In the Cisco Data Center Infrastructure 2.1 SRND (link below), this has been stated on page 2-3:
The data center core is interconnected with both the campus core and aggregation layer in a redundant
fashion with Layer 3 10 GigE links. This provides for a fully redundant architecture and eliminates a
single core node from being a single point of failure. This also permits the core nodes to be deployed
with only a single supervisor module.http://www.cisco.com/application/pdf/en/us/guest/netsol/ns107/c649/ccmigration_09186a008073377d.pdf
For a client redesign, I originally had planned out a dual-6509 dual-supervisor model at the condensed distribution/core layer and 3750's at the access layer. But I am confused - is that actually needed? Is the redundancy and resiliency between a dual 6509 dual-sup vs. dual 6509 only very marginal? I prefer technical explanations to just telling the customer that he would really me more redundant with buying more product.
Also, the SRND does seem to suggest that 'it's okay' going with two 6509's with single sup's in each. Unless I am reading their language wrong.
As for the client's requirement, they do want redundancy in case a supervisor or chassis fails but what's the likelihood of both happening at the same time?