3Com CoreBuilder 7000 Operation Manual page 75

PNNI Routing 75
while complete information is transmitted downstream. This information
is stored in the topology database. In this network topology, you can also
see that there are 14 border nodes.
Physical Links and Logical Links
In the lowest level hierarchy, any connection between two nodes is a
physical link. This connection can also be a VPC (Virtual Private Channel)
between two nodes. Links between two nodes in the same peer group
are not aggregated. Links inside a peer group are horizontal links,
whereas links between two peer groups are outside links.
When a link becomes operational, the nodes attached to the link initiate
an exchange of information via a well-known VCC (VPI=0, VCI=18). This
exchange is known as the PNNI Routing Control Channel or RCC. Once
this channel is open, messages between the nodes are sent.
Any link between the lowest level hierarchy and the upper levels of
hierarchy is called a logical uplink. Any link between logical nodes is a
logical link.