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Version 3.3-01

Watson Ethernet

Operating Manual

Revision: 2010-01-20

3-17

The spanning tree network protocol provides a loop free topology for any bridged

LAN. It detects/disables network loops and provides backup links between

bridges. If a loop is detected then the protocol blocks one or more redundant

ports.

The bridges continually exchange information. When they recognize a change in

network topology then RSTP automatically reconfigures bridge ports to avoid

network failure.

Bridge Protocol Data Units (BPDUs)

The bridges in a network continuously exchange RSTP information carried in

special frames called bridge protocol data units (BPDUs). BPDUs are exchanged

regularly at configurable intervals (the Hello time, 2 seconds by default) and ena-

ble bridges to keep track of network changes and activate or disable ports as re-

quired.

Topology Change Notification (TCN) BPDUs are used to inform other bridges of

port changes. TCNs are injected into the network by a non-root bridge and prop-

agated to the root. Upon reception of the TCN the root bridge will set a Topology

Change flag in its normal BPDUs. This flag is propagated to all other bridges to

instruct them to rapidly age out their forwarding table entries.

Port states

Each port on a bridge can be in one of three states:

 Learning - the port processes BPDUs and awaits possible new information

that would cause it to return to the blocking state. While the port does not yet

forward frames it does learn source addresses from frames received and

adds them to the filtering database.

 Blocking – the port processes BPDUs but discards all other frames.

 Forwarding – the port forwards frames and processes BPDUs.

When a bridge port is initialized it will not immediately start to forward frames. It

will instead go into the Learning state while it processes BPDUs and determines

the topology of the network.

The port will go into Forwarding state after expiry of a timer (the forwarding delay

timer) unless RSTP determines that this port would cause a loop in the network.

In this case the port will transition to Blocking state.

RSTP Protocol operation

 Root bridge election: one bridge in the network is selected as the root bridge.

The root bridge is the root of the tree spanning the entire network once the

RSTP has converged.

Selection is based on the Bridge Identifier (BID) which uniquely identifies

each bridge. A BID is composed of a configurable bridge priority and the

MAC address of the bridge. BPDU exchange results in the bridge with the

numerically lowest BID to be selected as root bridge.

The bridge priority is normally left at its default value (32768) but can be re-

configured to a lower number if the network administrator wishes a particular

bridge to be elected as root bridge.

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