Cabletron Systems SmartSTACK ELS100 - Manual (Page 94)

80 Spanning Tree Concepts ELS100-24TX
Communicating Between Bridges
Periodically, all devices running the Spanning Tree Protocol on a network
transmit packets to each other “in care of” the Bridge Group Address
which all bridges share. When a bridge receives a frame sent to the
Bridge Group Address, the bridge’s Spanning Tree Protocol processes
the packet. Application software and other LAN segments ignore the
packet. Bridges communicate between each other in order to determine
the Root Bridge.
Selecting a Root Bridge and Designated Bridges
During communication between bridges, one bridge is determined to
have the lowest bridge identifier. This bridge becomes the Root Bridge.
After the Root Bridge has been selected, each LAN segment looks for the
bridge that has the lowest cost relative to the Root Bridge. These bridges
become Designated Bridges.
Selecting Designated Ports
Each Designated Bridge selects a Designated Port. This port is
responsible for forwarding packets to the Root Bridge.
Handling Duplicate Paths
When the active topology of the network is determined, all packets
between any two nodes in the network use only one path. Where a
duplicate path exists, the non-designated port is put into a blocking state.
Remapping Network Topology
If there is a change in the network topology due to a failure or the removal
or addition of any active components, the active topology also changes.
This may trigger a change in the state of some blocked ports.
There are five (5) states that the ports can be in for spanning tree:
• Blocking: A port in this state does not participate in the transmission
of frames, thus preventing duplication arising through multiple paths
existing in the active topology of the bridged LAN.
• Listening: A port in this state is preparing to participate in the
transmission of frames. The transmission of frames is temporarily
disabled in order to prevent temporary loops, which may occur in a
bridged LAN during the lifetime of this state as the active topology of
the bridged LAN changes.
• Learning: A port in this state is preparing to participate in the
transmission of frames.
• Forwarding: A port in this state is participating in the transmission of
frames.
• Disabled: A port in this state does not participate in the transmission
of frames or the operation of the spanning tree process.