This variable indicates the partner's group number, as
received through capabilities exchange.
Peer groups are arbitrary groups of routers with one or
more designated border peers. Border peers form peer
connections with every router in their group and with
border peers in other groups. The role of a border peer
is to forward explorers on behalf of other routers.
A value of zero means no group has been set and the
DLSw is not a group member.