Spanning Tree Protocol

2.5.4.5 Spanning Tree Protocol

The Spanning Tree Protocol (STP), defined in the IEEE Standard 802.1D, creates a The Spanning Tree Protocol (STP), defined in the IEEE Standard 802.1D, creates a spanning tree within a mesh network of connected layer-2 bridges (typically Ethernet spanning tree within a mesh network of connected layer-2 bridges (typically Ethernet switches) and disables the links which are not part of that tree, leaving a single active path switches) and disables the links which are not part of that tree, leaving a single active path between any two network nodes.

between any two network nodes.

Multiple active paths between network nodes cause a bridge loop. Bridge loops create Multiple active paths between network nodes cause a bridge loop. Bridge loops create several problems. First, the MAC address table used by the switch or bridge can fail, since several problems. First, the MAC address table used by the switch or bridge can fail, since the same MAC addresses (and hence the same network hosts) are seen on multiple ports.

the same MAC addresses (and hence the same network hosts) are seen on multiple ports.

Second, a broadcast storm occurs. This is caused by broadcast packets being forwarded in Second, a broadcast storm occurs. This is caused by broadcast packets being forwarded in an endless loop between switches. A broadcast storm can consume all available CPU automatic backup paths if an active link fails,

automatic backup paths if an active link fails, without the danger of bridge loops, without the danger of bridge loops, or the needor the need for manually enabling/disabling these backup links.

for manually enabling/disabling these backup links.

To provide faster spanning tree convergence after a topology change, an evolution of the To provide faster spanning tree convergence after a topology change, an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP), introduced by IEEE with Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP), introduced by IEEE with document 802.1w. RSTP, is a refinement of STP; therefore, it shares most of its basic document 802.1w. RSTP, is a refinement of STP; therefore, it shares most of its basic operation characteristics. This essentially creates a cascading effect away from the root operation characteristics. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it

bridge where each designated bridge proposes to its neighbors to determine if it can make acan make a rapid transition. This is one of the major elements allowing RSTP to achieve faster rapid transition. This is one of the major elements allowing RSTP to achieve faster convergence times than STP.

convergence times than STP.

Select

Select Spanning TreeSpanning Tree ProtocolProtocol from the from the Switch ManagementSwitch Management menu and then two options menu and then two options will be displayed as follows.

will be displayed as follows.

1. STP Switch Settings:

1. STP Switch Settings: To set up system priority, MAX Age, Hello Time, etc.To set up system priority, MAX Age, Hello Time, etc.

2. STP Port Settings:

2. STP Port Settings: To set up port cost and priority.To set up port cost and priority.

2.5.4.5.1 STP Switch Settings 2.5.4.5.1 STP Switch Settings

Select the option

Select the option STP Switch SettingsSTP Switch Settings fromfrom Spanning Tree ProtocolSpanning Tree Protocol  menu, then the  menu, then the following screen appears.

following screen appears.

Spanning Tree Protocol:

Spanning Tree Protocol: Enable or disable Spanning Tree Protocol.Enable or disable Spanning Tree Protocol.

Designated Root:

Designated Root: View-only field that shows the MAC address of the designated root. View-only field that shows the MAC address of the designated root.

Root Path Cost:

Root Path Cost: View-only field that shows the path View-only field that shows the path cost of the designated root.cost of the designated root.

Root Port:

Root Port: View-only field that shows the root port.View-only field that shows the root port.

Root Max Age:

Root Max Age: View-only field that shows the max age of the designated root.View-only field that shows the max age of the designated root.

Root Hello Time:

Root Hello Time: View-only field that shows the hello time of the designated root.View-only field that shows the hello time of the designated root.

Root Forward Delay:

Root Forward Delay: View-only field that shows the forward delay time of the designatedView-only field that shows the forward delay time of the designated root.

root.

Root Bridge Priority:

Root Bridge Priority: View-only field that shows the bridge priority value of the designatedView-only field that shows the bridge priority value of the designated root.

root.

Max Age:

Max Age: If another switch in the spanning tree does not send out a hello packet for a long If another switch in the spanning tree does not send out a hello packet for a long period of time, it is assumed to be disconnected. This default timeout is 20 seconds.

period of time, it is assumed to be disconnected. This default timeout is 20 seconds.

Hello Time:

Hello Time: Periodically, a hello packet is sent out by the Root Bridge and the Designated Periodically, a hello packet is sent out by the Root Bridge and the Designated Bridge that are used to communicate information about the topology throughout the entire Bridge that are used to communicate information about the topology throughout the entire Bridged Local Area Network. each switch has a relative priority and cost that

each switch has a relative priority and cost that is used to decide what the shortest path is is used to decide what the shortest path is toto forward a packet. The lowest cost path is always used unless the other path is down. If you forward a packet. The lowest cost path is always used unless the other path is down. If you have multiple bridges and interfaces then you may need to adjust the priorities to achieve have multiple bridges and interfaces then you may need to adjust the priorities to achieve optimized performance. The switch with the lowest priority

optimized performance. The switch with the lowest priority will be selected as the will be selected as the root bridge.root bridge.

The root bridge is the “central” bridge in the spanning tree.

The root bridge is the “central” bridge in the spanning tree.

2.5.4.5.2 STP Port Settings

STP Port State (Off/On): Use the spacebar to turn on or off STP mode. Use the spacebar to turn on or off STP mode.

STP Port Cost (0-65535):

STP Port Cost (0-65535): This parameter is used by the STP to determine the best path This parameter is used by the STP to determine the best path between devices. Therefore, lower values should be assigned to ports attached to faster

STP Port Priority (0-255): Choose a value between 0 and 255 to  Choose a value between 0 and 255 to set the priority for the portset the priority for the port interface. A higher priority will designate the interface to forward packets first. A lower interface. A higher priority will designate the interface to forward packets first. A lower number denotes a higher priority.

number denotes a higher priority.