When the initiator of a v2 connection tries to connect to a v1 node, the connection fails because the v1 node sends back an error message.
The analogical opposite when a v1 node initiates a connection to v2 node succeeds with a v2 node falling back to v1.
===Example===
This can be rarely observed in Basic csit test suite:
https://logs.opendaylight.org/releng/jenkins092/sxp-csit-1node-basic-all-carbon/421/