabilities. You can use the MLS for LAN switching and routing, however if you have huge network with big routing tables, you will not be able to use MLS since it has less capabilities than routers.

Typically you are going to want a router at your network edge to the Internet because of its better support for WAN technologies. Layer 3 switches are nice for core and distribution layers of your network to route traffic between VLANs. Layer 3 switches typically have more interfaces than routers. Layer 3 switches allow you to have both switch-port interfaces and have routing enabled all in one device, whereas routers only have routing interfaces and can only be configured with one interface per network subnet. For example, you cannot have 2 interfaces in a router belong to the same network subnet.

I undestand and thanks for all , but what about Catalyst 6500 cisco define this device as switch (multilayer core switch ) and when you looking in data sheet you see it is support WAN module and interfaces , also support rich of QOS features , I agree with (pr0d16y )