INTRODUCTION
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.
1.5MULTIPLE 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.
1.5.1The 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.
