[c-nsp] Looking for recommended IOS for 7600 RSP720CXL

[Frank Bulk]

We need to upgrade to an IOS release that supports DHCPv6 relay chaining
(i.e. there's already a relay in DHCPv6 request).  We're currently running
12.2(33)SRE2 and it's been very stable.

We have redundant RSP720-3C's, three WS-X6748 and two WS-X6704-10GE
linecards, all with DFCs.

I'm leaning towards 15.2(4)S4a -- anyone have reason to discourage us?  Any
upgrade surprises?

Regards,

Frank Bulk