There are two things I'd consider. The main problem you have is that a spanning tree event on your campus network would knock out both 6509's unless you have layer 3 functionality to your distribution switches. 3550-12G's are ideal to allow you to route between them and the core preventing any single spanning tree instance from existing campus wide.
1. Define a transit vlan between your 7200 and 6509's and a separate vlan for the campus network. The MSFC would be used to route between the campus subnet and the internet. The transit vlan would exist only between the 7200 and the 6509's and would be removed from the trunks that go between the 6509's and distribution layer switches. End result, a separate spanning tree instance on the transit vlan which is not proporgated around your campus network.
2. Define the connections between the 7200 and 6509 as port based vlans. ie. A separate vlan used for each connection between each individual 6509 and the 7200 which again would be removed from all trunk links out of the core switches expect the specific link between the 7200 and 6509. Use the MSFC to route between the 7200 and campus network. Using a routing protocol, each MSFC would see two routes to the 7200 - one locally and the other through the second 6509. The advantage of this configuration over option 1 is that there is no possiblity of a spanning tree loop between the 6509's and 7200 as different vlans' exist between the 7200 and each 6509.
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