Interlogix GE-DSSG-244-POE User Manual - Manual (Page 115)

IFS NS3601-24P/4S GE-DSSG-244 and 244-POE User Manual
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Spanning Tree Protocol
Theory
The Spanning Tree protocol can be used to detect and disable network loops, and to provide backup links between switches,
bridges or routers. This allows the switch to interact with other bridging devices in your network to ensure that only one route exists
between any two stations on the network, and provide backup links which automatically take over when a primary link goes down.
The spanning tree algorithms supported by this switch include these versions:
„ STP – Spanning Tree Protocol (IEEE 802.1D)
„ RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w)
„ MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
The IEEE 802.1D Spanning Tree Protocol and IEEE 802.1W Rapid Spanning Tree Protocol allow for the blocking of links between
switches that form loops within the network. When multiple links between switches are detected, a primary link is established.
Duplicated links are blocked from use and become standby links. The protocol allows for the duplicate links to be used in the event
of a failure of the primary link. Once the Spanning Tree Protocol is configured and enabled, primary links are established and
duplicated links are blocked automatically. The reactivation of the blocked links (at the time of a primary link failure) is also
accomplished automatically without operator intervention.
This automatic network reconfiguration provides maximum uptime to network users. However, the concepts of the Spanning Tree
Algorithm and protocol are a complicated and complex subject and must be fully researched and understood. It is possible to cause
serious degradation of the performance of the network if the Spanning Tree is incorrectly configured. Please read the following
before making any changes from the default values.
The Switch STP performs the following functions:
„ Creates a single spanning tree from any combination of switching or bridging elements.
„ Creates multiple spanning trees – from any combination of ports contained within a single switch, in user specified
groups.
„ Automatically reconfigures the spanning tree to compensate for the failure, addition, or removal of any element in the
tree.
„ Reconfigures the spanning tree without operator intervention.
Bridge Protocol Data Units
For STP to arrive at a stable network topology, the following information is used:
„ The unique switch identifier
„ The path cost to the root associated with each switch port
„ The port identifier
STP communicates between switches on the network using Bridge Protocol Data Units (BPDUs). Each BPDU contains the following
information:
„ The unique identifier of the switch that the transmitting switch currently believes is the root switch
„ The path cost to the root from the transmitting port
„ The port identifier of the transmitting port
The switch sends BPDUs to communicate and construct the spanning-tree topology. All switches connected to the LAN on which the
packet is transmitted will receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses the
information in the frame to calculate a BPDU, and, if the topology changes, initiates a BPDU transmission.
The communication between switches via BPDUs results in the following:
„ One switch is elected as the root switch
„ The shortest distance to the root switch is calculated for each switch
„ A designated switch is selected. This is the switch closest to the root switch through which packets will be forwarded to
the root.
„ A port for each switch is selected. This is the port providing the best path from the switch to the root switch.
„ Ports included in the STP are selected.
Creating a Stable STP Topology
It is to make the root port a fastest link. If all switches have STP enabled with default settings, the switch with the lowest MAC
address in the network will become the root switch. By increasing the priority (lowering the priority number) of the best switch, STP
can be forced to select the best switch as the root switch.
When STP is enabled using the default parameters, the path between source and destination stations in a switched network might
not be ideal. For instance, connecting higher-speed links to a port that has a higher number than the current root port can cause a
root-port change.
STP Port States
The BPDUs take some time to pass through a network. This propagation delay can result in topology changes where a port that
transitioned directly from a Blocking state to a Forwarding state could create temporary data loops. Ports must wait for new network
topology information to propagate throughout the network before starting to forward packets. They must also wait for the packet
lifetime to expire for BPDU packets that were forwarded based on the old topology. The forward delay timer is used to allow the