[c-nsp] ME3600X Stable IOS version

[James Bensley]

We have settled on 15.3(3)S6 for stability, we don't have a crazy feature set:

MP-BGP/LDP/OSPFv2 to provide MPLS L3 & L2 VPNs (VPLS, PWE3) and a dash
of MPLS-TE. They are metro PEs for us basically. They are pretty much
exclusively used with IPv4, we are using ASR920s for IPv6 or "anything
bigger", not the MEs.

We have stretched them and they seem to be stable, we have some
nearing the max number of bridge domains, SVIs and VLANs etc, max
route counts, circa 90+ BGP seesions on some. They will do what it
says on the tin and they seem stable for us on 15.3(3)S6.

Cheers,
James.