2 units of cat6509 switches, both with dual SUP720 engine. After I configured both switches with VSS, both switches standby SUP engine went into RMON. The VSS is working fine.
Why this happened? can't each chassis have one active and one standby SUP ?
What version of IOS are you running? SXH only support single SUP in each chassis of thre VSS. The newer SXI will support dual SUP in each chassis.