What results to connect 2 VSS routers on 10G card over Supervisor engine ports?

VSS presents both chassis as a single logical switch to the network, so inter-chassis links are treated as internal fabric rather than external links that would trigger STP loop detection.

Cisco VSS best practices require VSL links to be distributed across multiple line cards to achieve hardware diversity. If both VSL connections run through the supervisor engine ports and that supervisor fails or is reloaded, the entire VSL goes down, potentially causing a split-brain scenario or complete switch-block outage. Using dedicated line cards for VSL ensures that a single hardware component failure does not sever the inter-chassis link.