Cabletron Systems NB20E - Manual (Page 11)

INTRODUCTION
1-5
This feature is referred to as Aging Time. Aging Time can be altered
through Cabletron Systems’ Remote LANVIEW Network Control
Management for the Cabletron Systems' NB20E and NB25E.
1.5 MULTIPLE BRIDGE ENVIRONMENTS
Both the NB20E and NB25E Two Port Bridges are designed to promote
maximum network use in multiple bridge environments. A bridge learns
the bridge topology of its network from bridge protocol data that is
transmitted onto the network by each bridge. The bridges then apply a
Spanning Tree Algorithm to select a root bridge and to determine primary
data paths within potential data loop configurations. A Spanning Tree
Algorithm, as defined earlier, is a hierarchy (or tree) of priorities that is
established between bridges. This hierarchy guarantees that primary and
redundant data paths will be clearly defined at all times, so that the
network is available to users at all times.
1.5.1 The Root Bridge
One bridge in the network is established as the root bridge. This bridge
has priority over all other bridges. Each bridge in a Spanning Tree must
learn which bridge is the root and then determine its own relative priority
within the network.
Determining the root bridge begins with the broadcast of Bridge Protocol
Data Units (BPDUs) across the network. When the bridges go on line,
each bridge sends out a BPDU which contains its priority field and its
unique identifier or Ethernet address. Through the broadcast of these
identifiers across the network, the bridges learn which bridge has the
highest priority, or lowest identifier. This bridge becomes the root bridge
for the network. Each bridge then determines its own priority in relation
to this bridge.
The network manager can, however, override this process for determining
the root. Using the root switch, the manager can manually select a
different bridge to be the root by applying a selectable switch at the
appropriate bridge. This bridge transmits BPDUs to inform the other
bridges that it is now the root. With this information, the bridges
immediately learn their new priority in relation to the new root, and the
system of priorities is maintained.