This parameter is used whenever SRTG is bridging packets
between TB and SR domains. Before forwarding packets from
a TB domain, SRTG specifically adds this 'virtual' ring
number and its own bridge number to the source route
information of the destination station retrieved from the
SRDB. Thus from a source routing station's point of
view, the entire transparent bridge LAN appears as a single
source routed ring. If this parameter has the value 0,
no virtual ring number is added.