Spanning Tree Protocol

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The Spanning Tree Protocol’s purpose is preventing bridging loops and allows a network to quick recover if the primary path fails. STP was invented by Radia Perlman from the Digital Equipment Corporation.

Spanning-Tree Protocol is a way to have redundant paths and stop loops from having. All Ethernet networks only work well if there is only one path between two points. Having many different paths will cause loops in a network. This cause frames to be duplicated. The Spanning-Tree protocol will define all switches in the network. The algorithm will automatically change the STP topology by activating or inactivating certain paths, so that only one path will be present between two points. The end-devices do not know they are connected to one network or many networks.

Election of the Root Switch The switches in the network gather information with the exchange of data messages. These are called bridge protocol data units (BPDUs). There is an election for a root switch on every LAN segment. To stop loops STP by putting redundant switches ports into a backup state. The root switch is the place where the STP algorithm runs. All other paths not needed to reach the root switch are put in to backup mode

Root Switch Variables

Variable	Description
Hello Time	How often a hello is broadcasted to the other switches, so the switch knows the other switches are functioning.
Maximum Age Timer	Defines how long a bridge or switch should wait after the last received hello message before believing that the network topology has changed.
Forward Delay Timer	Time the switch has spent learning/listening before it begins forwarding traffic.

Information a BPDU contains

• MAC addresses
• Switch priority
• Port priority
• Port cost

STP Steps Elects a root bridge Only one bridge can exist per VLAN Designated ports send and receive traffic and configuration trafic called BPDUs The switch with the lowest priority will become the root bridge

Selects the root port for all other switches One port on all switches, that are not root bridges, This port is the lowest cost path to the root bridge. If two ports have the same priority, the lowest port number will become the root port.

STP Steps

• Elects a root bridge
  • Only one bridge can exist per VLAN
  • Designated ports send and receive traffic and configuration trafic called BPDUs
  • The switch with the lowest priority will become the root bridge

• Selects the root port for all other switches
  • One port on all switches, that are not root bridges,
  • This port is the lowest cost path to the root bridge.
  • If two ports have the same priority, the lowest port number will become the root port.

Port Roles on a Nondesignated Switch

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Port	Description
Root	The port is on nonroot bridges. It is considered to be the best path. The root port is able to send traffic in the direction of the root bridge. This port will add entries to its MAC address table. There is only one root port per bridge.
Designated	This port is on the root bridge and on the nonroot bridges. All switch ports are designated ports. There can only be one designated port per segment, unless it’s the root bridge where all the ports are designated ports. If there is more than one switch in the segment, an election will be held.
Nondesignated	The nondesignated port is in the blocking state. It will not add new entries to its MAC address table.
Disabled	The port is shutdown.