Hi,
wouldn't you agree that two chassis, located in different fire compartements and with independent infrastructure in each compartement can increase the availability?
Intra-chassis redundancy (2 SEs in a single standalone chassis) is of course better than a single SE in terms of availability, but when you want to offer a high availability solution to your customers combined with the appropriate SLAs, this might not be enough (and the cost of such a SLA should justify the extra chassis on long term). A second chassis can also increase the system bandwidth (depending on the design).
From the Cat6500 Nonstop Forwarding with Stateful Switchover Whitepaper:
In most cases one SE per chassis will be enough when you have two chassis. An interesting design variant is VSS. From a control-plane perspective it works exactly like intra-chassis redundancy (active-standby control-plane) but the two SEs are located in two different chassis since they now can communicate using Ethernet instead of the backplane of a single chassis. But the data-plane, in contrast, works now in active-active mode which doubles the system bandwitdh in normal operation. The main advantage of combining intra-chassis and inter-chassis redundancy is that in the rare event of a single SE failure the switching capacity of the affected chassis (and consequently the total system) is fully maintained because you don't loose the linecards of that box.
HTH
Rolf
