Cabletron Systems ELS100-24TXG Installation And User Manual

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Cabletron Systems ELS100-24TXG Installation And User Manual Manual pdf 94 page image

80 Spanning Tree Concepts ELS100-24TXGCommunicating Between BridgesPeriodically, all devices running the Spanning Tree Protocol on a networktransmit packets to each other “in care of” the Bridge Group Addresswhich all bridges share. When a bridge receives a frame sent to theBridge Group Address, the bridge’s Spanning Tree Protocol processesthe packet. Application software and other LAN segments ignore thepacket. Bridges communicate between each other in order to determinethe Root Bridge.Selecting a Root Bridge and Designated BridgesDuring communication between bridges, one bridge is determined tohave the lowest bridge identifier. This bridge becomes the Root Bridge.After the Root Bridge has been selected, each LAN segment looks for thebridge that has the lowest cost relative to the Root Bridge. These bridgesbecome Designated Bridges.Selecting Designated PortsEach Designated Bridge selects a Designated Port. This port isresponsible for forwarding packets to the Root Bridge.Handling Duplicate PathsWhen the active topology of the network is determined, all packetsbetween any two nodes in the network use only one path. Where aduplicate path exists, the non-designated port is put into a blocking state.Remapping Network TopologyIf there is a change in the network topology due to a failure or the removalor 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 transmissionof frames, thus preventing duplication arising through multiple pathsexisting in the active topology of the bridged LAN.• Listening: A port in this state is preparing to participate in thetransmission of frames. The transmission of frames is temporarilydisabled in order to prevent temporary loops, which may occur in abridged LAN during the lifetime of this state as the active topology ofthe bridged LAN changes.• Learning: A port in this state is preparing to participate in thetransmission of frames.• Forwarding: A port in this state is participating in the transmission offrames.• Disabled: A port in this state does not participate in the transmissionof frames or the operation of the spanning tree process.